Percent Composition by Mass Calculator
Calculate the percent composition by mass of each element in a compound.
Enter up to 4 elements and get molar mass plus mass percentages for each.
How Percent Composition Is Calculated
Percent composition tells you what fraction of a compound’s total mass comes from each element. It’s fundamental to stoichiometry, empirical formula determination, and quality control in chemistry.
Formula:
% Element = (Mass of Element in 1 mol / Molar Mass of Compound) × 100
Steps:
- Write the molecular formula
- Find the molar mass of each element present (from periodic table)
- Multiply by the number of atoms of that element
- Divide by total molar mass of compound
- Multiply by 100
Worked Example, Water (H₂O):
- Molar mass of H = 1.008 g/mol × 2 = 2.016 g/mol
- Molar mass of O = 15.999 g/mol × 1 = 15.999 g/mol
- Total molar mass = 18.015 g/mol
- % H = (2.016 / 18.015) × 100 = 11.19%
- % O = (15.999 / 18.015) × 100 = 88.81%
- Check: 11.19 + 88.81 = 100% ✓
Worked Example — Glucose (C₆H₁₂O₆), MW = 180.16 g/mol:
- C: (6 × 12.011) / 180.16 × 100 = 40.00%
- H: (12 × 1.008) / 180.16 × 100 = 6.71%
- O: (6 × 15.999) / 180.16 × 100 = 53.28%
Applications:
- Verifying compound purity in pharmaceutical manufacturing
- Determining empirical formulas from combustion analysis
- Nutrition labels (macronutrient percentages by mass)
- Mining: ore grade expressed as percent metal composition
Entering formulas with parentheses
This calculator takes element-and-count pairs rather than a formula string, so anything in brackets has to be multiplied out first. Calcium hydroxide, Ca(OH)₂, is entered as Ca 1, O 2, H 2, giving a molar mass of 74.09 g/mol. Aluminium sulfate, Al₂(SO₄)₃, becomes Al 2, S 3, O 12.
Watch the oxygen count in that second one. The subscript 4 inside the bracket multiplies by the 3 outside it, which catches people out far more often than the metal does.
Why measured composition drifts from the theoretical figure
If you run a combustion analysis and the percentages come back a few points off, the usual culprit is water rather than a bad measurement. Many salts crystallise with water locked into the lattice, and that water counts toward the mass. Copper sulfate pentahydrate, CuSO₄·5H₂O, is 36% water by mass, so its copper percentage is 25.5% against the 39.8% you would calculate for the anhydrous salt.
To handle a hydrate here, enter the water’s hydrogen and oxygen along with everything else: CuSO₄·5H₂O is Cu 1, S 1, O 9, H 10.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
SuperGlobalCalculator is independently built and maintained. See how we build and verify our calculators.