Parts per million (ppm)

Convert between parts per million, percentage strength, and ratio strength.

What is this for?

Parts per million expresses extremely dilute concentrations — fluoride in drinking water, trace contaminants, preservative residues. One ppm is one part in one million parts, which is a shift of four decimal places from percentage strength.

How to use it

Choose whether you are converting from ppm or from a percentage.
Enter the numeric value.
The mg/L equivalent is valid for dilute aqueous solutions, where 1 litre weighs approximately 1 kilogram.

Worked example

A municipal water supply is fluoridated to 0.7 ppm. Express this as a percentage strength and in mg/L.

Answer: 0.7 ÷ 10,000 = 0.00007%. In dilute aqueous solution this is 0.7 mg/L, which is the current US Public Health Service recommended fluoridation level.

Clinical pearls & pitfalls

The mental shortcut is the decimal shift: percentage to ppm moves the point four places right, ppm to percentage four places left.
For dilute aqueous solutions ppm is numerically equal to mg/L, because one litre of water weighs about one million milligrams. This equivalence fails for non-aqueous vehicles and concentrated solutions.
Fluoride supplementation decisions depend on the water fluoride concentration in ppm and the child's age; check the local water report before recommending a supplement.

Assumptions & limitations

The mg/L equivalence assumes a density of 1 g/mL. Do not apply it to oils, syrups, or alcoholic vehicles.
Some analytical contexts define ppm on a volume basis (ppmv), particularly for gases. This calculator uses the mass/mass convention standard in pharmacy.

References

US Public Health Service. Recommendation for fluoride concentration in drinking water for prevention of dental caries. Public Health Rep. 2015;130(4):318-331.
United States Pharmacopeia. General Notices, 8.140 Percentage Measurements.