What a TDS meter can (and can't) tell you about your water

Pure water is a poor conductor. As the USGS puts it, water "stops being an excellent insulator once it starts dissolving substances." Dissolved salts split into charged particles (ions), and those carry a current. In drinking water, the main ones are calcium, magnesium, sodium, potassium, bicarbonate, chloride, sulfate and nitrate, according to the WHO.

A TDS meter sends a small current between two probes, measures how well the water conducts it, then multiplies by a conversion factor to estimate TDS. The WHO notes that factor "varies with the type of water." HM Digital's TDS-3, for example, uses an average factor of 0.5 based on sodium chloride, according to its spec sheet. That's why two meters can show slightly different numbers for the same glass. For drinking water, ppm and mg/L mean the same thing, so a reading of 120 ppm is 120 mg/L.

The catch: a meter only sees what conducts electricity. It adds everything up into one number and can't tell you what any of it is.

The EPA lists TDS among its secondary standards, at 500 mg/L. The EPA calls these "non-mandatory" and says it does not enforce them. They exist to help water systems manage taste, color and odor, and the EPA says these contaminants "are not considered to present a risk to human health" at that level. For TDS above 500, the effects it lists are hardness, deposits, colored water, staining and a salty taste.

The WHO goes further: "No health-based guideline value is proposed for TDS," and it says reliable data on health effects from TDS in drinking water "are not available." What it does report is how TDS tastes. Here's how taste panels rated water by TDS level:

Very low TDS can go the other way. The WHO says water with extremely low TDS "may also be unacceptable because of its flat, insipid taste." That's a taste note, not a warning. Plenty of people prefer RO water exactly because it tastes plain.

You'll see charts online calling 50 to 150 "ideal." Those bands don't come from the EPA or the WHO, so we don't use them.

Home RO systems certified to NSF/ANSI 58 are tested by feeding them water at 750 mg/L TDS (plus or minus 40). To pass, the water coming out can't be above 187 mg/L, which means at least 75% lower. You can see that test on any certified system's performance data sheet, like this one from Navien, which lists an 84.2% average reduction for its system. Makers often claim more: APEC says its RO membranes reject 90 to 99%.

What that looks like in practice: if the water going into a system reads 300, a 90% reduction gives you 30, and 99% gives you 3. Your numbers will depend on your water and your system.

That same Navien data sheet says RO product water "shall be tested periodically to verify that the system is performing properly." A TDS meter is the easy way to do that at home. Test the water going in and the water coming out. If the gap shrinks over weeks or months, the membrane or filters may be due. One exception: if your system adds minerals back after the membrane, like an alkaline stage, the final reading will be higher by design. Test before that stage if you can.

Thinking about a home system at all? Our breakdown of whether reverse osmosis is worth it compares refill stations, home systems and bottled water by cost per gallon.

A TDS meter reads the total, not the ingredients. Some contaminants that matter most show up in amounts far too small to move the needle. The EPA's limit for arsenic is 0.010 mg/L. A typical pocket meter reads in steps of 1 ppm (APEC's spec), so arsenic right at the legal limit is 100 times smaller than the smallest change the meter can show. Bacteria and viruses don't register, and microplastics are tiny particles rather than dissolved salts.

This matters at refill machines too. A 2026 study of US water kiosks traced lead to plumbing parts placed after the RO step, something a TDS reading wouldn't flag. We summarize it in our refill station safety checklist.

So a low reading after RO is a good sign, not proof. If you want to know what's in a specific water, you need a certified lab test. If you want to know what RO is designed to remove, we cover it in Does RO remove PFAS, fluoride and microplastics?

Basic pocket meters don't cost much. APEC lists one at $18.99 (checked October 27, 2026).

At a refill machine you can't test the water going in, so you can't work out a percentage. What you can do is take a reading every time you fill up, with the same meter. A steady number is what you want to see. If it jumps well above your usual reading, tell the operator, and fill up somewhere else until it settles.

A TDS reading is one check among several. The safety checklist linked above covers what else to look at before you fill, from a clean nozzle to an operator name or service sticker on the machine.

Also check which water you're buying. Reverse osmosis and deionized water read low. Alkaline and mineral-added water reads higher on purpose, and spring water keeps its natural minerals, so a higher number there isn't a problem. More than 21,000 stations in our directory list reverse osmosis, so you can usually compare a couple near you.

A TDS meter can't tell you whether water is alkaline or high-pH. For that you need pH strips or a pH meter. If you're choosing between alkaline, RO and deionized water, our distilled vs. reverse osmosis vs. spring water guide explains how each is made.

Reverse osmosis · Deionized · Alkaline · All cities · Listed refill prices