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Vapour Pressure of Water Calculator

Ready to calculate
Antoine equation.
NIST coefficients.
7 pressure units.
100% Free.
No Data Stored.

How it Works

01Enter Temperature

Water temperature in °C, °F, or K. Range: −100 °C to +374 °C (critical point).

02Apply Antoine Equation

log P = A − B/(C + T) with NIST coefficients. Two regimes: 1-100 °C and 99-374 °C; ice sublimation below 0 °C.

03Phase Classification

Solid (ice — sublimation), Liquid (vapor-liquid equilibrium), or Supercritical (above 374 °C / 220 bar).

04Get P in 7 Unit Systems

Pa, kPa, hPa = mbar, bar, atm, mmHg = torr, psi — pick whichever your application needs.

What is a Vapour Pressure of Water Calculator?

Saturation vapor pressure of water is the equilibrium pressure exerted by water vapor over liquid water (or ice) at a given temperature — and it's one of the most-used reference quantities in physical chemistry, atmospheric science, food engineering, and HVAC design. From dew-point and humidity calculations in air-conditioning to autoclave sterilization at 121 °C / 2 bar to supercritical hydrothermal synthesis above 374 °C, water vapor pressure governs every thermal-chemical equilibrium that involves H₂O. Our calculator implements the standard Antoine equation with NIST WebBook coefficients across the full liquid range and switches to the Tetens / Goff-Gratch sublimation formula for ice below 0 °C.

The Antoine equation has the deceptively simple form log₁₀(P) = A − B/(C + T), with three coefficients fit to experimental vapor-pressure data. For water, NIST publishes two coefficient sets covering 1-100 °C (A = 8.07131, B = 1730.63, C = 233.426 with P in mmHg) and 99-374 °C (A = 8.14019, B = 1810.94, C = 244.485) — the calculator automatically selects the appropriate set. Output: vapor pressure in 7 pressure unit systems simultaneously — Pa (SI), kPa, hPa = mbar, bar, atm, mmHg = torr, and psi — so you can pick whatever your reference document or instrument expects without manual conversion. The result panel also includes a phase-regime classification (solid ice with sublimation pressure, liquid water with vapor-liquid equilibrium, or supercritical above the critical point) and smart warnings for above-1-atm vapor pressures (sealed-vessel territory) and high-pressure regimes where the simple Antoine deviates from research-grade IAPWS-IF97 steam tables.

Designed for chemistry students learning vapor-pressure curves and the Clausius-Clapeyron relation, atmospheric scientists computing dew point and frost point, HVAC engineers sizing humidification / dehumidification systems, food and pharmaceutical engineers designing autoclaves and freeze dryers, brewers and distillers tracking boiling-point shifts at altitude, and any researcher working with water-vapor equilibria — runs entirely in your browser, no account, no data stored.

Pro Tip: Pair this with our Molarity Calculator for solution chemistry, our Partial Pressure Calculator for gas mixtures, our Molality Calculator for colligative-property work, or our Gibbs Phase Rule Calculator for multi-phase equilibria.

How to Use the Vapour Pressure of Water Calculator?

