Concentration Converter
Same value in every unit
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ppm and mg/L are not the same thing
They are treated as interchangeable constantly, and in water they very nearly are — a litre of water weighs about a kilogram, so a milligram in a litre is one part per million by mass. Outside water that reasoning collapses. In a solvent half as dense, the same mg/L is twice the ppm; in a gas the relationship depends on molecular weight, temperature and pressure.
This converter handles only the dimensionless ratios, which are exact. Converting to or from mg/L needs a density you have to supply, so pretending otherwise would just produce confident wrong answers.
The ratios, which are exact
- 1% = 10,000 ppm
- 1‰ (per mille) = 1,000 ppm
- 1 ppm = 1,000 ppb = 1,000,000 ppt
- 1 pcm = 10 ppm
Watch out for ppt
It means parts per trillion in most environmental and analytical work, and parts per thousand in some older texts and in parts of the drinks industry. Those differ by a factor of a billion. Where the document does not say, the magnitude of the surrounding numbers usually settles it — but check rather than assume.
By mass or by volume
For solids and liquids, ppm normally means by mass. For gases it normally means by volume, written ppmv. The unit looks identical either way, so the basis has to come from the context.
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Frequently Asked Questions
Only in water, and only approximately. The equivalence holds because a litre of water weighs almost exactly one kilogram, so a milligram in a litre is one part in a million by mass. In any other liquid, or in a gas, the density is different and the two are not interchangeable — this converter deliberately does not offer mg/L for that reason.
Ten thousand. One percent is one part in a hundred, and a ppm is one part in a million, so the ratio is 10,000. Going the other way, 250 ppm is 0.025%.
Usually parts per million by volume rather than by mass, often written ppmv. Air quality and gas detection figures are almost always by volume, and converting them to a mass concentration needs the molecular weight and the temperature and pressure.
Per cent mille — parts per hundred thousand, so 10 ppm. It turns up in nuclear engineering and in some metallurgy, and rarely anywhere else.
Because it is used for both parts per trillion and parts per thousand, which differ by a factor of a billion. This tool uses it for trillion and offers per mille (‰) separately for thousand. If you meet ppt in a document, check which one is meant before using the number.