What is TDS in water?
Your water report or countertop meter shows a “TDS” number, and the internet is full of confident thresholds for what's “safe” or “normal” — most of which aren't real standards.
Here's the straight version: what TDS actually measures, how it's read, the one real EPA figure, and what measured US water looks like — plus the correction that matters most, TDS is not hardness.
TDS — total dissolved solids — is the combined concentration of everything dissolved in water: minerals, salts, and ions, measured in mg/L (equivalent to ppm). Across measured US samples, the national median is about 226 mg/L. The EPA's 500 mg/L guideline is an aesthetic standard, not a health limit — no federal health-based TDS limit exists.
Definition
Dissolved, not suspended
Total dissolved solids are the substances dissolved in water — small enough to pass a 2-micron filter — as opposed to suspended particles that make water cloudy. They are organic and inorganic: calcium, magnesium, sodium, bicarbonate, chloride, sulfate, and traces of many others. TDS is their combined concentration, in mg/L (equivalent to ppm).
Measurement
How TDS is measured
The reference method is gravimetric: evaporate a measured volume of water and weigh what remains. Consumer TDS meters read electrical conductivity and multiply by a conversion factor — an estimate, not a gravimetric (dried-and-weighed) measurement. That is why two meters can disagree, and why a lab number is the one to trust — see the testing guide.
Limits
The regulatory truth
There is no health-based federal limit on TDS, and no minimum. What exists is the EPA's secondary standard of 500 mg/L — a non-enforceable, aesthetic guideline about taste, color, and staining, not safety (EPA National Secondary Drinking Water Regulations).
The World Health Organization describes palatability rather than setting a limit: water below about 300 mg/L rates as excellent, 300–600 good, 600–900 fair, 900–1,200 poor, and above 1,200 mg/L unacceptable in taste (WHO, attributed).
Beyond these, the thresholds you'll meet online — an “unsafe above 1,000 ppm” rule, a “50–150 optimal” band — are not federal standards; a source citing them is describing a rule that does not exist. What matters more than the total is what the dissolved solids are made of.
Rankings
Measured TDS in US water
Across measured US samples, the national median TDS is about 226 mg/L — the median of all measured values, the same kind of figure as the site's hardness numbers.
| Highest (state median) | TDS | Samples |
|---|---|---|
| Montana | 2,210 | 28,989 |
| North Dakota | 848 | 8,101 |
| South Dakota | 764 | 14,597 |
| Wyoming | 637 | 8,933 |
| Nebraska | 482 | 4,442 |
| Lowest | TDS | Samples |
|---|---|---|
| Maine | 26.5 | 192 |
| South Carolina | 53 | 813 |
| Hawaii | 65 | 235 |
| Georgia | 66 | 7,287 |
| North Carolina | 73 | 4,835 |
State medians in mg/L, from measured samples. Medians reflect where agencies measure, which often targets known problem areas — so a high state figure signals heavily-sampled mineral-rich basins, not every tap.
Normal levels
What's a “normal” TDS?
There isn't a single normal — it depends on your local geology — but there are useful reference points. Most US tap water runs from roughly 100 to 400 mg/L, and the measured national median is about 226 mg/L. The WHO frames it by taste, not safety: below about 300 mg/L rates as excellent, 300–600 good, up to 900 fair.
Reverse-osmosis water is the low outlier at 10–50 mg/L because the membrane strips most ions. A number well outside your local norm — a sudden jump, or a reading far above nearby wells — is the signal worth chasing, not the absolute figure alone.
Comparison
TDS vs hardness — the correction that matters
This is the one people get wrong most: TDS is not hardness. Hardness is a subset of TDS — only the calcium and magnesium — while TDS counts every dissolved ion, sodium and chloride and sulfate included.
So the two correlate but can diverge sharply. Sodium-rich groundwater can read high TDS while staying soft, because the sodium lifts the total without adding hardness; softened water is the same story, having swapped calcium for sodium at nearly the same total. This is also why a cheap TDS meter can't tell you your hardness — it reads the whole dissolved load, not the calcium-and-magnesium slice. For that slice, see what water hardness is; to test it properly, the testing guide.
Sources
Where dissolved solids come from
Geology is the biggest source — water dissolves minerals from the rock and soil it moves through.
Treatment and softening adjust the mix; ion exchange, for instance, swaps calcium for sodium without changing the total much.
Runoff adds road salt, fertilizer, and industrial residue to surface water.
Plumbing contributes small amounts from pipes and fittings over time.
Reverse osmosis
Why RO water reads so low
Reverse osmosis forces water through a membrane that rejects most dissolved ions, so RO water typically reads 10–50 mg/L. That's far below any tap norm, which is the point for some uses — but it also strips the minerals that give water its flavor, so very-low-TDS water can taste flat. Some systems add a measured amount back afterward for taste.
Questions
Frequently asked
- What does TDS stand for and mean?
- TDS is total dissolved solids — everything dissolved in water that passes a 2-micron filter: minerals, salts, and other ions, organic and inorganic, reported in mg/L (the same as ppm).
- What is a normal TDS for drinking water?
- Most US tap water runs from roughly 100 to 400 mg/L; across measured US samples the national median is about 226 mg/L. There is no single “normal” — it depends on local geology.
- Is there a safe TDS limit?
- There is no health-based federal limit. The EPA's 500 mg/L figure is a secondary, aesthetic standard about taste and appearance, not safety — and what the dissolved solids are made of matters more than the total.
- Does high TDS mean hard water?
- No. Hardness counts only calcium and magnesium, a subset of TDS; water can be high in TDS yet soft when sodium or other ions dominate. They are correlated, not identical.
- Why is my RO water's TDS so low?
- Reverse osmosis rejects most dissolved ions, so RO water typically reads 10–50 mg/L. Some systems remineralize afterward for taste.
By Marko Visic, MPharm, BSc Physics. Source: EPA/WHO standards as cited; measured medians from USGS/EPA params layer. Data last verified . Methodology.
Cite this page: “What Is TDS in Water?.” Hard Water Map. Retrieved 2026-07-27, https://hardwatermap.com/what-is-tds/