Enter the Water Temperature: in °C, °F, or K. The calculator handles unit conversion internally. Range: −100 °C to +374 °C (the critical point of water). Below 0 °C, the calculator uses the SUBLIMATION pressure of ice (ice ⇌ vapor); above 0 °C, the standard Antoine vapor pressure of liquid water.
Apply the Antoine Equation: log₁₀(P_mmHg) = A − B / (C + T_°C), with NIST coefficients automatically selected by temperature regime. 1-100 °C: A = 8.07131, B = 1730.63, C = 233.426. 99-374 °C: A = 8.14019, B = 1810.94, C = 244.485.
Convert to Multiple Pressure Units: the calculator returns Pa, kPa, hPa = mbar, bar, atm, mmHg = torr, and psi simultaneously. Conversions: 1 atm = 760 mmHg = 101.325 kPa = 1.01325 bar = 14.696 psi.
Read the Phase Regime: Solid (ice) below 0 °C — sublimation pressure (ice → vapor). Liquid 0-374 °C — standard Antoine regime, vapor-liquid equilibrium. Supercritical above 374 °C and 220.6 bar — water is a single supercritical fluid, no longer separate liquid + vapor.
Use for Psychrometric Calculations: Relative humidity (%RH) = 100 × (actual partial pressure of water vapor) / (saturation pressure at that T). For dew point: at the dew point T_dp, the actual partial pressure equals the saturation pressure — so given air at T = 25 °C with 50% RH, dew point is the T at which P_sat = 0.50 × 23.76 mmHg = 11.88 mmHg, which corresponds to ~14 °C.
Cross-Check Against Reference Values: 0 °C: 4.58 mmHg (= 0.611 kPa). 25 °C: 23.76 mmHg (= 3.169 kPa). 37 °C body temp: 47.07 mmHg (= 6.275 kPa). 100 °C boiling at 1 atm: 760.0 mmHg (= 101.32 kPa = 1 atm). 121 °C autoclave: ~1500 mmHg (= 200 kPa = 2 atm).
Adjust for Solutes (Raoult's Law): for solutions, P = P°(water) × x_water, where x_water is the mole fraction of water and P°(water) is the pure-water vapor pressure from this calculator. Example: 0.5 mol fraction sucrose solution has water vapor pressure = 0.5 × P°(water).
For High-Precision Steam-Table Work: the Antoine equation is accurate to ~0.5% in 1-100 °C and ~1% in 100-374 °C; for research-grade precision (e.g. NIST-traceable thermophysical work, supercritical-water reactor design), use the IAPWS Industrial Formulation 1997 (IF97) directly via NIST REFPROP or IAPWS-95 software.

How is water vapor pressure calculated?

The water vapor pressure curve is one of the most-cited thermophysical references in chemistry — the Antoine equation gives a fast, accurate empirical fit, while the IAPWS steam tables provide research-grade precision for engineering applications.

References: NIST Chemistry WebBook; Antoine, Comp. Rend. 107 (1888) 681; Goff & Gratch (1946); IAPWS Industrial Formulation 1997 (IF97); CRC Handbook of Chemistry and Physics.

Antoine Equation

log₁₀(P) = A − B / (C + T)

Where P is the saturation vapor pressure (in mmHg), T is the temperature (in °C), and A, B, C are empirical coefficients fit to experimental data for each substance. For water across two T ranges:

  • 1 ≤ T ≤ 100 °C: A = 8.07131, B = 1730.63, C = 233.426. Accurate to ~0.5%.
  • 99 ≤ T ≤ 374 °C: A = 8.14019, B = 1810.94, C = 244.485. Accurate to ~1%.

Worked Example — Vapor Pressure at Room Temperature

T = 25 °C; first-range coefficients.

  • log₁₀(P) = 8.07131 − 1730.63 / (233.426 + 25) = 8.07131 − 1730.63 / 258.426 = 8.07131 − 6.69624 = 1.37507.
  • P = 10^1.37507 = 23.76 mmHg = 3.169 kPa = 0.0313 atm.
  • Reference value: 23.756 mmHg at 25 °C (CRC Handbook). Agreement < 0.1%.

Worked Example — Boiling Point at 1 atm

At T = 100 °C, vapor pressure should equal 1 atm = 760 mmHg.

  • log₁₀(P) = 8.07131 − 1730.63 / (233.426 + 100) = 8.07131 − 1730.63 / 333.426 = 8.07131 − 5.18999 = 2.88132.
  • P = 10^2.88132 = 761.0 mmHg = 1.0013 atm.
  • Off by 0.13% from exactly 760 mmHg = 1.000 atm — a small offset because Antoine fits are not perfect; precise definition of 100 °C as boiling point at 1 atm fixes the small discrepancy.

Worked Example — Autoclave Conditions

Standard autoclave: 121 °C for 15 min sterilization. Use the second-range coefficients (T > 99 °C).

  • log₁₀(P) = 8.14019 − 1810.94 / (244.485 + 121) = 8.14019 − 1810.94 / 365.485 = 8.14019 − 4.95491 = 3.18528.
  • P = 10^3.18528 = 1532 mmHg = 204.3 kPa = 2.016 atm = 1.49 psi gauge above 14.7 psi atmospheric.
  • Reference: standard autoclave operates at ~15 psi gauge (~30 psi absolute) at 121 °C — close to our calculation.

