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To calculate parts per million (ppm), divide the mass of the substance of interest by the total mass of the sample and multiply by 1,000,000. In symbols, ppm = (mass of solute ÷ mass of whole sample) × 106, with both masses in the same unit. For a dilute water-based solution the same number is, to a good approximation, the concentration in milligrams per litre (mg/L). This guide gives the formula, worked examples, the unit shortcuts that follow from it, and the case where the shortcut stops being safe.
What you need
- The mass of the substance you are measuring (the solute or analyte), in any mass unit.
- The mass of the whole sample or solution, in the same unit. If you only have a volume, you also need the density of the solution.
- A calculator. No other equipment is involved.
Steps: how to calculate parts per million
- Write both masses in the same unit (for example, both in grams, or both in milligrams).
- Divide the solute mass by the total mass of the sample. The result is a dimensionless fraction.
- Multiply the fraction by 1,000,000. The answer is in ppm.
- Check the size: 1 ppm is one part in a million, so a fraction of 0.000003 is 3 ppm.
Worked example 1: both quantities are masses
A sample of 1,500 g contains 0.0045 g of the substance. The fraction is 0.0045 ÷ 1,500 = 0.000003. Multiplying by 1,000,000 gives 3 ppm.
Worked example 2: milligrams per kilogram
The US EPA states that 1 milligram per kilogram (mg/kg) equals 1 part per million parts, or 1 ppm. A 2 kg soil sample containing 3 mg of a contaminant is therefore 3 mg ÷ 2 kg = 1.5 mg/kg = 1.5 ppm.
Worked example 3: from a percentage
A percentage is parts per hundred, so multiply a mass-percent value by 10,000 to get ppm. A 1% (w/w) solution is 10,000 ppm, and a 0.05% (w/w) solution is 500 ppm. For how mass percent is defined and prepared, see how to make a mass percent solution.
Converting mg/L to ppm, and when not to
EPA’s data-reporting guidance explains the shortcut used in most water chemistry: 1 litre of water weighs approximately 1 kilogram if you ignore the small density differences between pure water and the majority of environmental samples, so 1 mg/L is equivalent to 1 ppm. By the same logic, 1 microgram per litre (µg/L) or 1 microgram per kilogram (µg/kg) is equivalent to 1 part per billion (ppb).
| Reported as | Equals | Condition |
|---|---|---|
| 1 mg/kg | 1 ppm | Mass per mass; no assumption needed |
| 1 mg/L | about 1 ppm | Only if 1 L of the sample weighs about 1 kg (dilute water-based samples) |
| 1 µg/L or 1 µg/kg | 1 ppb | Same condition for the per-litre form |
| 1% (w/w) | 10,000 ppm | Arithmetic: 1 ÷ 100 × 1,000,000 |
Why the shortcut is a water assumption
ppm is a ratio of masses, while mg/L divides by a volume. The two agree only when a litre of the sample has a mass of one kilogram. The USGS Water Science School lists the density of pure water as 1.00000 g/cm3 at 4.0 °C and 0.99802 g/cm3 at 21 °C, so for water the error is small. For a denser or lighter liquid it is not.
Illustrative arithmetic (the density is an assumed example value, not a measured one): a solution of density 1.20 g/mL has a mass of 1.20 kg per litre. If it contains 10 mg of solute per litre, the mass concentration is 10 mg ÷ 1.20 kg = 8.33 mg/kg, or about 8.3 ppm, not 10 ppm. Treating mg/L as ppm would overstate the value by 20%. When the sample is not dilute and water-like, convert through density, or weigh the sample and work in mg/kg directly.
What can go wrong
- Mixing mass and volume. Dividing a mass by a volume and calling the result ppm silently assumes a density of 1 g/mL.
- Different units top and bottom. 5 mg in 2 kg is 5 mg ÷ 2,000,000 mg, not 5 ÷ 2. Convert first.
- Wrong denominator. ppm is relative to the whole sample or solution, not to the solvent alone. For very dilute samples the difference is negligible; for concentrated ones it is not.
- Confusing ppm and ppb. Per EPA, µg/L is ppb and mg/L is ppm: a factor of 1,000.
- Ambiguous wording. NIST points to ambiguity in the names of numbers as one of the reasons why the use of ppm, ppb and ppt is deprecated.
Why NIST discourages ppm, and what to write instead
NIST Special Publication 811 (Guide for the Use of the SI), section 7.10.3, says ppm, ppb and ppt are not acceptable for use with the SI to express the values of quantities. It also recognizes that a law or regulation may require them. Where you are free to choose, write the quantity in an unambiguous form: mg/kg, mg/L, µg/L, or a pure number such as 3 × 10-6. Where a regulation, method or label states a limit in ppm, report in ppm and state what the ratio is based on (mass per mass or mass per volume).
Related calculations on CASRAI: how to calculate a serial dilution for stepping a stock down to a target concentration, and how to make a 10% bleach solution, where disinfectant strength is expressed as ppm available chlorine. Exposure limits expressed in ppm are discussed in permissible exposure limits (PELs, TLVs, RELs) in the laboratory.
When to get professional help
The arithmetic itself is simple, but a ppm value that feeds a regulatory report, a method validation or a safety decision should be checked against the governing method or limit document, and by your laboratory’s quality or safety lead. If the sample is not dilute and water-like, or the matrix is unusual, ask an analytical chemist how the method defines the reporting basis before you convert.
FAQ
How to calculate parts per million?
Divide the mass of the substance by the total mass of the sample, both in the same unit, and multiply by 1,000,000. For example, 0.0045 g in 1,500 g is 3 ppm. In mg/kg terms, 1 mg/kg is 1 ppm.
Is 1 mg/L the same as 1 ppm?
Approximately, for dilute water-based samples, because a litre of water weighs about a kilogram (EPA). For denser or lighter liquids, convert with the density.
How do I convert a percentage to ppm?
Multiply the percentage by 10,000. So 0.05% is 500 ppm and 1% is 10,000 ppm.
Sources
- NIST Special Publication 811, Guide for the Use of the SI, Chapter 7, section 7.10.3 “ppm, ppb, and ppt”: https://www.nist.gov/pml/special-publication-811/nist-guide-si-chapter-7-rules-and-style-conventions-expressing-values (checked 2026-10-29).
- US EPA (archive), Data Reporting Questions: https://archive.epa.gov/epawaste/hazard/testmethods/web/html/faqs_data.html (checked 2026-10-29).
- USGS Water Science School, Water density (density values only): https://www.usgs.gov/special-topics/water-science-school/science/water-density (checked 2026-10-29).
