How to calculate ppm

Written by michael keenan
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How to calculate ppm
Calculating the parts per million (PPM) of a chemical in a substance is done for many reasons. (Calculator image by Alhazm Salemi from

PPM stands for "parts per million" and is used to describe the amount of a certain chemicals in a substance. These measurements often are used by farmers to calculate the strength of fertilisers. For example, nitrogen is sold as part of a solid compound, but farmers need to make liquid fertilisers of different strengths, measured in PPM. PPM calculations also are used by government agencies to determine whether air, soil or water is polluted enough to be hazardous to human health. For example, the Environmental Protection Agency (EPA) intervenes when the amount of copper in water exceeds 1.3 PPM.

Skill level:

Things you need

  • Calculator
  • Solvent
  • Solute
  • Scale

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  1. 1

    Determine the mass of the solvent in kilograms. If the solvent is water, 1 litre equals 1 kilogram. If you plan to dissolve fertiliser into 5 litres of water, the mass of your solvent would be 5 kilograms.

  2. 2

    Determine the mass of the solute in milligrams. If the solute is part of a compound, determine how much of the compound is the solute. For example, if you add 10 grams of fertiliser that is 20 per cent nitrogen, multiply 1 gram times 0.2 (20 per cent) and you get 0.2 grams. To get from grams to milligrams, multiply by 1,000 and you get 200 milligrams.

  3. 3

    Divide the mass of the solute in milligrams by the mass of the solvent in kilograms. In this example, divide 200 by 5 and you get 40 parts per million (PPM).

  1. 1

    Find the volume of the solution. For example, if you were trying to determine the concentration of salt in water, determine how much water is in your container.

  2. 2

    Remove the substance you are taking the concentration of from the solvent. The process will depend on the substance. For example, if you are trying to measure the concentration of salt in salt water, boil the water until only the salt remains.

  3. 3

    Find the mass of the solute in milligrams. In this example, measure the mass of the salt crystals that are left.

  4. 4

    Divide the mass of the solute in milligrams by the mass of the solvent in kilograms. For example, if you started with 2 litres of water and were left with 3 milligrams of salt, you would convert the volume of water to mass. Because the mass of 1 litre of water is 1 kilogram, your total mass would be 2 kilograms. Then divide 3 milligrams by 2 kilograms to get 1.5 parts per million.

Tips and warnings

  • Make sure you use kilograms for the solvent and milligrams for the solute or your parts per million concentration won't be accurate.

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