Reference Vapor Pressures (CRC Handbook, NIST)

  • −40 °C (ice): 0.097 mmHg = 12.9 Pa (sublimation).
  • −20 °C (ice): 0.776 mmHg = 103 Pa.
  • 0 °C (triple point): 4.58 mmHg = 611.7 Pa = 0.0061 atm.
  • 10 °C: 9.21 mmHg = 1.228 kPa.
  • 20 °C: 17.55 mmHg = 2.339 kPa.
  • 25 °C (room): 23.76 mmHg = 3.169 kPa.
  • 30 °C: 31.84 mmHg = 4.246 kPa.
  • 37 °C (body T): 47.07 mmHg = 6.275 kPa.
  • 50 °C: 92.55 mmHg = 12.34 kPa.
  • 75 °C: 289.1 mmHg = 38.55 kPa.
  • 100 °C (1 atm boiling): 760.0 mmHg = 101.32 kPa = 1 atm.
  • 121 °C (autoclave): ~1532 mmHg ≈ 2.0 atm.
  • 200 °C: ~11,659 mmHg ≈ 15.5 atm.
  • 300 °C: ~64,432 mmHg ≈ 85.9 atm.
  • 374 °C (critical): ~165,488 mmHg ≈ 220.6 bar = critical pressure P_c.

Pressure Unit Conversions

  • 1 atm = 760 mmHg = 760 torr = 101,325 Pa = 101.325 kPa = 1.01325 bar = 1013.25 hPa = 14.696 psi.
  • 1 bar = 750.06 mmHg = 100,000 Pa = 14.504 psi.
  • 1 mmHg = 1 torr ≈ 133.322 Pa.
  • 1 hPa = 1 mbar = 100 Pa.
  • 1 psi = 6894.76 Pa = 51.715 mmHg.
Real-World Example

Worked Example — Compute Boiling Point at Mountain Altitude

Question: A hiker is camping on Mount Everest at altitude where atmospheric pressure is 240 mmHg (vs 760 mmHg at sea level). At what temperature does water boil?

Step 1 — Boiling Point Definition. Water boils when its saturation vapor pressure equals the ambient atmospheric pressure: P_sat(T_boil) = P_ambient.

  • At sea level (760 mmHg): boiling at 100 °C ✓.
  • At Mount Everest (240 mmHg): boiling at lower T because lower atmospheric pressure means lower required vapor pressure.

Step 2 — Solve Antoine for T Given P = 240 mmHg.

  • log₁₀(240) = 8.07131 − 1730.63 / (233.426 + T).
  • 2.380 = 8.07131 − 1730.63 / (233.426 + T).
  • 1730.63 / (233.426 + T) = 5.691.
  • 233.426 + T = 304.10.
  • T = 70.7 °C.

Step 3 — Verify with Calculator. Enter T = 70.7 °C → vapor pressure should be ~240 mmHg.

  • log₁₀(P) = 8.07131 − 1730.63 / (233.426 + 70.7) = 8.07131 − 1730.63 / 304.126 = 8.07131 − 5.69052 = 2.38079.
  • P = 10^2.38079 = 240.4 mmHg ✓.

Step 4 — Practical Implications.

  • Cooking implications: at 70 °C, eggs don't fully cook in boiling water; pasta and rice take much longer; brown rice may not soften adequately. Mountaineers use pressure cookers to maintain 100 °C+ at altitude.
  • Medical: sterilization by boiling is INADEQUATE at high altitude — standard protocol requires 100 °C for 10+ minutes; at 70 °C many spore-formers survive.
  • Brewing & distillation: shifts the equilibrium of volatile compounds; ethanol BP also shifts at altitude (78 °C at sea level → ~70 °C at 4000 m).

Step 5 — Altitude-Boiling-Point Reference.

  • Sea level (760 mmHg): 100 °C.
  • Denver, CO (1600 m, ~630 mmHg): ~95 °C.
  • La Paz, Bolivia (3650 m, ~490 mmHg): ~88 °C.
  • Mt. Everest base camp (5300 m, ~395 mmHg): ~83 °C.
  • Mt. Everest summit (8849 m, ~240 mmHg): ~70 °C.
  • Rule of thumb: boiling point drops ~1 °C per 280 m (920 ft) of altitude gained.

Who Should Use the Vapour Pressure Calculator?

