Every water test answers one narrow question, so first work out which question you have. On city water, start with your utility’s published results, which cost you nothing. On a private well, nobody tests for you, and EPA recommends a yearly test for total coliform, nitrates, total dissolved solids (TDS) and pH. A home meter can’t tell you the water is safe: that takes a lab. This page shows which test fits which question, how to think about TDS and PFAS, and which guide to read at each step.
This is general information, not medical or professional advice. Take well results and health worries to your state or county health department or your doctor. WaterPlain has no lab: everything here comes from published sources.
Start with who supplies your water
City water. Your utility’s annual water quality report comes first. For PFAS, EPA’s UCMR 5 Data Finder lets you search final 2023 to 2025 results by system. Those results don’t show compliance, and not every small system is in them, so ask the utility if yours is missing.
Private well. EPA says private domestic wells are not regulated under the Safe Drinking Water Act, and the owner is responsible for the water’s safety. Federal limits are written for public water systems, though they remain a useful yardstick. Test right away if you notice a change in odor, color or taste. Our guide to well water tests explains what to order and what a kit can and can’t do, and the well water section covers problems and treatment.
Match the question to the test
| What you want to know | Use this |
|---|---|
| Is my tap water safe? | Your utility’s report. For a well, a state-certified lab |
| Do I have lead or PFAS? | A lab test for that contaminant |
| Has my well changed? | A TDS meter log, plus a lab test if a rise holds |
| Is my reverse osmosis membrane still working? | A TDS meter, before and after |
| Is my water hard? Is my softener working? | A hardness test kit, with the result in gpg or ppm. Our hardness converter handles the units |
| Someone at home is ill and you suspect the water | Your doctor and your state or county health department |
A meter answers two of those six questions, and neither is about safety.
TDS: useful, within limits
TDS is everything dissolved in a sample of water, mostly mineral salts such as calcium, magnesium, sodium, bicarbonate, chloride and sulfate. Begin with our explainer on TDS in water.
EPA lists 500 mg/L as a secondary level for public systems. It is tied to things like staining and salty taste, and EPA doesn’t enforce it. WHO says “No health-based guideline value is proposed for TDS.” Water with a TDS of 50 can still hold too much lead, and water at 700 can pass every federal health standard.
The total hides what matters because many contaminants are regulated at tiny amounts. Using EPA’s drinking water limits, a 1 ppm meter step is 100 times bigger than the arsenic limit and 250,000 times bigger than the PFOA limit. One high-reading caution: EPA says water high in sulfate or TDS should not be used to prepare powdered infant formula. It doesn’t define “high”, so ask your pediatrician or health department.
Then, if you own a reverse osmosis (RO) system or want to track a well, read our guide to TDS meters. Three things to keep in mind:
- A meter gives an estimate. It measures conductivity and multiplies by a factor. The same water can read 500 ppm on a meter set to 0.5 and 700 ppm on one set to 0.7.
- It suits two jobs. For RO, compare the water going in with the water at the RO faucet. GE says to replace the membrane below 75% reduction, on its own systems, so follow your manual. For a well, log readings and treat a rise that holds as a reason to go to a lab. A steady reading doesn’t clear a well.
- It can’t judge softeners or carbon. Montana State University says “Water softening does not reduce total dissolved solids”, and carbon’s usual targets barely register on a meter. An unchanged reading doesn’t show that either one has failed.
PFAS: get the number, then choose a filter
Read these two guides in order, because the filter decision depends on the result. EPA says that if PFAS have not been measured in your water, or are below federal limits, “a filter may not be useful to you.”
Step 1: test or look it up
Our PFAS testing guide covers who needs a test, costs and reading the report. On city water, a paid test mainly makes sense to check a filter, or if your small system has no data. On a private well near a likely source, such as an airfield, fire training area, landfill or field spread with sludge, test now. With no known source, agencies disagree: MassDEP recommends testing all private wells, while Minnesota doesn’t recommend testing every one. In that case, test bacteria and nitrate first.
Published lab prices we found start at $320 plus handling, and MassDEP says tests run up to $500. Some states test wells for free, so call yours before you pay anyone. For a well, our pick is EPA Method 533 with a reporting limit of 2 ppt or lower.
As of 2 November 2026, the federal limit is 4.0 parts per trillion (ppt) each for PFOA and PFOS, and EPA’s health goal for both is zero. EPA has proposed more time for eligible systems on those two and removing the other PFAS limits. Neither is final. Compare your result with the federal limit and your state’s, and use the lower one.
Step 2: read the label closely
Our guide to what PFAS filter certification proves starts from an awkward fact: we found no filter certified to reach 4 ppt. Certification today means a filter kept treated water under 70 or 20 ppt in a lab test, depending on the version of the standard, and EPA said in May 2026 that the standard “does not currently meet” its limits. That doesn’t make these filters useless, since EPA expects they can go below 20 ppt during much of their use. It means the label can’t tell you how far yours goes.
- Work out the removal you need. It is 1 − (target ÷ your level). Getting 10 ppt down to 4 takes 60%, and 40 ppt takes 90%.
- Check the exact model in a certifier’s directory, then read its newest data sheet for the level it was tested to.
- Look at the field evidence. In a 2020 study of filters in North Carolina homes, under-sink RO and two-stage filters did best, while single-stage carbon and pitchers varied widely. Four of six whole-house carbon systems had more PFAS after the filter than before it. It was one state and small groups.
- Test the filtered water if the result matters. A lab sample is the only direct answer.
Reading the units on a result
| On the report or display | What it means |
|---|---|
| mg/L or ppm | Effectively the same for drinking water. TDS is reported this way |
| ng/L or ppt | The same thing on a PFAS report: parts per trillion |
| µg/L or ppb | Multiply by 1,000 to get ppt, so 0.004 µg/L is 4 ppt |
| “ppt” on a TDS meter | Parts per thousand, or 1,000 ppm, as NDSU Extension explains. It has nothing to do with parts per trillion |
| ND | Not detected above the lab’s limit. An ND with a reporting limit of 5 ppt can’t tell you whether you are under 4.0 ppt |
Claims to question
- “This reading shows your water is unsafe.” The Minnesota Department of Health warns of in-home tests that “may simply indicate the presence of naturally occurring minerals in the water.” Ask which contaminant the number is, and what certified lab result supports it.
- “Your softener handles it.” Minnesota says sampling data show whole-house softeners “do NOT remove PFAS”.
- “Just boil it.” Minnesota says boiling will not remove PFAS, and boiling doesn’t remove TDS either.
- “Meets the 4 ppt limit.” That is the seller’s claim, not certification. Ask for the lab report.
If a result worries you
- Check the reading. Retest with a meter. Confirm a lab result with a second sample before you pay for treatment.
- Call your state or county health department before you buy anything. Ask what the result means, what help exists, and what to use for drinking and formula in the meantime.
- Choose treatment from the lab result, not a total. High hardness, high sodium and high nitrate are different problems. On a well, match the treatment to your test.
- Test the treated water afterward.
Facts on this page come from the guides linked above, which were checked against primary sources on 2 November 2026.
