If you’ve ever looked at a water test or a TDS meter, you’ve seen a number in “ppm.” Here’s what Total Dissolved Solids actually measures, what’s a normal range, and why the number alone doesn’t tell you whether your water is safe.
What TDS is
Total Dissolved Solids (TDS) is the combined weight of everything dissolved in your water — minerals, salts, and metals — measured in milligrams per liter (mg/L), often shown as parts per million (ppm). A handheld TDS meter estimates it by measuring the water’s electrical conductivity, since dissolved ions conduct electricity (USGS).
What counts as “normal”
The EPA lists TDS as a secondary standard — a guideline for taste and aesthetics, not health — recommending below 500 ppm (EPA). Most U.S. tap water falls somewhere between roughly 50 and 500 ppm. Very low TDS tastes “flat”; very high TDS can taste salty, metallic, or bitter and may leave spots and scale.
The big caveat: TDS is not a safety score
This is the key thing meters don’t tell you. TDS measures the quantity of dissolved material, not what it is. Water with a healthy dose of harmless calcium and magnesium can read the same as water carrying something you’d rather not drink. And crucially, TDS meters do not detect many of the contaminants people worry about most — lead, PFAS, bacteria, nitrate, and pesticides can be present at concerning levels while the TDS number looks fine. A low TDS reading is not a clean bill of health, and a moderate reading isn’t automatically bad.
How to use a TDS meter well
- Verify a reverse osmosis system is working. This is the meter’s best use: compare your tap water to the RO output. A big drop (often to single or low double digits) confirms the membrane is doing its job. A rising RO number over time signals the membrane is due for replacement.
- Track changes. A sudden jump from your baseline is worth investigating with a real lab test.
- Don’t use it to judge safety. For that, you need an actual water-quality test.
Curious how RO changes your number? See our reverse osmosis systems, or grab a simple tester from water testing.
Get the real picture
For what’s actually in your water — not just how much — send us your ZIP or utility report for a free water analysis, or take the Find Your System quiz.
Sources
- USGS — Dissolved Solids: usgs.gov/water-science-school
- U.S. EPA — Secondary Drinking Water Standards: epa.gov/sdwa