Percent Composition Calculator
Enter a chemical formula and see what share of its mass each element contributes. Brackets and hydrate dots are understood.
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What percent composition tells you
Percent composition breaks a compound down by mass rather than by atom count. It answers a practical question: out of every 100 g of this substance, how many grams are each element?
Percent of an element = (mass of that element in one mole ÷ molar mass) × 100
The number of atoms and the share of the mass are quite different things. In water, two of the three atoms are hydrogen, so hydrogen makes up 67% of the atoms. By mass, though, hydrogen is only 11.19%, because each oxygen atom weighs roughly sixteen times as much as a hydrogen atom.
Worked example
Find the percent composition of calcium carbonate, CaCO3.
Step 1. Work out the mass each element contributes to one mole.
Ca: 1 × 40.078 = 40.078 g
C: 1 × 12.011 = 12.011 g
O: 3 × 15.999 = 47.997 g
Step 2. Add them for the molar mass.
40.078 + 12.011 + 47.997 = 100.086 g/mol
Step 3. Divide each contribution by the total and multiply by 100.
Ca: (40.078 ÷ 100.086) × 100 = 40.04%
C: (12.011 ÷ 100.086) × 100 = 12.00%
O: (47.997 ÷ 100.086) × 100 = 47.96%
The three add to 100.00%, which is a good check on your arithmetic.
Turning percent into grams
Once you know the percentages, scaling to a real sample is straightforward. Multiply the sample mass by each percentage.
A 25.0 g sample of calcium carbonate contains 25.0 × 0.4004 = 10.0 g of calcium. This is exactly how you work out how much of a nutrient is in a fertilizer, or how much metal sits in an ore. Enter a sample mass above and the calculator adds that column.
Percent composition of some common compounds
| Compound | Formula | By mass |
|---|---|---|
| Water | H2O | 11.19% H, 88.81% O |
| Carbon dioxide | CO2 | 27.29% C, 72.71% O |
| Sodium chloride | NaCl | 39.34% Na, 60.66% Cl |
| Glucose | C6H12O6 | 40.00% C, 6.71% H, 53.28% O |
| Ammonia | NH3 | 82.24% N, 17.76% H |
| Iron(III) oxide | Fe2O3 | 69.94% Fe, 30.06% O |
Percent composition and hydrates
Hydrates are where this calculation earns its keep. Copper sulfate pentahydrate, CuSO4·5H2O, is 36.08% water by mass. Heat the blue crystals and that water drives off, leaving a white powder that weighs barely two thirds of what you started with.
Working backwards from that mass loss is a standard laboratory experiment. You heat a known mass of hydrate to constant mass, and the percentage lost tells you how many water molecules were attached.
The link to empirical formulas
Percent composition and the empirical formula are two directions of the same journey. Going from a formula to percentages, as this page does, has exactly one answer. Going the other way does not, because several compounds can share the same percentages.
Glucose (C6H12O6) and acetic acid (C2H4O2) have identical percent composition, since both reduce to CH2O. To tell them apart you need the molar mass as well.
Mistakes worth avoiding
- Confusing atom percent with mass percent. Water is two thirds hydrogen by atoms but only about a ninth by mass.
- Forgetting the subscript. The oxygen in CaCO3 contributes three times its atomic mass.
- Leaving out the water in a hydrate. It is often a third of the total mass.
- Not checking the total. The percentages should add to 100 within rounding.
- Rounding the molar mass early. Keep the digits until the final division.
Frequently asked questions
How do I calculate percent composition?
Work out the total mass each element contributes to one mole of the compound, divide that by the compound's molar mass, then multiply by 100. Repeat for every element and the results should add up to 100 percent.
What is the percent composition of water?
Water is 11.19 percent hydrogen and 88.81 percent oxygen by mass. Hydrogen makes up two of the three atoms but very little of the mass, because an oxygen atom is about sixteen times heavier than a hydrogen atom.
How do I find the mass of one element in a sample?
Multiply the sample mass by that element's percentage as a decimal. A 25.0 g sample of calcium carbonate, which is 40.04 percent calcium, contains 25.0 times 0.4004, which is 10.0 g of calcium.
Why do my percentages not add to exactly 100?
Small gaps come from rounding the atomic masses. Anything within about 0.1 percent is normal. A larger gap usually means an element was left out of the formula.
Can two compounds have the same percent composition?
Yes. Glucose and acetic acid have identical percent composition because both reduce to the empirical formula CH2O. You need the molar mass to tell them apart.
How do I find the percent of water in a hydrate?
Multiply the number of attached water molecules by 18.015, divide by the full molar mass of the hydrate, then multiply by 100. Copper sulfate pentahydrate works out to 36.08 percent water.
Related terms
Next: reverse the process on the empirical formula calculator.