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Percentage by mass calculator

Free TutorAura tool · no sign-up · Updated

Type a chemical formula and the calculator works out the relative formula mass (MrM_r) and the percentage by mass of each element:

% by mass=Ar×number of atomsMr×100\%\text{ by mass} = \frac{A_r \times \text{number of atoms}}{M_r} \times 100

It uses the relative atomic masses on the GCSE periodic table and understands brackets and water of crystallisation, such as Ca(OH)₂ and CuSO₄.5H₂O.

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Worked examples

  • Hydrogen in water (H₂O): Mr=2+16=18M_r = 2 + 16 = 18. H: 2÷18×100=11.1%2 \div 18 \times 100 = 11.1\%; O: 88.9%.
  • Calcium in calcium carbonate (CaCO₃): Mr=40+12+48=100M_r = 40 + 12 + 48 = 100. Ca: 40÷100×100=40%40 \div 100 \times 100 = 40\%.
  • Nitrogen in ammonium nitrate (NH₄NO₃): Mr=80M_r = 80. There are two nitrogen atoms: 28÷80×100=35%28 \div 80 \times 100 = 35\%.

Count every atom

The most common slip is missing atoms. NH₄NO₃ has two N atoms (one in each part), and Ca(OH)₂ has two O and two H atoms because the bracket is doubled. The calculator lists each element with its count so you can check against your own working.

Why it's useful

Fertiliser makers compare compounds by their percentage of nitrogen; metal extraction compares ores by their percentage of metal (iron in Fe₂O₃ is 70%). Percentages by mass are also where empirical formula questions start: given the percentages, you work back to the formula.

Mass of an element in a sample

Multiply the sample's mass by the percentage. A 50 g sample of CaCO₃ contains 50×0.40=2050 \times 0.40 = 20 g of calcium. This is a quick check on reacting-mass answers.

Common mistakes

  • Using the atomic number instead of the relative atomic mass.
  • Forgetting the number of atoms of the element.
  • Lower-case slips: Co is cobalt, CO is carbon monoxide.

Practise with our percentage by mass questions, and get MrM_r on its own with the relative formula mass calculator.

Exam tips

  • Write the MrM_r calculation out in full, showing the number of each atom.
  • Give the percentage to 3 significant figures unless the question says otherwise.
  • For hydrated salts, include the water: CuSO₄.5H₂O has an MrM_r of 249.5, and water makes up about 36% of its mass.
  • Check your percentages add up to 100%.

Percentage composition of common compounds

CompoundMrM_rElement% by mass
Carbon dioxide, CO₂44C27.3%
Sodium chloride, NaCl58.5Na39.3%
Iron(III) oxide, Fe₂O₃160Fe70.0%
Ammonia, NH₃17N82.4%
Sulfuric acid, H₂SO₄98S32.7%
Copper(II) sulfate, CuSO₄159.5Cu39.8%

These use the relative atomic masses on the GCSE periodic table (Cl 35.5, Cu 63.5). Type any of them into the calculator to see the full working.

Frequently asked questions

How do you calculate percentage by mass?

Multiply the element's Ar by the number of its atoms in the formula, divide by the compound's Mr, then multiply by 100.

What is the percentage of nitrogen in ammonium nitrate?

35%. NH4NO3 has an Mr of 80 and contains two nitrogen atoms (28), and 28 ÷ 80 × 100 = 35%.

Do the percentages add up to 100?

Yes, the percentages of all the elements in a compound add up to 100% (allowing for rounding).

Is anything I type sent to a server?

No. The calculation runs in your browser and nothing is stored.

Sources

  1. GCSE Chemistry (8462) specification: subject content. AQA.
  2. Cambridge IGCSE Chemistry 0620 syllabus for 2026, 2027 and 2028. Cambridge International.

Keep reading

Percentage by mass questions and answers

8 GCSE and IGCSE questions with worked answers, checked as you type.

Relative formula mass (Mr) calculator

Mr for any formula, including brackets and water of crystallisation, with a breakdown table.

Empirical formula calculator with working

The simplest whole-number formula from masses or percentages, with the working shown.

Relative formula mass (Mr): how to calculate it

Mr step by step, brackets, percentage by mass and conservation of mass, with examples.