Concentration in ppm:
reading and calculating parts per million

What one part per million looks like

Parts per million expresses a very small share of something. One ppm is one part of the solute in a million parts of the whole, the way 1 percent is one part in a hundred. It is used when the amount is too tiny for percentages to be readable, such as chlorine in tap water or nitrate in a river.

To picture it: one ppm is about one drop of ink in 50 litres of water, or one second in roughly 11.6 days. It sounds negligible, yet many contaminants are regulated and many nutrients are dosed at that level.

Strictly, ppm is a ratio of mass to mass, milligrams of solute per kilogram of mixture. Water chemistry takes a convenient shortcut, which is explained next.

The mg/L shortcut and the formula behind it

A litre of pure water has a mass of about one kilogram. So for a dilute watery solution, milligrams of solute per kilogram of solution is, to a good approximation, milligrams per litre. That is why water reports use mg/L and ppm almost interchangeably.

The shortcut fails for solutions that are much denser or lighter than water, such as concentrated brine, syrups or oils. In those cases convert using the actual density, or work with mass of solution in kilograms.

Worked example: 25 mg in 5 litres

A hobbyist dissolves 25 mg of a nutrient salt in a 5-litre aquarium refill. What is the concentration in ppm?

Going the other way helps with dosing. To reach 2 ppm in a 500 L tank you need 2 × 500 = 1,000 mg, which is 1 g of the active substance. If the product is only 40 percent pure, the weight to add rises to 1 ÷ 0.40 = 2.5 g.

Diluting a stock solution follows C₁ × V₁ = C₂ × V₂. A 100 ppm stock diluted to 5 ppm in 1 L needs V₁ = 5 × 1,000 ÷ 100 = 50 mL of stock topped up with water.

Quick conversions between ppm, mg/L and percent

Seawater, at about 35,000 ppm of dissolved salts, is the one row where the density assumption starts to slip slightly, because seawater is a little heavier than fresh water. For everyday purposes, though, the mg/L figure remains close.

Gas concentrations in air also use ppm, but there it usually means a volume ratio (ppmv), not mass. Do not mix the two without checking the definition.

Where ppm figures need more care

• Density far from 1 kg/L: use mass of solution, not volume.
• Temperature: volume changes with temperature, so a mg/L value can drift slightly even when the mass of solute does not.
• Different species: 5 ppm of nitrate as NO₃ and 5 ppm of nitrate as nitrogen are different amounts. Reports state the basis, so read it.
• Solids and soils: ppm there means mg per kg of dry material, not per litre.

Reading a lab report and dosing a pool

Reports sometimes use smaller units. One ppm equals 1,000 parts per billion (ppb), and 1 µg/L is 1 ppb in water. A result of 5 ppm may therefore appear as 5,000 µg/L or 5,000 ppb; the quantity is the same, only the unit differs.

A practical dosing sum: a pool holds 50,000 L and you want to raise a disinfectant by 2 ppm. Each litre needs 2 mg, so 2 × 50,000 = 100,000 mg, or 100 g of the active substance. If the product contains 65 percent active ingredient, you would weigh 100 ÷ 0.65 = 154 g.

• Always confirm the volume of water; a rough guess is the biggest source of dosing error.
• Add chemicals gradually and retest, since reactions and losses to sunlight change the final level.
• Follow the product label and local guidance for safe limits; targets differ by use and by authority.

A test result in ppm only has meaning relative to what is being measured and in what form. Ask what the lab quotes the figure 'as', and keep that note with the number.

What the figure might actually refer to

Because ppm is shorthand, the same number can hide different bases. Ask four questions when you meet one: is it by mass or by volume, is it of the element or of the compound, is it per litre or per kilogram, and is it wet or dry basis for solids.

A nutrient label may list nitrogen at 150 ppm 'as N', which is a different amount from 150 ppm of the nitrate ion. Converting between them needs the molar masses, with nitrate weighing about 4.4 times as much as the nitrogen inside it.

When no basis is given, assume the figure is a mass ratio in a dilute watery solution, and confirm with the source if the decision matters.

A two-minute sanity check before trusting a result

1. 1Convert the solute mass to milligrams (1 g = 1,000 mg).
2. 2Convert the volume to litres (1,000 mL = 1 L).
3. 3Divide mg by litres.
4. 4If the answer is over 10,000 ppm, ask whether the solution is still dilute enough for the shortcut and whether a percentage would be clearer.

A general chemistry calculator is useful for handling such quantity conversions and cross-checking, but the density caveat is something only you can judge for your own solution.