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Percent Composition Calculator

Ready to calculate
%X = (n_X·M_X) / Σ × 100.
39 Elements (IUPAC 2021).
Up to 5 Element Slots.
100% Free.
No Data Stored.

How it Works

01Pick Up to 5 Elements

Choose elements from the 39-element library; defaults are H, C, O, N, S — change as needed

02Enter Atom Counts

Each element's count from your molecular formula (e.g., glucose C₆H₁₂O₆: C=6, H=12, O=6)

03Compute Total Molar Mass

Σ(n_i × M_i) — sum each element's contribution to total mass using IUPAC 2021 atomic weights

04Get % per Element

%X = (n_X × M_X) / total × 100 — visual bars + reference table comparison for 12 known compounds

What is a Percent Composition Calculator?

Percent composition by mass is one of the very first quantitative concepts in stoichiometry — the answer to "how much of each element is in this compound, by weight?" The formula is the foundation of empirical-formula determination, combustion analysis, pharmaceutical assays, and any time a chemist needs to translate a molecular formula into a mass-fraction view of the molecule: %X = (nX × MX) / Σ(ni × Mi) × 100, where nX is the count of element X in the formula and MX is its standard atomic weight. Our Percent Composition Calculator handles any compound with up to 5 distinct elements via slot-based input — pick each element from a 39-element library and enter its atom count, and the calculator instantly returns the molecular formula in subscript form, the total molar mass, and the mass percent of every element with color-coded visual bars.

Just enter atom counts for each element slot (defaults are H, C, O, N, S — the five most common in biological molecules), and select different elements via dropdown if needed. The calculator multiplies each element's count by its IUPAC 2021 standard atomic weight, sums to get total molar mass, then divides each element's mass contribution by the total and multiplies by 100. Output: the formula displayed in pretty subscript notation (e.g., C₆H₁₂O₆), the total molar mass in g/mol, individual element cards with mass percent (color-coded with gradient bars), automatic match against 12 reference compounds when your formula is recognized, and a complete step-by-step calculation breakdown.

Designed for general chemistry students learning stoichiometry, organic chemistry students computing percent composition for combustion-analysis verification, pharmaceutical scientists computing API content in formulations, food scientists quantifying nutrient ratios, materials scientists characterizing alloy and ceramic compositions, and forensic chemists matching unknowns against composition databases, the tool runs entirely in your browser — no data is stored or transmitted.

Pro Tip: Pair this with our Molar Mass Calculator for the related "given a formula, find molar mass" problem, or our Combustion Analysis Calculator to reverse-engineer the empirical formula from CO₂ and H₂O combustion masses.

How to Use the Percent Composition Calculator?

Pick Up to 5 Elements: Each slot has a dropdown selector covering 39 common elements with their IUPAC 2021 atomic weights. Defaults: H, C, O, N, S — the most common in biological compounds. Change any slot to a different element (e.g., Cl, Fe, Na, Cu) for inorganic compounds.
Enter Atom Counts: Type the number of each element from your molecular formula. Glucose C₆H₁₂O₆: H=12, C=6, O=6. Sodium chloride NaCl: Na=1, Cl=1. Aspirin C₉H₈O₄: H=8, C=9, O=4. Leave count blank or 0 to skip a slot.
Compute Each Element's Contribution: Mass contribution = atom count × atomic weight. For glucose: H = 12 × 1.008 = 12.10 g/mol; C = 6 × 12.011 = 72.07 g/mol; O = 6 × 15.999 = 95.99 g/mol.
Sum to Get Total Molar Mass: Σ(n × M) = total grams per mole of compound. Glucose: 12.10 + 72.07 + 95.99 = 180.16 g/mol — the textbook value. ✓
Compute Each Mass Percent: %X = (n × M) / total × 100. Glucose: %H = 12.10/180.16 × 100 = 6.71%; %C = 72.07/180.16 × 100 = 40.00%; %O = 95.99/180.16 × 100 = 53.29%. Should sum to 100% within rounding.
Press Calculate: Get formula in subscript notation, total molar mass, color-coded element bars, automatic reference-compound match, and step-by-step calculation breakdown.

How is percent composition calculated?

Percent composition is conceptually simple but operationally critical — it underlies empirical-formula determination, drug content assays, and food-label nutrient calculations. Here's the complete framework:

The conservation-of-mass foundation (Lavoisier 1789) guarantees that the masses of all elements in a compound must sum to the compound's total mass. Percent composition just expresses this as a percentage rather than absolute mass.

