What Is TDS in Water, and Why Does It Matter?

If you've ever tested your water with a handheld meter and seen a number pop up — 60, 200, 450 — you've measured TDS. It's one of the simplest water quality tests there is, and also one of the most misunderstood. Here's what that number actually tells you.

What TDS Actually Measures

TDS stands for total dissolved solids — the combined weight of everything dissolved in your water that isn't the water itself. That includes minerals like calcium, magnesium, and potassium, salts like sodium chloride and sulfates, and trace metals. It's measured in parts per million (ppm), where 1 ppm equals about 1 milligram of dissolved material per liter of water.

A TDS meter doesn't identify any of these substances individually. It measures electrical conductivity — pure water conducts electricity poorly, while water carrying dissolved minerals and salts conducts it much better — and converts that reading into a ppm estimate. It's a fast, useful snapshot, but it's a total, not a breakdown.

TDS Is Not the Same Thing as Hardness

This is the mix-up we hear most often. Hardness, measured in grains per gallon (GPG), refers specifically to the concentration of calcium and magnesium in your water — the minerals responsible for scale buildup, dry skin, and that chalky residue on glassware. TDS is a much broader number that includes hardness minerals as just one piece, alongside sodium, chlorides, and everything else dissolved in the water.

In practice, that means two homes could have identical TDS readings with very different water — one high in hardness minerals, the other high in sodium or sulfates instead. The TDS number alone won't tell you which. If scale and hard water damage are your concern, GPG is the number that actually matters; TDS is a different, broader measurement entirely.

What Counts as a Good TDS Reading

Most water quality guides put the ideal range for drinking water somewhere between 50 and 150 ppm — often called the "sweet spot," where there's enough dissolved mineral content for a pleasant taste without it becoming heavy or metallic. A reading like 60 ppm sits comfortably inside that range. Water below 50 ppm can taste flat or empty, since it's missing the trace minerals most people are used to. Water above 300 ppm often starts to taste noticeably different — salty, bitter, or mineral-heavy, depending on what's driving the number up.

The EPA has set a secondary standard of 500 ppm for TDS, but it's worth understanding what that actually means: it's a non-enforceable, aesthetic guideline about taste and appearance, not a health-based safety limit.

Is High TDS Dangerous?

Not necessarily — and this is the part most filter marketing glosses over. TDS is an aggregate number, and it can't tell you whether what's driving it up is harmless (like natural calcium and magnesium in mountain spring water) or something worth investigating (like nitrate contamination from agricultural runoff). A low TDS reading doesn't guarantee safety either; some contaminants of real concern, including certain heavy metals and nitrates, can be present at meaningful levels without moving the TDS number much at all.

The honest takeaway: TDS is a helpful, quick indicator, but it isn't a substitute for knowing what's actually in your water. If a reading changes suddenly, or if you want the full picture rather than one aggregate number, that's worth digging into further.

Where to Go From Here

Our homepage includes a free lookup tool powered by the Environmental Working Group's tap water database — enter your zip code to see what's actually been detected in your area, beyond just a TDS number.

If you want lower, more consistent TDS at the tap, reverse osmosis is the most effective way to get there. Our Evince ClarityX under-sink system is a tankless RO unit that reduces a wide range of dissolved solids at the point of use — your Cove rep can test your water and show you exactly what your own before-and-after TDS numbers would look like.