It is the same kind of thing on a different scale. Per cent is parts per hundred, ppm is parts per million, so 1 % is 10,000 ppm and 1 ppm is 0.0001 %. The larger scale exists because writing 0.0001 % invites a misplaced decimal point while 1 ppm does not, and the practice of switching units to keep numbers between one and a thousand is the same instinct that gives us millimetres and kilometres. Convert between them by multiplying or dividing by 10,000, and check the direction by asking which number should be bigger.

Why do some reports say mg/kg instead?

Because it is unambiguous where ppm is not. A milligram per kilogram states the ratio and both its units, so nobody has to work out whether a mass or a volume was meant. For soil, sediment, food and anything solid it is the standard form, since a litre of soil is not a defined quantity. Where a report offers both, prefer the one with units attached, and where it offers only ppm, look for a sentence in the method section saying whether the ratio is by mass or by volume — a good report always has one.

One Milligram per Litre Is One ppm — in Water, and Only There

Published 10/6/2026 · 3 min read · Health calculators

Sofia Nunes — Health & wellness writer at OneKitly

Nutrition · Hydration

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In short

Parts per million is a ratio of a part to a whole, multiplied by a million, and it carries no units of its own. The reason milligrams per litre and ppm are used interchangeably for water is a coincidence of the metric system: a litre of water weighs a kilogram, which is a million milligrams, so 50 mg in a litre is 50 parts in a million by mass. Take the same 50 mg into a litre of seawater, which is denser, and the answer drops to about 48.8 ppm; take it into a solvent lighter than water and it rises. The equivalence is a property of water's density and not of the notation, and treating it as a definition is how a laboratory figure gets carried into a context where it means something else.

50 mg in a litre is 50 ppm, because a litre of water weighs a million milligrams. Change the liquid, or move to air, and the two units stop agreeing.

In air, ppm usually means by volume

A gas concentration quoted in ppm is normally a ratio of volumes or of molecules, not of masses, and that makes it a different quantity from the water case with the same name. Converting one to the other needs the molar mass of the substance, the temperature and the pressure, which is why occupational exposure values are published in both ppm and milligrams per cubic metre with a stated conversion condition, and why the two columns are not proportional to each other across substances. Reading a number in ppm without knowing which ratio it is makes a comparison between an air figure and a water figure meaningless.

ppb is a thousandth of a ppm, not a hundredth

Water reports mix milligrams and micrograms per litre freely, and the step between them is a factor of a thousand: 1 mg/L is 1 ppm and 1 µg/L is 1 ppb. A value transcribed from one column into the other is out by three orders of magnitude, which is the kind of error that turns a routine result into an alarm or the reverse. The safeguard is to carry the unit through every step of any copying, and to look twice at any figure that seems surprising by roughly a thousandfold — that ratio is much more often a unit than a discovery.

50 mg of solute, three carriers
Carrier	Mass of 1 L	Concentration
Fresh water	1.000 kg	50.0 ppm
Seawater	1.025 kg	48.8 ppm
Air	about 0.0012 kg	a different quantity entirely

Worked with our own calculator

PPM calculator

Given

Solute (mg): 50
Solution (litres/kg): 1

Result

Concentration (ppm): 50

These figures are produced by the calculator below, not typed in by hand — they are recomputed whenever the tool changes.

Run it on your own figures →

Frequently asked questions

Is ppm a percentage?: It is the same kind of thing on a different scale. Per cent is parts per hundred, ppm is parts per million, so 1 % is 10,000 ppm and 1 ppm is 0.0001 %. The larger scale exists because writing 0.0001 % invites a misplaced decimal point while 1 ppm does not, and the practice of switching units to keep numbers between one and a thousand is the same instinct that gives us millimetres and kilometres. Convert between them by multiplying or dividing by 10,000, and check the direction by asking which number should be bigger.
Why do some reports say mg/kg instead?: Because it is unambiguous where ppm is not. A milligram per kilogram states the ratio and both its units, so nobody has to work out whether a mass or a volume was meant. For soil, sediment, food and anything solid it is the standard form, since a litre of soil is not a defined quantity. Where a report offers both, prefer the one with units attached, and where it offers only ppm, look for a sentence in the method section saying whether the ratio is by mass or by volume — a good report always has one.

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This article explains a unit, not a safety threshold. What counts as an acceptable concentration of anything in drinking water, air or food is set by your national authority and varies by substance and by country; nothing here is a limit, a diagnosis or a reason to act on a test result. If a measurement worries you, take it to the body that regulates it or to a doctor.

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