1
Compute saturation vapor pressure for relative humidity, dew point, frost point, and cloud microphysics calculations. Standard quantity in psychrometric and climate-modeling work.
2
Size humidification / dehumidification systems by matching saturation vapor pressure at design T to target indoor humidity. Standard in psychrometric chart calculations and energy-balance modeling.
3
Standard autoclave at 121 °C runs at ~2 atm (saturation vapor pressure of water at 121 °C). Pressure cookers achieve faster cooking at 1.5-2 atm by raising water boiling point to 110-120 °C.
4
Predict vapor-liquid equilibria in distillation column design; understand boiling-point shifts at altitude for brewing, distilling, and food preparation.
5
Below 0 °C the ice sublimation pressure determines lyophilization (freeze-drying) cycle parameters; primary drying typically occurs at sub-mbar shelf pressure.
6
Sterilization protocols, water-activity calculations (a_w = P_water_solution / P_water_pure), and shelf-life modeling all depend on water vapor pressure across the storage T range.
7
Standard exercise for the Clausius-Clapeyron relation, vapor-pressure curves, and phase diagrams. The water example is the most-cited reference case.

Technical Reference

Antoine Equation Origin. Louis Charles Antoine (1825-1897), French chemist, proposed his empirical vapor-pressure equation in 1888 (Comp. Rend. Acad. Sci. 107, 681 and 836). The three-parameter form A − B/(C + T) is a refinement of the earlier two-parameter Clausius-Clapeyron form (which assumes constant heat of vaporization). The Antoine constants are not derived from first principles; they are fit to experimental data and span specified T ranges. For water, the standard NIST WebBook coefficients are taken from Bridgeman & Aldrich (1964, NBS J. Res.) and refined data sets. Modern usage often uses the extended Antoine form (5 parameters) for higher accuracy across wider T ranges, but the 3-parameter form remains the textbook standard.

Clausius-Clapeyron Relation. The Antoine equation's thermodynamic foundation is the Clausius-Clapeyron equation: dP/dT = ΔH_vap × P / (R × T²), where ΔH_vap is the molar heat of vaporization. For water at 100 °C, ΔH_vap = 40.65 kJ/mol; at 25 °C, ΔH_vap = 43.99 kJ/mol (slightly higher because the liquid is more cohesive at lower T). The Antoine equation is an empirical solution of the Clausius-Clapeyron differential equation that approximates the T-dependence of ΔH_vap.

Critical Point of Water. T_c = 374.15 °C = 647.30 K; P_c = 220.64 bar = 22.064 MPa = 217.7 atm; ρ_c = 322 kg/m³. Above the critical point, water is a single supercritical fluid with no separate liquid + vapor phases. The vapor pressure curve technically ends at the critical point — above T_c, "vapor pressure" is not defined; instead use density and enthalpy directly from IAPWS equations of state. Supercritical water is used in: (1) supercritical-water oxidation (SCWO) for waste destruction; (2) hydrothermal synthesis of nanoparticles; (3) supercritical Rankine cycle power plants (~600 °C, 250 bar, 45% efficiency); (4) chromatography (supercritical CO₂ is more common but supercritical water has been used for special separations).

Triple Point of Water. T_t = 0.01 °C = 273.16 K; P_t = 6.117 mbar = 4.587 mmHg = 611.7 Pa. The water triple point was the SI definition of the kelvin from 1954 to 2019 (now redefined via Boltzmann's constant). At the triple point, ice + liquid water + water vapor coexist at equilibrium. Below this T (and 0 °C is the standard atmospheric ice melting point because of pressure: actually slightly > 273.15 K), water is solid (ice); above, water is liquid up to the BP curve.

Ice Vapor Pressure Below 0 °C. The Tetens / Goff-Gratch / Murphy-Koop formulas give the sublimation pressure of ice over the −100 to 0 °C range:

  • Tetens (simplified): P_ice (hPa) = 6.1078 × 10^(9.5 × T / (T + 265.5)).
  • Murphy-Koop (more accurate): ln P_ice = 9.550426 − 5723.265/T + 3.53068 ln T − 0.00728332 T (T in K, P in Pa).
  • Reference values: −40 °C: 12.9 Pa; −20 °C: 103 Pa; 0 °C: 611.7 Pa.
  • Used for: frost-point hygrometry, freeze-drying (lyophilization), polar atmospheric chemistry, cryoprotectant formulation.

