What is a good TDS for drinking water?

The EPA secondary limit for TDS is 500 mg/L, which is based on taste and aesthetics, not health. Most US tap water falls between 100 and 400 mg/L. Water above 500 mg/L may taste salty or mineral-heavy. There is no established health-based TDS limit for drinking water.

Is low TDS water better for you?

Not necessarily. Low TDS means fewer dissolved minerals, which are mostly harmless and some, like calcium and magnesium, are beneficial. RO-filtered water has very low TDS but is perfectly safe to drink. The relationship between TDS and health is not direct.

Does TDS measure water safety?

No. TDS is a measure of total dissolved ion concentration, not a safety test. Lead at 10 ppb, arsenic at 5 ppb, PFAS at 4 ppt, and bacteria don't register meaningfully on a TDS meter. Water can have a low TDS and still contain dangerous contaminants at levels that require action.

How do I lower TDS in my drinking water?

Reverse osmosis is the most effective method, typically reducing TDS by 90 to 99 percent. Distillation works similarly. Water softeners trade calcium and magnesium for sodium, which may change TDS modestly but doesn't reduce it significantly. Carbon filters do not meaningfully reduce TDS.

What TDS does a reverse osmosis system produce?

A properly functioning RO system typically produces water with TDS between 10 and 40 mg/L from a source water of 200 to 400 mg/L. If RO output TDS climbs above 80 to 100 mg/L, the membrane is likely degraded and should be replaced.

What Is TDS in Water? What the Number Means (and Doesn't Mean)

TDS stands for total dissolved solids. A TDS meter sends a tiny electrical current through a water sample. Dissolved ions conduct that current. The more ions, the higher the conductivity, the higher the reading. Results come in parts per million (ppm) or milligrams per liter (mg/L), which are the same value for water.

It’s one of the most misread numbers in water quality.

What Actually Contributes to TDS

The dissolved substances that make up most TDS readings are calcium, magnesium, sodium, potassium, bicarbonate, sulfate, chloride, and silica. These are naturally occurring minerals. Most are harmless. Calcium and magnesium are the minerals that cause hard water, and both are beneficial nutrients in normal dietary amounts.

In a typical US municipal water supply, these minerals make up the vast majority of the TDS reading.

What TDS Doesn’t Detect

This is the part that matters most.

Lead at 10 ppb. Arsenic at 5 ppb. PFAS at 4 ppt. Nitrates at 8 mg/L. Bacteria. Viruses.

These contaminants are either present at concentrations too small to move a TDS meter or don’t conduct electricity in a way a TDS meter picks up. A water sample can show TDS of 150 mg/L, well below the EPA secondary limit, and still contain arsenic or PFAS at levels worth addressing.

Using TDS as a proxy for water safety is the most common mistake people make with these meters.

The EPA Secondary Limit: 500 mg/L

The EPA’s secondary maximum contaminant level for TDS is 500 mg/L. Secondary standards are non-enforceable, based on taste and aesthetics rather than health. Water above 500 mg/L often tastes salty, brackish, or heavily mineral. That threshold is about palatability, not safety.

What Different TDS Ranges Actually Look Like

Under 50 mg/L: very soft water. Pacific Northwest municipal systems, rainwater, RO output. Minimal mineral taste.
50 to 150 mg/L: low TDS. Most soft-water cities fall here. Neutral, clean taste.
150 to 300 mg/L: moderate. The typical range for US tap water. Taste is generally fine.
300 to 500 mg/L: moderately high. Common in hard water regions. Some taste effects at the higher end.
500 to 1,000 mg/L: high. Noticeable mineral or salty taste. Scale buildup on fixtures.
Above 1,000 mg/L: very high. Brackish or salty. Agricultural irrigation limits start at about 3,000 mg/L depending on crop type.

When TDS Actually Is Useful

Monitoring RO membrane performance. A functioning reverse osmosis system reduces TDS by 90 to 99 percent. Output should be 10 to 40 mg/L from typical source water. When output TDS starts climbing toward 80 to 100 mg/L, the membrane is degrading and needs replacement. A $12 TDS meter is the cheapest and easiest way to check this monthly.

ZeroWater filter replacement timing. ZeroWater filters to near-zero TDS (under 6 ppm) and ships a TDS meter with every pitcher for this reason. When output rises above 6 ppm, the filter is exhausted. Higher source TDS means the filter exhausts faster. If your tap water is 400 mg/L, expect shorter filter life than someone with 150 mg/L source water.

Quick hardness estimates. TDS correlates roughly with hardness in most freshwater. A TDS of 300 mg/L usually means moderately hard water. That’s not precise enough to size a water softener, but it gives you a fast directional indicator before investing in a full test.

When TDS Is Misleading

The association between low TDS and “clean” water trips people up constantly.

RO water has low TDS. So does rainwater. So does water that had lead removed by filtration. These are all very different situations but produce similar TDS readings.

Mineral-rich spring water has high TDS but is among the most popular bottled water in the world. Hard well water in a pristine aquifer has high TDS but no health hazard.

The TDS number has no direct relationship to whether drinking water is safe for the specific contaminants that actually harm people.

The Bottom Line

Buy a TDS meter ($10 to $15) if you have an RO system or a ZeroWater pitcher. For those two specific uses, it’s the best cheap maintenance tool available.

Don’t use TDS to evaluate whether your tap water is safe. If that’s the question, use certified lab testing.

See how to test water at home for a full rundown of testing options, or go straight to the best under-sink RO systems if you’re already thinking about filtration.