How to Find Percentage Purity

Updated 6 November 2026 · Standard formula, shown below

This guide explains how to calculate the percentage purity of a substance, typically a chemical sample. You will learn the standard formula, how to apply it using mass data from an experiment or given values, and how to interpret the result. The steps work for any situation where you know the mass of the pure component and the total mass of the sample.

What You’ll Need

The mass of the pure substance (in grams, kg, or any consistent unit)
The total mass of the sample (in the same unit as the pure mass)
A calculator (optional, but helpful for large numbers)

Step-by-Step

Identify the pure mass. This is the mass of the substance you are interested in, excluding any impurities. In chemistry, this often comes from a titration, gravimetric analysis, or a given specification. For example, if you have a sample of iron filings that contains 4.5 g of actual iron, the pure mass is 4.5 g.
Identify the total sample mass. This is the mass of the entire sample as weighed, including impurities. Using the same example, if the iron filings sample weighs 5.0 g in total, that is the total mass.
Ensure units match. Convert both masses to the same unit before dividing. If one is in grams and the other in kilograms, convert one to the other. For instance, 0.005 kg becomes 5 g.
Divide the pure mass by the total mass. Use the formula: percentage purity = (pure mass / total mass) × 100. For the iron example: (4.5 g / 5.0 g) = 0.9.
Multiply by 100 to get a percentage. 0.9 × 100 = 90%. So the iron filings are 90% pure.
Check your answer. The result should be between 0% and 100%. If it is above 100%, you likely used the wrong mass or mixed units. If it is below 0%, check for negative values (which shouldn’t occur in a real mass measurement).

Common Mistakes

Mixing units: Dividing grams by kilograms without converting gives a wildly wrong answer. Always convert to the same unit first.
Using the wrong mass: Confusing the pure mass with the total mass. The total mass always includes impurities; the pure mass is only the substance of interest.
Forgetting to multiply by 100: The formula yields a decimal (like 0.9). Without the ×100, you get 0.9% instead of 90%.
Rounding too early: Round intermediate values only at the final step to avoid small errors. For example, if pure mass is 2.333 g and total is 2.500 g, dividing gives 0.9332, not 0.93.

Tips

Use the same precision for both masses. If one is measured to two decimal places, the other should be too, for consistency.
In titration problems, the pure mass is often calculated from the volume and concentration of a standard solution. Make sure you have the correct stoichiometry before plugging in numbers.
If you are working with a hydrate, remember that the water of crystallization counts as part of the total mass but is not part of the pure anhydrous substance.
For quick checks, remember that if pure mass is half of total, purity is 50%; if pure mass equals total, purity is 100%.

Troubleshooting

Result is above 100%: Double-check that the pure mass is not actually larger than the total mass. This can happen if you used the mass of a product from a reaction instead of the original sample. Re-read the problem.
Result is negative: Negative masses are impossible in practice. You may have subtracted masses somewhere incorrectly. Start over with the raw data.
Units don’t match: If you have the pure mass in milligrams and total in grams, convert milligrams to grams by dividing by 1000, or grams to milligrams by multiplying by 1000.
You don’t know the pure mass: In some lab scenarios, you might have the mass of a precipitate that corresponds to the pure substance. Use stoichiometry to convert that precipitate mass to the mass of the substance of interest. For example, if you precipitate BaSO₄ to find sulfate, convert moles of BaSO₄ to moles of sulfate, then to mass of sulfate.

Frequently Asked Questions

What is percentage purity?

Percentage purity is the mass of the pure substance divided by the total mass of the sample, expressed as a percentage. It tells you how much of a sample is actually the desired compound versus impurities.

Can percentage purity be over 100%?

No, not in a correct calculation. A value over 100% usually indicates an error, such as using the wrong mass or having a contaminant that reacts in a way that inflates the measured pure mass (e.g., in titration, if an impurity also reacts with the titrant).

How do I find the pure mass if I only have the total mass and the percentage purity?

Rearrange the formula: pure mass = (percentage purity / 100) × total mass. For example, if you have a 10 g sample that is 95% pure, the pure mass is (95/100) × 10 = 9.5 g.

What if my sample is a mixture of two known substances?

If you know the masses of each component, add them to get the total mass, and the pure mass is the mass of the component you are interested in. The formula works the same way.

Does percentage purity have units?

No, it is a dimensionless number expressed as a percentage. The masses cancel out in the division, so the unit (grams, kg, etc.) does not matter as long as they are the same.

Why is percentage purity important in industry?

It determines the actual amount of active ingredient in a product, affecting dosage in pharmaceuticals, quality of materials in manufacturing, and cost calculations. A lower purity means more impurities and often a lower value.

Method: the standard formula for this calculation, rounded as each result states. See how the calculators work or report an error.