Psychrometric Calculations. Relative humidity (%RH) = 100 × actual partial pressure of water vapor / saturation pressure at that T. Dew point: the T at which saturation pressure equals the actual partial pressure (cooling air to dew point causes condensation). Frost point: below 0 °C, the T at which saturation pressure over ICE equals the actual partial pressure (cooling causes frost formation, not liquid condensation). Wet-bulb temperature: psychrometric T defined by adiabatic saturation; differs slightly from dew point. Standard psychrometric charts (ASHRAE Handbook) use saturation vapor pressure as the foundation.

Raoult's Law for Solutions. For an ideal solution containing a non-volatile solute, the partial pressure of water vapor is reduced: P_water (solution) = x_water × P°_water, where x_water is the mole fraction of water and P°_water is the pure-water vapor pressure at that T. For non-ideal solutions, multiply by an activity coefficient: P = γ_water × x_water × P°. Practical: a 0.5 mole-fraction sucrose solution has water vapor pressure = 0.5 × P°. For NaCl solutions, the apparent mole fraction includes both ions (Na⁺ + Cl⁻), so a 1 m NaCl solution has effective x_water ≈ 0.965 → vapor pressure ≈ 0.965 × P°.

IAPWS Steam Tables. For research-grade and engineering-grade precision, use the International Association for the Properties of Water and Steam (IAPWS) formulations. The Industrial Formulation 1997 (IF97) is the standard for power-plant and process engineering; the IAPWS-95 fundamental equation of state is the standard for scientific work. Both are accurate to < 0.01% across the full water phase diagram, including the supercritical region. Software: NIST REFPROP, IAPWS-Cool, FluidProp, SteamTables.com, and many others.

Practical Reference Values (CRC Handbook of Chemistry and Physics). Memorize: 0 °C → 4.58 mmHg = 0.611 kPa; 25 °C → 23.76 mmHg = 3.169 kPa; 37 °C → 47.07 mmHg = 6.275 kPa (body T); 100 °C → 760 mmHg = 101.32 kPa = 1 atm (BP at 1 atm); 121 °C → ~1500 mmHg ≈ 2 atm (autoclave); 200 °C → ~12000 mmHg ≈ 16 atm; 300 °C → ~65000 mmHg ≈ 86 atm; 374 °C → 220 bar (critical point). References: NIST Chemistry WebBook; Antoine (1888); Bridgeman & Aldrich (1964); Goff & Gratch (1946); IAPWS-IF97 (1997); CRC Handbook of Chemistry and Physics (annual).

Conclusion

Water vapor pressure as a function of temperature is one of the most-tabulated reference quantities in physical chemistry — and the Antoine equation gives a fast, accurate empirical formula across the full liquid range (1-374 °C). Memorize the milestones: 0 °C: 4.58 mmHg; 25 °C: 23.76 mmHg; 100 °C: 760 mmHg = 1 atm; 121 °C: ~2 atm (autoclave); 374 °C: 220.6 bar (critical point). These five reference points anchor every vapor-pressure problem you'll encounter in chemistry, atmospheric science, food/pharmaceutical engineering, and HVAC.

Two operational reminders: (1) The Antoine equation is empirical — accurate to ~0.5% in 1-100 °C, ~1% in 100-374 °C. For research-grade precision (NIST-traceable thermophysical work, supercritical-water reactor design), use IAPWS Industrial Formulation 1997 (IF97) via NIST REFPROP or equivalent software. (2) For solutions and mixtures, apply Raoult's law — the actual vapor pressure of water in solution is reduced by the mole fraction of water (P = x_water × P°_water). For seawater (about 0.97 mole fraction water), vapor pressure is ~3% lower than pure water at the same T. The calculator gives PURE water reference values; multiply by x_water for solution applications.

Frequently Asked Questions

What is the Vapour Pressure of Water Calculator?
It implements the standard Antoine equation (NIST WebBook coefficients) for the saturation vapor pressure of water across the full liquid range 1-374 °C, with automatic extension to ice sublimation pressure below 0 °C via the Tetens / Goff-Gratch formula. Input: temperature in °C, °F, or K. Output: vapor pressure in 7 unit systems (Pa, kPa, hPa = mbar, bar, atm, mmHg = torr, psi) plus phase classification (solid / liquid / supercritical).

