What Is TDS in Water? Total Dissolved Solids Explained

A TDS number tells you how much is dissolved in your water, not what it is. According to the World Health Organization, it’s mostly inorganic salts, such as calcium, magnesium, sodium and potassium paired with carbonate, bicarbonate, chloride, sulfate and nitrate, plus small amounts of organic matter.

What is a good TDS level for drinking water?

The EPA’s secondary standard for TDS is 500 mg/L. It’s a non-enforceable guideline for taste and appearance: above it, the EPA lists hardness, deposits, colored water, staining and a salty taste as possible effects.

The WHO cites taste panels that rated water below 300 mg/L as excellent, 300 to 600 as good, and anything above 1,200 as unacceptable. Water under 1,000 mg/L is usually acceptable to people, it says, and water with very little TDS can taste flat. High TDS (above 500 mg/L) also means more scale in pipes, water heaters and kettles.

Is high TDS bad for your health?

There’s no good evidence that TDS itself is. The WHO says reliable data on health effects of TDS in drinking water are not available, and it proposes no health-based guideline value. What matters is what makes up the number: a few of those substances, such as nitrate and fluoride, have their own limits. If your TDS is very high, the WHO suggests identifying the individual substances, which means a lab test, not a meter.

Should I test TDS, and does reverse osmosis lower it?

On a private well, yes: the CDC lists TDS among the tests to run at least once a year, along with coliform bacteria, nitrate and pH. Our guide to well water testing covers the rest.

Reverse osmosis is the usual way to cut TDS at the tap. The standard for RO systems, NSF/ANSI 58, requires every certified system to reduce TDS by at least 75 percent in testing. If your water is under 500 mg/L and tastes fine, you don’t need to lower it. More in how reverse osmosis works.