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VPD Calculator

Ready to calculate
Tetens-Magnus Formula.
Air + Leaf VPD.
Stage Classification.
100% Free.
No Data Stored.

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How it Works

01Enter Temperature

Air or leaf temperature in °C or °F.

02Enter Humidity

Relative humidity as a percentage.

03Compute VPD

Tetens-Magnus equation gives VPD in kPa.

04Stage Classification

Propagation, vegetative, flowering, or stress range.

What is a VPD Calculator?

Vapor Pressure Deficit (VPD) is the difference between the amount of moisture air can hold at saturation and the amount it actually holds. It drives plant transpiration, nutrient uptake, and stomatal behavior — making it the single most important environmental metric in commercial greenhouses, indoor cannabis grows, and controlled-environment agriculture.


The calculator takes air temperature, relative humidity, and (optionally) leaf temperature and returns VPD in kilopascals (kPa). Recommended ranges: 0.4–0.8 kPa for clones/seedlings, 0.8–1.2 kPa for vegetative growth, 1.2–1.6 kPa for flowering. Too low → poor transpiration → calcium deficiency, fungal pressure. Too high → stomatal closure, nutrient lockout, wilting.

How to Use the Calculator

Enter air temperature: °C or °F. Use the canopy-level reading, not ambient room.
Enter relative humidity (%): measured at canopy level with a calibrated hygrometer.
Optional: leaf temperature: measured with IR thermometer. Leaf is typically 2–3 °C below air under transpiration.
Calculate: Returns Air VPD (using air temp) and Leaf VPD (using leaf temp) in kPa.
Compare to target range for your growth stage and adjust environment.

The Math Behind It

Saturation vapor pressure (Tetens equation):


SVP(T) = 0.6108 × exp(17.27 × T / (T + 237.3)) [kPa, T in °C]


Actual vapor pressure: AVP = SVP(T_air) × RH/100


Air VPD: VPD_air = SVP(T_air) − AVP = SVP(T_air) × (1 − RH/100)


Leaf VPD: VPD_leaf = SVP(T_leaf) − AVP — uses leaf temp for saturation but air's actual vapor pressure. This is the physiologically meaningful VPD because it drives stomatal water loss.

Real-World Example

Worked Example

Vegetative cannabis at 25 °C, 60% RH, leaf at 23 °C:

  • SVP(25) = 3.169 kPa · SVP(23) = 2.810 kPa
  • AVP = 3.169 × 0.6 = 1.901 kPa
  • VPD_air = 3.169 − 1.901 = 1.27 kPa
  • VPD_leaf = 2.810 − 1.901 = 0.91 kPa ← within 0.8–1.2 kPa veg target

Who Uses It

1
🌿 Cannabis Cultivators: Dial in clone, veg, and flower environment.
2
🌷 Greenhouse Growers: Manage transpiration for tomatoes, peppers, cucumbers.
3
🍓 Vertical Farms: Calibrate HVAC and dehumidification for leafy greens.
4
🌱 Tissue Culture Labs: Acclimate plantlets from 100% RH to outside environment.
5
🍄 Mushroom Cultivators: Maintain high humidity (low VPD) for fruiting.
6
🔬 Plant Researchers: Standardize growth chamber conditions.

Technical Reference

VPD targets by stage:

  • Clones / seedlings: 0.4–0.8 kPa (high humidity, low VPD)
  • Vegetative: 0.8–1.2 kPa
  • Early flower: 1.0–1.4 kPa
  • Late flower: 1.2–1.6 kPa (push transpiration, reduce mold risk)
  • Tomatoes (greenhouse): 0.5–1.0 kPa typically

Common reference points (air VPD):

  • 20°C / 70% RH → 0.70 kPa
  • 25°C / 60% RH → 1.27 kPa
  • 28°C / 50% RH → 1.89 kPa
  • 30°C / 40% RH → 2.55 kPa (stress range)

Key Takeaways

VPD captures the actual driving force for plant transpiration in a single number that combines temperature and humidity. Use leaf VPD (with IR-measured leaf temperature) for the most physiologically meaningful reading. Targets: 0.4–0.8 kPa clones, 0.8–1.2 veg, 1.2–1.6 flower. Cannabis growers especially: VPD beats RH alone as the environmental control variable.

Frequently Asked Questions

Air VPD or leaf VPD — which one?
Leaf VPD is more accurate physiologically because stomatal evaporation is driven by leaf-temperature saturation. Air VPD is easier to measure (no IR thermometer needed) and works as a proxy when leaf is near air temperature. Best practice: use leaf VPD when CO₂ is enriched or LED lighting is intense (leaf can run several °C above air).
My RH meter is unreliable. Now what?
Get a calibrated hygrometer (Sensirion-based, ±2% RH) or run a salt test. A $15 cheap meter can be 10% off, which throws VPD off by 0.3+ kPa. For commercial use, sensor-grade instruments (~$100+) pay back quickly in avoided crop losses.
Why does too-low VPD cause calcium deficiency?
Calcium moves only via transpiration (xylem flow). Low VPD = low transpiration = low Ca delivery to growing tips → blossom-end rot (tomato), tip burn (lettuce), bud rot precursors (cannabis). Symptoms appear in young growth even with adequate Ca in the root zone.
What about CO₂ enrichment?
Higher CO₂ allows plants to tolerate higher VPD because stomata close partially while still maintaining photosynthesis. Cannabis growers running 1200–1500 ppm CO₂ can push VPD into 1.5–1.8 kPa for late flower without yield loss.
How fast should I change VPD?
Plants adapt to environmental setpoints over 2–7 days. Sudden swings (e.g., AC turning on/off cycling RH from 80% to 40%) cause stress. Use proportional control on dehumidification.
Is VPD relevant outdoors?
Yes, but you have less control. Mid-day VPD in arid climates routinely exceeds 3 kPa, causing midday stomatal closure and reduced photosynthesis. Shade cloth, misting, and mulching all reduce effective VPD at the canopy.

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The ToolsACE Team

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Disclaimer

VPD targets are general guidelines based on common cultivar responses. Individual genetics, lighting intensity, CO₂ levels, and root-zone conditions all interact with optimal VPD. Adjust based on observed plant response (transpiration rate, leaf curl, stomatal conductance if measurable).