What TDS actually is
Total dissolved solids is the combined amount of dissolved minerals and salts, such as calcium, magnesium, sodium, chloride and bicarbonate. The EPA lists TDS among its secondary, non-enforceable standards, with a guideline of 500 mg/L, because high TDS can cause hardness, deposits, staining and a salty taste. It is an aesthetic measure, not a health verdict.
Most handheld meters do not measure dissolved solids directly. They measure electrical conductivity and convert it into an estimate, usually in ppm.
What a TDS meter is good for
- Tracking reverse osmosis performance. Measure tap water and RO water the same day. The percentage reduction should stay fairly steady over time.
- Spotting a change. A rising RO reading compared with your own earlier readings often points to a membrane or seal issue. See when to replace RO filters.
- A rough mineral picture. Higher readings usually mean more dissolved minerals, which often goes along with hardness.
What it cannot tell you
| Question | Can TDS answer it? |
|---|---|
| Is there lead in my water? | No. Lead at levels of concern is far too small to show up; use a certified lab test |
| Are there bacteria? | No. Bacteria do not register on a TDS meter |
| Are there PFAS? | No. Requires lab analysis |
| How much chlorine or chloramine? | No. Use a chlorine test |
| Exact hardness? | No. Use a hardness test |
| Is my water good to drink? | No single number answers that |
The CDC advises testing for the specific germs or chemicals you are concerned about and choosing a filter certified to address them.
Why a whole-home filter may barely change TDS
Carbon filters mainly reduce chlorine, chloramine and certain organic compounds, which make up little of the TDS number. Softeners swap hardness minerals for sodium or potassium, so TDS can stay about the same even though hardness drops. Alkaline RO systems add some minerals back after the membrane, so their reading is higher than plain RO by design. None of these means the system is not working.
Be wary of TDS demonstrations
A TDS reading can look dramatic, but it measures only dissolved solids. Do not let a single number, high or low, be the reason you buy or skip a system. Ask for a test that covers what you care about and a system certified for those specific claims.
How to take a good reading
- Rinse the probe with the water you are testing.
- Use a clean glass and let the reading settle.
- Test tap and filtered water the same day.
- Write the numbers down with the date.
- Calibrate or replace the meter per its instructions.
Jorge's free in-home test goes beyond TDS to cover basics like hardness and chlorine or chloramine, and he explains what each result means. See what happens during a home water assessment.
https://www.epa.gov/sdwa/secondary-drinking-water-standards-guidance-nuisance-chemicals
https://www.cdc.gov/drinking-water/prevention/about-choosing-home-water-filters.html
https://www.epa.gov/system/files/documents/2025-01/ws-products-home-water-treatment-guide_v2_508.pdf
https://www.epa.gov/watersense/point-use-reverse-osmosis-systems
Updated 2026-10-17 · Living Waters · Jorge Canela, Water Quality Specialist