What Does TDS Mean? TDS in Water, Explained

TDS means total dissolved solids: the combined amount of minerals, salts and metals dissolved in a sample of water. It is one number that tells you how much "stuff" is dissolved in your water, but not what that stuff is.

That distinction matters. A TDS reading is useful, quick and cheap to take, and it is also one of the most misunderstood numbers in home water. This guide covers what TDS in water is, what a normal water TDS level looks like, how to run a TDS test with a meter, and what a TDS meter can't tell you.

What is TDS in water?

TDS in water is everything dissolved in it that isn't water itself and that would stay behind if the water evaporated. Most of it is made up of charged particles called ions. The most common ones are:

• Minerals: calcium and magnesium (the main causes of hard water), plus potassium.
• Salts: sodium, chloride, sulfate and bicarbonate.
• Nutrients: nitrate and phosphate, often from fertilizer or septic runoff.
• Metals: iron, manganese, and in small amounts, metals like lead or copper from plumbing.

Most TDS comes from nature. Water picks up minerals as it moves through soil and rock. Road salt, farm runoff, industrial discharge and household plumbing can add more.

What TDS means, in plain terms

If you are looking up what TDS means because a number came back on a test or a filter display, think of it as a measure of dissolved concentration. A high number means a lot is dissolved. A low number means little is dissolved. On its own, it says nothing about whether the water is safe.

How TDS is measured: ppm and mg/L

TDS is usually reported in parts per million (ppm). For water, 1 ppm is roughly equal to 1 milligram per liter (mg/L), so you will see the two units used interchangeably. A reading of 200 ppm means about 200 milligrams of dissolved solids in every liter.

Labs measure TDS precisely by evaporating a filtered sample and weighing the residue. Handheld TDS meters estimate it instead, by measuring how well the water conducts electricity. Dissolved ions carry current, so more ions means higher conductivity and a higher reading.

Water TDS levels: what the numbers usually mean

There is no single "correct" TDS. Tap water across the U.S. varies widely depending on the source. The U.S. Environmental Protection Agency (EPA) sets a secondary standard of 500 mg/L for TDS. Secondary standards are non-enforceable guidelines based on taste, odor and appearance, not health.

TDS (ppm)	What it usually means
Under 50	Very low. Typical of reverse osmosis, distilled or deionized water. Tastes clean and flat to some people.
50–150	Low. Common for water from soft sources such as some surface water.
150–300	Moderate. A common range for municipal tap water. Minerals may start to show up in taste.
300–500	Higher. Often hard water. You may notice scale on fixtures and kettles and a stronger taste.
Over 500	Above EPA's 500 mg/L secondary guideline. Salty, bitter or metallic taste and heavy scaling are more likely.
Over 1,000	High. Often a sign of brackish sources, heavy salt intrusion or a well that needs investigation.

These ranges are general. Use them to understand your number, not to decide whether your water is safe to drink.

How to do a TDS test with a TDS meter

A TDS test at home takes about a minute with an inexpensive handheld TDS meter. Here is how to get a reading you can trust:

1. Use a clean glass. Rinse it with the water you are about to test so leftover residue doesn't skew the result.
2. Run the tap briefly. Let cold water run for several seconds, then fill the glass.
3. Let it reach room temperature. Conductivity changes with temperature. Many meters compensate automatically, but very hot or very cold water can still throw a reading off.
4. Remove the cap and turn the meter on. Dip the probe to the marked line, not past it.
5. Stir gently and wait. Move the probe to release air bubbles and wait for the number to settle.
6. Record the reading. Many meters have a hold button so you can read it out of the water.
7. Rinse and dry the probe. Put the cap back on so it stays accurate.

The most useful way to use a TDS meter is comparison. Test your tap water, then test the water from your filter. Retest the filtered water every few weeks. The trend tells you more than any single number.

What a TDS meter can't tell you

A TDS meter can't identify specific contaminants. It reports one combined number, and it only "sees" dissolved substances that carry an electrical charge. Several contaminants people worry about most are invisible to it:

• Lead. Lead matters at parts per billion, a thousand times smaller than the parts per million a TDS meter reads. Lead well above health guidelines won't move the number. See lead in drinking water.
• PFAS. These "forever chemicals" are measured in parts per trillion. A TDS meter has no way to detect them. See PFAS in water.
• Trihalomethanes (THMs). These disinfection byproducts are uncharged organic compounds, so they don't conduct electricity. See trihalomethanes.
• Chloramine and chlorine. Disinfectant levels barely register on a TDS meter. See chloramines.
• Bacteria and viruses. Living organisms aren't dissolved solids. A TDS meter says nothing about them.

That means a low TDS does not mean safe, and a high TDS does not automatically mean unsafe. Water full of harmless calcium can read high. Water with lead from an old service line or PFAS from a local source can read low.

Here is a real example. In an independent accredited lab test of a home in Arlington, VA, the kitchen tap had 58 ppb of total trihalomethanes. That is 0.058 ppm, far too small to show up on a TDS meter, and THMs don't conduct electricity anyway. Only a lab test caught it.

To learn which contaminants are in your water, look up what your utility reports with our zip code water quality lookup, and use a certified lab test for anything specific, such as lead. Our free water test (a kit mailed anywhere in the U.S., or in person in Northern Virginia) gives you a TDS reading and explains it.

How reverse osmosis lowers TDS

Reverse osmosis (RO) is the most common way to lower TDS at home. Water is pushed through a semipermeable membrane with pores small enough to hold back most dissolved ions, while water molecules pass through. The rejected minerals and salts are rinsed to the drain.

That is why RO water usually reads far lower on a TDS meter than tap water. Home RO systems also pair the membrane with sediment and carbon stages, which handle particles and chlorine-type chemicals that a membrane alone is not designed for. For a fuller explanation, read what is reverse osmosis water.

Other methods lower TDS too. Distillation and deionization both produce very low TDS water. Standard carbon pitchers and faucet filters generally don't change TDS much, because carbon isn't built to remove dissolved minerals. Our comparison of reverse osmosis vs a pitcher filter covers the difference.

Using TDS to watch filter performance

This is where a TDS number really earns its keep: tracking whether a filter is still doing its job. When an RO membrane or its cartridges wear out, more dissolved solids slip through and the TDS of the filtered water creeps up.

The Hydra under-sink reverse osmosis system has a real-time TDS readout built in, a display that shows the TDS of your filtered water. Instead of testing with a separate meter, you can see the number whenever you pour a glass. If it starts rising over time, that is a signal a cartridge may be due.

The Hydra filters through five stages: sediment, carbon, carbon block, a reverse osmosis membrane and a post-filter. It is tankless, filtering on demand at up to 800 gallons per day, and it comes with its own faucet. Both of its cartridges are replaced every 12 months, and they are sold together in the Filter Bundle. Our guide on how to change reverse osmosis filters walks through the swap.

Remember the limits above, though. A steady TDS reading tells you the membrane is holding back dissolved solids. It doesn't measure THMs or PFAS directly, which is why we also publish our lab results.

The bottom line on TDS

TDS is total dissolved solids, measured in ppm. It is a good quick check on hardness, taste and filter performance, and EPA's 500 mg/L secondary guideline is a useful reference point. It is not a safety test. Pair a TDS reading with a certified lab test for specific contaminants. If you want water with low TDS and a built-in way to watch it, take a look at the Hydra, or get a free TDS test first.