The Master Formula

For each element X in a compound:

%X = (nX × MX) / Σ(ni × Mi) × 100

where nX is the number of X atoms in the formula, MX is the standard atomic weight of X, and the denominator is the total molar mass of the compound (sum over all elements i).

Sanity Check: Percentages Sum to 100%

By construction, all percentages must sum to 100% (within rounding):

Σ %X = 100.00%

If your computed sum is > 0.5% off from 100, you've made an arithmetic error. The calculator does this check for you and reports the sum.

Why Use Mass Percent (Not Mole Percent)?

Two common ways to express composition:

  • Mass percent (this calculator): %X = (nX·MX)/(total mass) × 100. Used by chemists for stoichiometry, pharmacists for drug formulations, and food labels.
  • Mole percent / atomic percent: %X = nX/Σni × 100. Used by materials scientists for alloys and metallurgists for steel composition.

For glucose C₆H₁₂O₆: mass percents are %C=40.0, %H=6.7, %O=53.3. Mole percents are %C=25, %H=50, %O=25 (very different!). The two only coincide for elements of the same atomic mass.

Standard Atomic Weights (IUPAC 2021)

The atomic weights used are natural-abundance averages from the IUPAC 2021 commission. Common values:

  • H = 1.008, C = 12.011, N = 14.007, O = 15.999, F = 18.998
  • Na = 22.990, Mg = 24.305, P = 30.974, S = 32.06, Cl = 35.45
  • K = 39.098, Ca = 40.078, Fe = 55.845, Cu = 63.546, Br = 79.904
  • Ag = 107.868, I = 126.904, Au = 196.967, Hg = 200.592, Pb = 207.2

For isotopically labeled compounds (²H, ¹³C, ¹⁵N, ¹⁸O), substitute the exact isotope mass. Old IUPAC tables (pre-2007) gave slightly different values — modern tables differ by < 0.5%.

Connection to Empirical Formula Determination

Percent composition is the inverse of empirical-formula determination. Given mass percentages from combustion analysis:

  1. Assume 100 g of compound — % becomes grams.
  2. Divide each element's grams by its atomic weight to get moles.
  3. Divide all mole values by the smallest to get the mole ratio.
  4. Multiply by 1, 2, 3, 4, etc. to get integer subscripts (the empirical formula).

For molecular formula, multiply empirical by molar mass / empirical mass. Glucose has empirical CH₂O (M = 30.0); molecular C₆H₁₂O₆ (M = 180.0); ratio 6.

Edge Cases

  • Hydrates (CuSO₄·5H₂O): Treat as a single compound with all atoms — Cu=1, S=1, O=9 (from SO₄ + 5×H₂O), H=10 (from 5×H₂O). The hydrate-water mass IS part of the molecular weight.
  • Polyatomic ions (NH₄⁺, SO₄²⁻): Charge isn't part of mass — just count the atoms.
  • Polymers: Use the repeat unit's composition; molar mass varies with chain length.
  • Mixtures (alloys): Composition determined experimentally, not from formula. Brass = 67% Cu + 33% Zn (mass %), but Cu and Zn aren't bonded — it's a metallic solution.
Real-World Example

Percent Composition Calculator – Worked Examples

Example 1 — Water (H₂O). The simplest meaningful case.
  • H: 2 × 1.008 = 2.016 g/mol.
  • O: 1 × 15.999 = 15.999 g/mol.
  • Total: 2.016 + 15.999 = 18.015 g/mol (textbook value ✓).
  • %H = 2.016 / 18.015 × 100 = 11.19%.
  • %O = 15.999 / 18.015 × 100 = 88.81%.
  • Sum: 11.19 + 88.81 = 100.00% ✓.

Example 2 — Glucose (C₆H₁₂O₆). The textbook combustion-analysis target.

  • C: 6 × 12.011 = 72.066 g/mol.
  • H: 12 × 1.008 = 12.096 g/mol.
  • O: 6 × 15.999 = 95.994 g/mol.
  • Total: 180.156 g/mol (textbook 180.16 ✓).
  • %C = 72.066 / 180.156 × 100 = 40.00%.
  • %H = 12.096 / 180.156 × 100 = 6.71%.
  • %O = 95.994 / 180.156 × 100 = 53.29%.
  • Sum: 40.00 + 6.71 + 53.29 = 100.00% ✓.
  • Notice glucose's empirical formula is CH₂O (mole ratio 1:2:1) — the simplest carbohydrate. Percent composition is identical to formaldehyde (CH₂O) and acetic acid (C₂H₄O₂) — they all share the empirical CH₂O.

