What Do ppm, ppb and ppt Mean in Water Test Results?

The same water report can list nitrate in ppm, lead in ppb and PFAS in ppt. The units differ because the limits do: some substances matter at a few milligrams per liter, others at a few nanograms. Before you compare a result with a limit, make sure both are in the same unit.

Is mg/L the same as ppm?

For drinking water, yes. The EPA’s drinking water standards are listed in milligrams per liter and note that mg/L is equivalent to parts per million. The federal rule for water quality reports (40 CFR 141, Subpart O, Appendix A) defines the rest: ppb is micrograms per liter (µg/L), ppt is nanograms per liter (ng/L), and ppq, which you’ll rarely see, is picograms per liter.

How do I convert ppm to ppb or ppt?

Each step is a factor of 1,000. Multiply ppm by 1,000 to get ppb, and ppb by 1,000 to get ppt. That’s why arsenic’s legal limit of 0.010 mg/L shows up in reports as 10 ppb, and why the lead action level of 15 parts per billion is also written 0.015 mg/L. Under the Lead and Copper Rule Improvements, that action level drops to 0.010 mg/L (10 ppb) on November 1, 2027.

Which limits are in ppt?

The smallest. The EPA’s 2024 PFAS rule sets legal limits of 4.0 ppt for PFOA and PFOS, “also expressed as ng/L”. In mg/L that would be 0.000004.

At the other end, the 500 mg/L figure for total dissolved solids is a secondary standard: a guideline for taste and appearance that the EPA doesn’t enforce. Always check which kind of number you’re looking at. Our guide on how to read your water quality report walks through the columns, and well water testing lists the common limits for a well test.

Water hardness is the exception you’ll meet most: it’s often given in grains per gallon instead, where one grain per gallon is about 17.1 ppm.