How this calculator works
Parts-per notation expresses a small fraction without writing many zeros. One percent is one part in a hundred, so it is 10,000 parts per million. One per mille is one part in a thousand, or 1,000 ppm. Each step down the parts-per chain divides by a further thousand: ppm, ppb and ppt. The plain ratio is the fraction itself, so 1 ppm is 0.000001 and 100 percent is 1. The same numbers are used for mass fractions, volume fractions and sometimes mole fractions, so always record which quantity the ratio refers to.
Formula
ratio = value x F(unit), where F is 1 for a plain ratio, 0.01 for percent, 0.001 for per mille, 10^-6 for ppm, 10^-9 for ppb and 10^-12 for ppt. The value in another form is ratio ÷ F(that form).
Worked example
1 ppm is 0.0001 percent, 0.001 per mille, 1,000 ppb and 1,000,000 ppt. 1 percent is 10,000 ppm. 5 ppb is 0.005 ppm and 5,000 ppt.
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Frequently asked questions
Is 1 ppm the same as 1 mg/L?
Only for a dilute aqueous solution with a density close to 1 kg/L. Strictly, ppm is a mass or mole fraction, while mg/L is a mass concentration. Use the ppm to mg/L page to apply the density explicitly.
How many ppm are in one percent?
Ten thousand. One percent is 1/100 and one ppm is 1/1,000,000, so 1 percent is 10,000 times larger.
Is a part per trillion the same in every country?
The parts-per notation itself is the same, but a trillion always means 10^12 here. A US trillion is 10^12, while a historical British billion or trillion could mean other powers; this page uses the short scale throughout.