Example 3 — Caffeine (C₈H₁₀N₄O₂). A 5-element molecule from your morning coffee.

  • C: 8 × 12.011 = 96.088.
  • H: 10 × 1.008 = 10.080.
  • N: 4 × 14.007 = 56.028.
  • O: 2 × 15.999 = 31.998.
  • Total: 194.194 g/mol.
  • %C = 49.48%; %H = 5.19%; %N = 28.85%; %O = 16.48%. Sum = 100.00% ✓.
  • The high %N (28.85%) is characteristic of alkaloids — they contain multiple nitrogens. Combustion analysis of an unknown alkaloid showing %C ≈ 49, %H ≈ 5, %N ≈ 29 immediately suggests caffeine or a close relative.

Example 4 — Iron(III) Oxide (Fe₂O₃, Common Rust).

  • Fe: 2 × 55.845 = 111.69.
  • O: 3 × 15.999 = 47.997.
  • Total: 159.687 g/mol.
  • %Fe = 111.69 / 159.687 × 100 = 69.94%.
  • %O = 47.997 / 159.687 × 100 = 30.06%.
  • Iron ore quality is reported as %Fe — high-grade hematite ore approaches the theoretical 69.94% for pure Fe₂O₃; ore that's 65% Fe is excellent industrial-grade.

Example 5 — Sodium Chloride (NaCl). The simplest 1:1 ionic compound.

  • Na: 1 × 22.990 = 22.990.
  • Cl: 1 × 35.45 = 35.45.
  • Total: 58.44 g/mol.
  • %Na = 22.990 / 58.44 × 100 = 39.34%.
  • %Cl = 35.45 / 58.44 × 100 = 60.66%.
  • The 39.34% sodium content is what nutrition labels reference: 1 g of table salt contains 0.39 g of sodium. The FDA daily recommended max is 2,300 mg sodium = ~5.85 g of NaCl (about 1 teaspoon).

Who Should Use the Percent Composition Calculator?

1
General Chemistry Students: Solve textbook stoichiometry problems on percent composition; verify combustion-analysis empirical formulas; compute molar masses from formulas.
2
Organic Chemistry Students: Cross-check elemental analysis (CHN) results against proposed structures; identify unknowns from %C, %H, %N data.
3
Pharmaceutical Scientists: Compute API (active pharmaceutical ingredient) content in salt forms (HCl salts often have ~80% API by mass); design dosage calculations.
4
Food Scientists / Nutrition: Translate ingredient formulas into mass percent for label compliance; compute %sodium in salt-containing foods, %iron in fortified products.
5
Materials Scientists: Characterize alloy compositions (brass 67% Cu / 33% Zn), oxide ceramics, and intermetallic compounds; track stoichiometry in solid-state synthesis.
6
Mining / Metallurgy: Report ore grade as %metal (Fe₂O₃ rust = 69.94% Fe; cuprite Cu₂O = 88.82% Cu); compute extractable metal mass from ore tonnage.

Technical Reference

Theoretical Foundation. Percent composition follows directly from Antoine Lavoisier's 1789 law of conservation of mass — the total mass of a compound equals the sum of its constituent atoms' masses. The systematic determination of percent composition by combustion analysis was pioneered by Antoine Lavoisier and refined by Justus von Liebig (1831) with his Kaliapparat for CO₂ collection. Today's instrumental CHN analyzers (Perkin-Elmer 2400, LECO TruSpec, Elementar vario) automate the same principle to ±0.3% absolute precision.

IUPAC 2021 Standard Atomic Weights (Selected):

  • Period 1: H = 1.008, He = 4.003
  • Period 2: Li = 6.94, Be = 9.012, B = 10.81, C = 12.011, N = 14.007, O = 15.999, F = 18.998, Ne = 20.180
  • Period 3: Na = 22.990, Mg = 24.305, Al = 26.982, Si = 28.085, P = 30.974, S = 32.06, Cl = 35.45, Ar = 39.948
  • Period 4 (selected): K = 39.098, Ca = 40.078, Fe = 55.845, Ni = 58.693, Cu = 63.546, Zn = 65.38, As = 74.922, Br = 79.904
  • Heavier elements: Ag = 107.868, I = 126.904, Ba = 137.327, Au = 196.967, Hg = 200.592, Pb = 207.2, U = 238.029

