Parts per million is a way of expressing how much of something is present relative to everything else, and it turns up constantly on test results, meters and product packaging. The most useful thing to know about it is that for drinking water it is interchangeable with the other unit you will see, which removes most of the confusion.
What the unit means
One part per million means one unit of a substance for every million units of the whole. Because a litre of water weighs close enough to a kilogram, and a kilogram is a million milligrams, one milligram of a substance in a litre of water is one part in a million. So for practical water testing the two units are numerically identical and can be read interchangeably, which is why results sometimes switch between them without comment.
Where you will meet it
Hardness results are usually given in milligrams per litre as calcium carbonate, and the USGS classes water as soft at 0 to 60 mg/L, moderately hard at 61 to 120, hard at 121 to 180 and very hard above 180 mg/L. Iron and manganese results are given the same way, and Penn State Extension notes recommended maximums of 0.3 mg/L iron and 0.05 mg/L manganese as secondary aesthetic standards. Chlorine, nitrate and dissolved solids results all use it too.
The smaller units
Some contaminants matter at concentrations far below one part per million, so results use parts per billion or parts per trillion instead, and it is worth checking which before reading a number as reassuringly small. The EPA's 2024 rule set maximum contaminant levels of 4.0 parts per trillion for PFOA and PFOS and 10 parts per trillion for PFHxS, PFNA and GenX, with May 2026 proposals keeping PFOA and PFOS with a compliance option to 2031 and proposing to rescind the others.
Reading a result properly
Check the unit before the number, since a value that looks large in one unit is trivial in another. Check what the result is expressed as, since hardness given as calcium carbonate and hardness given as calcium are different figures for the same water. And note that softener settings are usually entered in grains per gallon rather than milligrams per litre, so a conversion is needed. Our softener guide covers what the hardness figure actually decides.
Questions people ask about what is ppm in water
Is parts per million the same as milligrams per litre?
For water, yes, numerically. A litre weighs about a kilogram, which is a million milligrams, so the two are interchangeable.
Why do some results use parts per billion or trillion?
Because some contaminants matter at concentrations far below one part per million, so a smaller unit is clearer.
What should I check before reading a number?
The unit, and what the result is expressed as, since hardness as calcium carbonate and as calcium differ for the same water.
Do softeners use these units?
Usually not. Softener settings are entered in grains per gallon, so a conversion from milligrams per litre is needed.