Pro Tip: Pair this with our Molarity Calculator.

What is the vapor pressure of water?
The equilibrium pressure exerted by water vapor over liquid water (or ice) at a given temperature. Symbol P_sat or P°_water; units Pa, mmHg, bar, atm, etc. Reference values: 0 °C: 4.58 mmHg = 0.611 kPa; 25 °C (room): 23.76 mmHg = 3.169 kPa; 37 °C (body): 47.07 mmHg = 6.275 kPa; 100 °C (boiling at 1 atm): 760 mmHg = 101.32 kPa = 1 atm. Vapor pressure roughly doubles every 10-12 °C in the typical 0-100 °C range.
What's the formula for water vapor pressure?
Antoine equation: log₁₀(P) = A − B / (C + T), with NIST coefficients for water: 1-100 °C: A = 8.07131, B = 1730.63, C = 233.426 (P in mmHg, T in °C). 99-374 °C: A = 8.14019, B = 1810.94, C = 244.485. For higher precision, use the IAPWS Industrial Formulation 1997 (IF97) or IAPWS-95 fundamental equation of state.
Why does water boil at 100 °C?
Because at 100 °C, water's saturation vapor pressure equals 1 atm. Boiling occurs when vapor pressure equals the ambient pressure: bubbles can form throughout the liquid (not just at the surface). At sea level, ambient is 1 atm = 760 mmHg, and the temperature where water's vapor pressure equals this is exactly 100 °C — by historical definition (the Celsius scale was originally calibrated this way; modern SI defines the kelvin via Boltzmann's constant, but the BP shift is < 0.01 °C). At higher altitudes (lower atmospheric pressure), water boils at lower T — Mt. Everest at ~240 mmHg gives BP = ~70 °C.
How does water boil at lower temperature at high altitude?
Atmospheric pressure decreases with altitude (about half at 5500 m, about a third at 8800 m). Boiling occurs when water's saturation vapor pressure equals the ambient atmospheric pressure — so at lower atmospheric pressure, less vapor pressure is needed, which corresponds to lower temperature. Reference table: sea level 100 °C; Denver 1600 m / 630 mmHg → ~95 °C; La Paz 3650 m / 490 mmHg → ~88 °C; Everest base camp 5300 m / 395 mmHg → ~83 °C; Everest summit 8849 m / 240 mmHg → ~70 °C. Rule of thumb: ~1 °C BP drop per 280 m altitude gain.
What is the autoclave temperature and pressure?
Standard autoclave: 121 °C and ~2 atm (200 kPa, 15 psi gauge). The 121 °C temperature is needed to kill heat-resistant bacterial spores (Geobacillus stearothermophilus) within the 15-minute sterilization window. To get water to 121 °C without boiling away, you must seal it under pressure equal to its saturation vapor pressure at that T, which is ~2 atm. Some applications use 134 °C / ~3 atm for faster sterilization (3-minute cycle, used in vacuum autoclaves for prion deactivation).
How do I compute relative humidity from vapor pressure?
%RH = 100 × actual partial pressure of water vapor / saturation pressure at that T. Example: air at 25 °C with measured water vapor partial pressure = 12 mmHg. P_sat at 25 °C = 23.76 mmHg. RH = 100 × 12 / 23.76 = 50.5%. Dew point is the T at which 12 mmHg becomes the saturation pressure (i.e. cooling causes condensation); from the calculator, that's ~14 °C. For frost point (below 0 °C), use the ice sublimation pressure at that T as the saturation reference instead of liquid-water Antoine.
What's the vapor pressure of water at 25 °C?
23.76 mmHg = 3.169 kPa = 0.0313 atm. Reference value at "room temperature" used universally in chemistry textbooks. From Antoine: log P = 8.07131 − 1730.63/(233.426 + 25) = 8.07131 − 6.69624 = 1.37507; P = 10^1.37507 = 23.76 mmHg. Other useful values: 20 °C: 17.55 mmHg; 30 °C: 31.84 mmHg.
What happens to water vapor pressure above 374 °C?
Above 374.15 °C and 220.64 bar — the critical point of water — water becomes a SUPERCRITICAL fluid. There is no longer a separate liquid + vapor phase; instead a single fluid with continuous density transitioning from gas-like to liquid-like behavior. The concept of "saturation vapor pressure" no longer applies. Supercritical water is used in: (1) Supercritical-water oxidation (SCWO) — waste destruction at 600 °C / 250 bar; organic compounds oxidize completely to CO₂, H₂O, mineral acids. (2) Hydrothermal synthesis — nanoparticle / inorganic-material synthesis. (3) Supercritical Rankine cycle power plants — modern coal/nuclear plants operate at 540-600 °C / 220-300 bar for ~45% thermal efficiency.
How accurate is the Antoine equation?
~0.5% in 1-100 °C and ~1% in 100-374 °C compared to high-precision experimental data and the IAPWS steam tables. Sources of error: (1) the 3-parameter form is empirical and cannot capture all the curvature of the real vapor-pressure-T relationship; (2) data scatter in the original fits. For research-grade precision (NIST-traceable, supercritical-water reactor design, climate-modeling), use IAPWS-IF97 or IAPWS-95 directly via NIST REFPROP or similar software. For most chemistry and engineering applications, the Antoine equation is sufficient.
How does dissolved solute change water vapor pressure?
Raoult's law: P_solution = x_water × P°_water. The dissolved solute reduces the mole fraction of water (x_water < 1), so the vapor pressure decreases proportionally. Examples: 0.5 mole fraction sucrose solution at 25 °C: vapor pressure = 0.5 × 23.76 = 11.88 mmHg. 1 m NaCl solution (which dissociates to Na⁺ + Cl⁻, so effective particle mole fraction is doubled): x_water_effective ≈ 0.965, vapor pressure ≈ 0.965 × 23.76 = 22.93 mmHg. Practical implication: seawater (3.5% w/w salt) has vapor pressure ~98% of pure water at the same T — basis of solar-still desalination. Higher-concentration solutions (saturated NaCl ~26%, syrups, sugar concentrates) have markedly lower vapor pressure.