Reference Compositions for Common Substances:

  • Water (H₂O): M = 18.015 — %H = 11.19, %O = 88.81
  • Methane (CH₄): M = 16.043 — %C = 74.87, %H = 25.13
  • Ammonia (NH₃): M = 17.031 — %N = 82.24, %H = 17.76
  • Carbon dioxide (CO₂): M = 44.009 — %C = 27.29, %O = 72.71
  • Glucose (C₆H₁₂O₆): M = 180.156 — %C = 40.00, %H = 6.71, %O = 53.29
  • Sucrose (C₁₂H₂₂O₁₁): M = 342.30 — %C = 42.11, %H = 6.48, %O = 51.42
  • Caffeine (C₈H₁₀N₄O₂): M = 194.194 — %C = 49.48, %H = 5.19, %N = 28.85, %O = 16.48
  • Aspirin (C₉H₈O₄): M = 180.158 — %C = 60.00, %H = 4.48, %O = 35.52
  • Sodium chloride (NaCl): M = 58.44 — %Na = 39.34, %Cl = 60.66
  • Hematite (Fe₂O₃): M = 159.687 — %Fe = 69.94, %O = 30.06
  • Magnetite (Fe₃O₄): M = 231.533 — %Fe = 72.36, %O = 27.64
  • Cuprite (Cu₂O): M = 143.09 — %Cu = 88.82, %O = 11.18
  • Calcium carbonate (CaCO₃): M = 100.087 — %Ca = 40.04, %C = 12.00, %O = 47.96

Pharmaceutical Salt Form Implications. Many drugs are formulated as their HCl, sulfate, or other salt forms — the "active" portion is only part of the dosed mass. Examples: Hydroxychloroquine sulfate (C₁₈H₂₆ClN₃O · ½H₂SO₄, MW 433.95) — only 77.5% is HCQ free base. Lisinopril dihydrate (C₂₁H₃₁N₃O₅·2H₂O, MW 441.52) — only 91.8% is anhydrous lisinopril. Dosage labels report "X mg of free base equivalent" to handle this — but you need percent composition to convert salt mass to active mass.

Food and Nutrition Reference. Salt (NaCl): 1 g salt = 0.39 g sodium → 1 tsp salt (~6 g) = 2,300 mg Na (FDA daily max). Iron in spinach (Fe in Fe₂O₃ form ~70%): 100 g spinach has 2.7 mg total Fe; bioavailability much lower than supplement Fe(II) salts. Calcium in milk (Ca in calcium phosphate / casein complex): ~120 mg Ca per 100 g milk, requiring ~1 g Ca per day from diet.

Mining / Ore Grade Standards.

  • Iron ore: Hematite (Fe₂O₃ pure) gives max 69.94% Fe; commercial high-grade ores 60-65% Fe; low-grade ores 30-50%.
  • Copper ore: Native Cu = 100%; chalcopyrite (CuFeS₂) = 34.6% Cu; cuprite (Cu₂O) = 88.82%; commercial ore 0.5-2% Cu (mostly gangue).
  • Bauxite (aluminum ore): Mostly Al(OH)₃ (gibbsite) = 34.6% Al; commercial bauxite 30-60% Al₂O₃ equivalent.
  • Gold ore: Native Au or AuTe₂ (calaverite, 56.4% Au); commercial ore 0.3-3 g/tonne (0.00003%).

Common Pitfalls. (1) Forgetting subscripts: for Fe₂O₃, n_Fe = 2, n_O = 3, NOT 1 each. (2) Hydrates: CuSO₄·5H₂O has Cu=1, S=1, O=9 (4 from sulfate + 5 from water), H=10. (3) Polyatomic ions: NH₄Cl has N=1, H=4, Cl=1 — count atoms, not the ion. (4) Charged species: charge doesn't add mass; SO₄²⁻ has the same molar mass as neutral SO₄. (5) Mixtures vs compounds: bronze (alloy) and brass (alloy) have variable composition — percent composition there means mass fraction, not stoichiometric.

Key Takeaways

Percent composition is the universal mass-fraction view of any compound: %X = (nX × MX) / Σ(ni × Mi) × 100. It's calculated by multiplying each element's atom count by its IUPAC 2021 atomic weight, summing to get total molar mass, and reporting each element's fraction as a percentage. Sanity check: percentages must sum to 100% within rounding. The classic landmarks: water = 11.19% H + 88.81% O; glucose = 40.00% C + 6.71% H + 53.29% O; NaCl = 39.34% Na + 60.66% Cl. Use the ToolsACE Percent Composition Calculator with up to 5 element slots, 39-element library, color-coded visual bars, automatic reference-compound matching against 12 known compounds, and full step-by-step breakdown. Bookmark it for chemistry coursework, combustion analysis verification, pharmaceutical formulation, food-label compliance, and any time you need to translate a formula into mass percent.

