Mass percent formula
Mass percent is the mass of solute divided by the total mass of the solution, times 100: mass% = (solute ÷ solution) × 100. The solution mass includes both solute and solvent. For very dilute solutions, the parts per million calculator is more practical.
How to use this calculator
Enter the solute mass and the total solution mass, both in grams. The result is the concentration by mass as a percentage. To express the same solution in mol/L instead, use the molarity calculator after finding moles.
Both figures must be masses, not volumes: weigh the solution rather than measuring it out, or convert volume to mass using the density first. A common slip is entering the mass of solvent alone in the second box, which understates the denominator and inflates the percentage.
Worked example
Dissolve 10 g of salt in water to make 100 g of solution. Mass percent = 10 ÷ 100 × 100 = 10%. Note the denominator is the whole solution (90 g water + 10 g salt), not just the solvent.
Because it depends only on masses, mass percent does not change with temperature — unlike molarity, whose volume term expands as a solution warms. That stability is why it is common on commercial reagent labels. Applied to a compound rather than a mixture, the same idea becomes percent composition.
Frequently asked questions
- What is mass percent?
- Mass percent is the mass of solute as a percentage of the total mass of the solution (solute plus solvent).
- Is solution mass the same as solvent mass?
- No — solution mass is solute plus solvent, so it is larger than the solvent mass alone.
- How is it different from molarity?
- Mass percent uses mass ratios; molarity uses moles per litre. Mass percent ignores volume and molar mass.
- Why must solution mass not be zero?
- Dividing by a total solution mass of zero is undefined, so a value is required.
- Can I enter masses in kilograms or milligrams instead of grams?
- Yes, provided both figures use the same unit. Mass percent is a ratio, so the units cancel — 10 kg in 100 kg gives the same 10% as 10 g in 100 g. Mixing units is the one thing that will produce a wrong answer.