Water QualityIntermediate9 min read•5/31/2026

Drinking Water Test Results Cheat Sheet: What Common Numbers Mean

A homeowner-friendly cheat sheet for interpreting common water test results, including units, action levels, and filter decision points.

Start With Units Before You Compare Numbers

Water test reports often mix mg/L, ug/L, ppm, ppb, pCi/L, hardness grains, and presence-absence bacteria results. Before you make a filter decision, confirm the unit and whether the number is a lab detection, a regulatory limit, or an advisory value.

For most dissolved chemicals, 1 mg/L is roughly equal to 1 ppm and 1 ug/L is roughly equal to 1 ppb in water. That conversion is useful, but you should still read the lab report notes because some results include method detection limits or qualifiers.

  • •Confirm the sample location and date.
  • •Match every result to the unit used by the lab.
  • •Flag anything reported as present, detected, or above a lab reporting limit.

Common Results and What They Usually Trigger

Coliform bacteria usually triggers immediate follow-up sampling, disinfection, or source inspection. Lead and copper are plumbing-related for many homes, so the first-draw sample method matters. Nitrate is especially important for infants and pregnant people. PFAS results require careful comparison with current EPA rules and local guidance.

No single table can replace a certified lab or local water authority, but a cheat sheet helps you ask better questions.

  • •Bacteria: confirm with the lab and local health authority.
  • •Lead: review plumbing, fixtures, and certified filters.
  • •Nitrate: act quickly if serving infants or vulnerable people.
  • •PFAS: compare against EPA MCLs and certified treatment options.

Source References to Keep Nearby

The EPA National Primary Drinking Water Regulations table is the best starting point for federally regulated contaminants. The EPA PFAS rule page is important because PFAS standards are measured at very low levels and use both individual limits and a hazard index for certain mixtures.

  • •EPA regulated drinking water contaminants: https://www.epa.gov/ground-water-and-drinking-water/national-primary-drinking-water-regulations
  • •EPA PFAS drinking water regulation: https://www.epa.gov/sdwa/and-polyfluoroalkyl-substances-pfas

How to Use the Cheat Sheet With a Filter

Choose a filter based on the contaminant you need to reduce, not the broadest marketing claim. Look for certification to the relevant NSF/ANSI standard where available, and check the product data sheet for the exact contaminant and reduction claim.

If you have a private well, retest after treatment so you know whether the installed system actually solved the problem at your tap.

  • •Match contaminant to technology.
  • •Check certification and test data.
  • •Retest after installation when results were concerning.

Drinking Water Test Results Cheat Sheet: What Common Numbers Mean - Frequently Asked Questions

Can I use a home strip test instead of a lab?

Home strips can be useful screening tools, but lab testing is more appropriate for health-relevant decisions and precise contaminant levels.

Does any one filter remove everything?

No. Filter choice depends on contaminants, concentration, flow rate, maintenance, and certification.

Related Resources

Continue with a few relevant reads plus trusted standards references.

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