Frequently Asked Questions

What is the Percent Composition Calculator?
It computes the mass percent of each element in a chemical compound using the universal stoichiometry formula %X = (nX × MX) / Σ(ni × Mi) × 100. Inputs: up to 5 element slots, each with a 39-element periodic-table dropdown (defaults H, C, O, N, S) and atom-count input. Output: molecular formula in pretty subscript form, total molar mass in g/mol, color-coded mass-percent cards with visual bars per element, automatic match against 12 reference compounds, and full step-by-step calculation breakdown. Verifies percentages sum to 100% within rounding.

Designed for general chemistry students learning stoichiometry, organic chemistry students cross-checking combustion-analysis results, pharmaceutical scientists computing API content in salt forms, food scientists for label compliance, materials scientists for alloys/ceramics, and mining engineers for ore-grade calculations. Runs entirely in your browser — no data stored.

Pro Tip: Use our Combustion Analysis Calculator to reverse-engineer the empirical formula from CO₂/H₂O combustion data.

What's the formula for percent composition?
%X = (nX × MX) / Σ(ni × Mi) × 100, where nX is the number of atoms of element X in the molecular formula and MX is its standard atomic weight (IUPAC 2021). The denominator is the total molar mass of the compound. Example for water (H₂O): %H = (2 × 1.008) / (2 × 1.008 + 1 × 15.999) × 100 = 2.016 / 18.015 × 100 = 11.19%; %O = 15.999 / 18.015 × 100 = 88.81%; sum = 100.00% ✓.
Why must the percentages sum to 100%?
Conservation of mass — Lavoisier's 1789 law. The total mass of a compound equals the sum of its constituent atoms' masses. So when you express each element's mass as a fraction of the total, the fractions must sum to exactly 1.0 (or 100%). If your computed sum is > 0.5% off from 100, you've made an arithmetic error: forgot a subscript, used the wrong atomic weight, or double-counted an element. The calculator does this sanity check and reports the sum.
What's the difference between mass percent and mole percent?
Mass percent (this calculator): %X = (nX·MX) / total mass × 100. Used by chemists, pharmacists, food scientists. Mole percent / atomic percent: %X = nX / total moles × 100. Used by materials scientists for alloys and metallurgists for steel. For glucose C₆H₁₂O₆: mass percents are %C=40.0, %H=6.7, %O=53.3 (heavier atoms get larger %); mole percents are %C=25, %H=50, %O=25 (just count atoms). The two only coincide when all atoms have the same mass — never true for real chemistry.
How do I handle hydrates like CuSO₄·5H₂O?
Treat the entire hydrate as a single formula and count ALL atoms including the water. CuSO₄·5H₂O has: Cu = 1, S = 1, O = 4 (from SO₄) + 5 (from 5 H₂O) = 9 total, H = 10 (from 5 H₂O). Total molar mass = 63.55 + 32.06 + 9 × 16.00 + 10 × 1.008 = 249.69 g/mol. The hydrate-water mass IS part of the molecular weight — that's what "copper sulfate pentahydrate" means by mass. The anhydrous form (CuSO₄, M = 159.61) has different percent composition.
Can I use this for organic functional-group analysis?
Yes — combustion analysis gives you %C, %H, %N from a CHN analyzer; comparing those to calculated values for a proposed structure verifies (or disproves) the structure. For acetic acid (C₂H₄O₂, M = 60.05): calculated %C = 40.00, %H = 6.71, %O = 53.29. If your CHN analysis gives 40.0% C, 6.7% H (within 0.3% tolerance), the molecular formula is consistent. The technique is routine for any new organic compound — JACS papers regularly include 'elemental analysis: calc'd C, x.x; H, y.y; N, z.z; found C, ...' as proof of identity.
Why are heavier elements always a larger percent than lighter ones?
Because mass percent weights each atom by its atomic mass. In sucrose C₁₂H₂₂O₁₁ (M = 342.30): C contributes 12 × 12.011 = 144.13 g/mol → 42.11%; H contributes 22 × 1.008 = 22.18 g/mol → only 6.48% even though there are 22 H atoms; O contributes 11 × 15.999 = 175.99 g/mol → 51.42%. Despite there being only 11 O atoms vs 22 H atoms, oxygen dominates by mass because each O is 16× heavier than each H. This is the fundamental difference between mass percent and atom percent.
What atomic weights does the calculator use?
IUPAC 2021 standard atomic weights for natural-abundance elements. These are the most current internationally agreed values, published by the IUPAC Commission on Isotopic Abundances and Atomic Weights. Recent updates: H = 1.008 (was 1.00794); C = 12.011; O = 15.999; N = 14.007; S = 32.06; Cl = 35.45. Pre-2007 textbooks may show slightly different values (typically within 0.5%). For isotopically labeled compounds (²H, ¹³C, ¹⁵N), substitute the exact isotope masses (²H = 2.014, ¹³C = 13.003, etc.) — the calculator's defaults are natural-abundance values.
How does this relate to empirical formula determination?
Percent composition is the INVERSE of empirical-formula determination. Given an unknown's %C, %H, %O from combustion analysis: (1) Assume 100 g sample so percent → grams. (2) Divide each element's grams by atomic weight to get moles. (3) Divide all moles by the smallest to get the mole ratio. (4) Multiply by 1, 2, 3 ... to get integer subscripts. Example: %C=40.00, %H=6.71, %O=53.29 → moles C=3.330, H=6.658, O=3.331 → ratio 1:2:1 → empirical CH₂O. To get the molecular formula, divide molar mass by empirical mass: glucose 180/30 = 6 → C₆H₁₂O₆.
Why doesn't pharmaceutical labeling always match my calculated %?
Because many drugs are formulated as salts (HCl salt, sulfate, mesylate) for solubility and stability. The labeled "100 mg dose" may refer to the FREE BASE equivalent, not the salt mass. Example: hydroxychloroquine sulfate (HCQ sulfate, M = 433.95) is only 77.5% HCQ free base; a 200 mg HCQ sulfate tablet contains 155 mg HCQ. Other examples: lisinopril dihydrate (91.8% anhydrous), levothyroxine sodium (96.6% T4 free acid). Always check whether your calculated % is for the SALT or the FREE drug, and which the label specifies.
Can the calculator handle ions and charged species?
Yes — charge doesn't add mass, so just count the atoms. SO₄²⁻ ion has the same composition as neutral 'SO₄ unit': %S = 33.39, %O = 66.61. NH₄⁺ has %N = 77.71, %H = 22.29 (same as ammonia NH₃ would have if formed; the 4th H came from H⁺ which is essentially massless on the scale of N and 3 H's). For ion pairs (CaCl₂, K₂SO₄), enter the full neutral salt formula. The 1+ or 2− charges don't change percent composition — they're just electronic features that don't contribute mass.

