What Does TDS Mean? Reading the Number on Your Water Meter Without Overreacting

A TDS meter is one of the most common tools people use to judge their water, and one of the most misread. It gives a single number in parts per million (ppm) that seems to say how clean the water is. It does not. Here is what the number really means.

What TDS measures

Total dissolved solids is the combined mass of everything dissolved in water: minerals such as calcium, magnesium, sodium, and potassium; salts such as chlorides, sulfates, and bicarbonates; and small amounts of dissolved organic matter. A handheld TDS meter does not weigh any of this. It measures electrical conductivity and converts it to an estimated ppm, because dissolved ions conduct electricity.

That conversion is the first thing to understand: the meter is only detecting charged particles. It gives a rough total, not a list.

What TDS does not tell you

• Which solids are present. 200 ppm of calcium and magnesium (harmless, even beneficial) reads the same as 200 ppm that includes lead or nitrates.
• Bacteria, viruses, or cysts. They are not dissolved ions and do not register at all.
• PFAS, pesticides, pharmaceuticals, or most organic contaminants. Present at parts per trillion and largely non-conductive; invisible to a TDS meter.
• Chlorine. Contributes very little to conductivity.
• Microplastics. Not dissolved.

A low TDS reading is not proof of safe water, and a high reading is not proof of unsafe water. The EPA sets a secondary (aesthetic) guideline of 500 ppm, but it is about taste and scaling, not health.

Typical readings

What TDS is actually good for

1. Checking a reverse osmosis system

This is the best use of a TDS meter. RO membranes remove 90 to 99 percent of dissolved solids, so comparing the reading before and after the system tells you whether the membrane is working. If tap water reads 300 ppm and the RO output reads 15 ppm, the membrane is fine. If output creeps up toward 60 or 80 ppm, the membrane is aging or a pre-filter is exhausted. Systems like the AquaVerde AV-RO-10S include a built-in TDS monitor for exactly this reason.

2. Spotting a change

If your well normally reads 400 ppm and one week reads 900 ppm, something changed: a new source of infiltration, a failed casing, road salt. It is a prompt to get a lab test, not a diagnosis.

3. Dialing in coffee and tea

Brewing associations recommend roughly 75 to 250 ppm for coffee. Pure RO water is too low for good extraction; remineralizing to around 100 to 150 ppm improves flavor. Our coffee water guide covers this in detail.

Is low TDS water unhealthy?

A common concern, usually overstated. Very low TDS water tastes flat and slightly more acidic, and it can be aggressive to metal plumbing over time, which is why RO systems are usually plumbed with plastic tubing. There is no credible evidence that drinking low-TDS water harms healthy adults who eat a normal diet, since the minerals in water are a minor contribution compared to food. If you prefer the taste of mineralized water, add a remineralization stage after the RO membrane.

Frequently asked questions

What is a good TDS for drinking water? There is no single correct number. Under 500 ppm meets EPA aesthetic guidance. Most people find 50 to 250 ppm tastes best.

Does a whole-house carbon filter lower TDS? No. Carbon removes chlorine and organics, which barely register on a TDS meter. Only RO, distillation, or deionization significantly lower TDS.

Does a water softener lower TDS? No, and it may raise it slightly, because it exchanges calcium and magnesium for sodium ion for ion.

Can TDS tell me if my water is hard? Only loosely. High TDS often correlates with hardness, but a proper hardness test measures calcium and magnesium specifically.

For a real picture of what is in your water, see our guide to home testing versus certified lab tests.