Author Spotlight

The ToolsACE Team - ToolsACE.io Team

The ToolsACE Team

Our ToolsACE chemistry team built this calculator to handle the saturation vapor pressure of water across the full liquid range using the standard <strong>Antoine equation</strong>: log₁₀(P) = A − B / (C + T). The calculator uses NIST WebBook coefficients in two T regimes: <strong>1-100 °C</strong> (A = 8.07131, B = 1730.63, C = 233.426) and <strong>99-374 °C</strong> (A = 8.14019, B = 1810.94, C = 244.485) — covering everything from cool-water laboratory work to high-pressure / high-temperature steam-table applications. Below 0 °C the calculator switches to the Tetens / Goff-Gratch sublimation pressure of ice, accurate to ~0.5% in the 0 to −40 °C range. Input temperature in °C, °F, or K; output in <strong>7 pressure unit systems</strong>: Pa (SI), kPa, hPa = mbar, bar, atm, mmHg = torr, and psi. Smart phase classification labels the result as solid (ice sublimation), liquid (normal vapor-liquid equilibrium), or supercritical (above the critical point T_c = 374 °C, P_c = 220.6 bar).

NIST Chemistry WebBook (NIST Standard Reference Database 69)Antoine, J. M. (1888) Comp. Rend.; Bridgeman & Aldrich (1964) NBS J. Res.IAPWS Industrial Formulation 1997 (IF97) for steam tables (cross-reference)

Disclaimer

The Antoine equation is empirical; accurate to ~0.5% in 1-100 °C, ~1% in 100-374 °C. For research-grade precision use IAPWS-IF97 or IAPWS-95 fundamental equation of state via NIST REFPROP or equivalent. Below 0 °C the calculator returns ICE SUBLIMATION pressure (Tetens / Goff-Gratch), not metastable supercooled liquid. Above 374 °C and 220.6 bar water is supercritical — no longer separate liquid + vapor; vapor pressure undefined. For solutions, apply Raoult's law: P = x_water × P°_water. The calculator gives PURE water reference values. References: NIST Chemistry WebBook; Antoine (1888); Bridgeman & Aldrich (1964); Goff & Gratch (1946); IAPWS-IF97 (1997); CRC Handbook of Chemistry and Physics.