Author Spotlight

The ToolsACE Team - ToolsACE.io Team

The ToolsACE Team

Our chemistry tools team implements the universal mass-percent formula every chemistry student learns in their first week of stoichiometry: <strong>%X = (n_X × M_X) / Σ(n_i × M_i) × 100</strong>. The calculator handles any compound with up to 5 distinct elements (defaults to H, C, O, N, S — the most common biological elements), with each slot offering a 39-element periodic-table dropdown using IUPAC 2021 standard atomic weights for natural-abundance elements. Output includes the molecular formula in pretty subscript form, the total molar mass in g/mol, the mass percent of every element with visual bar graphs and individual element cards (color-coded), an automatic match against 12 reference compounds (water, glucose, sucrose, ethanol, caffeine, aspirin, NaCl, etc.) when your input matches a known formula, and a complete step-by-step calculation breakdown showing each element's mass contribution and the final percent calculation. Sanity-check verification ensures the percentages sum to 100% within rounding.

StoichiometryMolecular Formula AnalysisSoftware Engineering Team

Disclaimer

Atomic weights are IUPAC 2021 standard values for natural-abundance elements. For isotopically labeled compounds (²H, ¹³C, ¹⁵N, ¹⁸O), use exact monoisotopic masses instead. Hydrate compounds (CuSO₄·5H₂O) require entering all atoms (Cu=1, S=1, O=9, H=10) — the calculator can't parse the dot-prefix notation directly. Percentages must sum to exactly 100% within rounding by conservation of mass; if not, an input error has occurred. The calculator handles up to 5 distinct elements per compound; for larger formulas split into two calculations or use a more general molar-mass tool.