Parts Per Million (ppm) Calculator

Parts per million (ppm) is one of the most widely used concentration units in science and environmental monitoring. It expresses the amount of one substance dissolved or dispersed in another as a ratio of one to one million by mass or by volume. This calculator covers two common tasks: working out the ppm of a solution from the mass of dissolved solute and the total mass of the solution, and converting a known ppm value into equivalent units including milligrams per litre (mg/L), milligrams per kilogram (mg/kg), parts per billion (ppb) and percentage by mass. For dilute aqueous solutions, 1 ppm is very close to 1 mg/L, because the density of water is approximately 1 kilogram per litre and the mass of the dissolved substance is negligibly small compared with the water. This is why water quality results in New Zealand and internationally are often reported interchangeably in ppm and mg/L. In soil science and agricultural contexts, ppm is typically expressed as mg/kg, meaning milligrams of substance per kilogram of dry soil or sample. The New Zealand Drinking Water Standards (NZDWS) published by the Ministry of Health set maximum acceptable values for common contaminants in mg/L (ppm), and these are the reference for public water supply compliance in New Zealand under the Water Services Act 2021. This calculator is intended for educational and planning purposes. Always refer to the relevant New Zealand standard or laboratory report for regulatory compliance.

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mg
g
ppm
5 ppm
concentration (parts per million)
Equivalent mg/L (dilute aq.)5 mg/L
Equivalent mg/kg5 mg/kg
Parts per billion (ppb)5,000 ppb
Percentage by mass0.0005%

ppm = (solute mg / solution g) x 1000 for mass/mass, or solute mg / solution mL for dilute aqueous solutions. For regulatory water quality limits refer to the NZ Drinking Water Standards.

How it works

When you enter solute mass (mg) and solution mass (g), ppm is calculated as: ppm = (solute mass in mg / solution mass in g) x 1000. This converts mg/g (milligrams per gram) to mg/kg (milligrams per kilogram), which equals ppm by mass. For dilute aqueous solutions, mg/L is numerically equal to ppm because 1 litre of water has a mass of approximately 1 kg. Parts per billion (ppb) is ppm x 1000. Percentage by mass is ppm / 10,000. If you enter a ppm value directly in the third field, all conversions update immediately.

Worked example

A water sample contains 5 mg of dissolved iron in 1000 g (1 litre) of water. The ppm is (5 / 1000) x 1000 = 5 ppm. This is equivalent to 5 mg/L, which can be compared directly with the NZ Drinking Water Standards aesthetic guideline for iron of 0.2 mg/L. The same concentration is 5 mg/kg, 5,000 ppb, or 0.0005% by mass. These match the default values above.

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Frequently asked questions

What does ppm mean?

Parts per million (ppm) is a way of expressing very small concentrations. One ppm means one part of a substance per one million parts of the total mixture, by mass or by volume. For dilute aqueous solutions, 1 ppm is approximately equal to 1 milligram per litre (mg/L), because the density of water is about 1 kg per litre. In soil testing, 1 ppm means 1 milligram of substance per kilogram of dry soil (mg/kg).

How do I convert ppm to mg/L?

For dilute aqueous solutions, 1 ppm is approximately equal to 1 mg/L. This approximation holds when the density of the solution is close to 1 kg/L (which is true for most water-based solutions at concentrations below a few thousand ppm). For other solvents or high concentrations, use ppm = (mass of solute / mass of solution) x 1,000,000 and then convert using the actual solution density.

What are typical ppm limits in NZ drinking water?

The New Zealand Drinking Water Standards (NZDWS) set maximum acceptable values (MAVs) for many contaminants in ppm or mg/L. For example, the MAV for nitrate is 11.3 mg/L (as nitrogen), for fluoride is 1.5 mg/L, and for arsenic is 0.01 mg/L. These limits are based on health guidelines and are enforced for public water supplies under the Health Act and the Water Services Act.