Testing
TDS chart: what total dissolved solids does and does not tell you
Quick answer
The EPA secondary standard for TDS is 500 mg/L, an aesthetic guideline rather than a health limit. TDS measures conductivity and estimates how much is dissolved, saying nothing about what, so it does not measure hardness and is not a safety result on its own.
A TDS meter is cheap, instant, and measures something real, which is exactly why it gets asked to answer questions it cannot. Total dissolved solids is a single number derived from how well water conducts electricity, and conductivity rises with almost anything dissolved in water: calcium, sodium, chloride, nitrate, arsenic, and plain table salt all move the needle in the same direction. A high reading tells you something is dissolved. It does not tell you what.
That gap matters most in the two directions people least expect. Low TDS is not a safety result, since it says nothing about specific contaminants that can be present at meaningfully low concentrations. High TDS is not automatically a problem either, since some genuinely fine drinking water simply carries more naturally occurring minerals than average. The one place TDS earns its keep without ambiguity is confirming a reverse osmosis membrane is still rejecting, which this chart covers alongside the published EPA standard and typical TDS ranges.
The EPA secondary standard, and why it is aesthetic, not a health limit
The secondary standard for TDS is 500 mg/L, an aesthetic guideline for taste, scale and corrosion rather than an enforceable health limit.
Published figure A published standard, regulatory limit, unit conversion or manufacturer specification. It does not change because somebody disagrees with it.
| Contaminant | Limit | Kind of limit | Effect above the limit |
|---|---|---|---|
| Total dissolved solids | 500 mg/L | Secondary (aesthetic) | Taste, scale, corrosion |
Secondary standards describe where a substance starts to become a nuisance, not a health threshold. A utility can exceed 500 mg/L TDS without violating any enforceable law the way exceeding a primary maximum contaminant level would. See the secondary drinking water standards chart for the rest of the aesthetic guidelines.
Typical TDS ranges, and what they do and do not tell you
Meter manufacturers and the water treatment trade commonly describe TDS results in bands like these. None of them are an EPA classification; only the 500 mg/L secondary figure above is a published regulatory number.
Water above roughly 1,000 mg/L commonly develops a noticeable taste, but the number alone still says nothing about which dissolved substance is responsible.
Rule of thumb Water treatment trade convention rather than a published standard. Taught everywhere, useful for planning, and not a specification. No standards body publishes it.
| TDS range | Common description | What it does NOT tell you |
|---|---|---|
| 0 to 50 mg/L | Very low, typical of distilled or fresh RO permeate | Whether any specific contaminant is present or absent |
| 50 to 300 mg/L | Low to moderate, common for many treated municipal supplies | Hardness specifically, chlorine, or any regulated contaminant |
| 300 to 500 mg/L | Moderate to the edge of the secondary guideline | Whether the dissolved load is minerals, salts, or something else |
| 500 to 1,000 mg/L | Above the EPA secondary guideline, taste often noticeable | Safety; many well and mineral rich waters sit here without a health issue |
| Above 1,000 mg/L | High, taste and scale usually pronounced | The specific cause without further testing; could be minerals, sulfate, or something worth a full panel |
These bands are trade convention rather than a regulatory classification. A certified laboratory test, not a TDS band, is what identifies what is actually contributing to any of these readings.
What TDS reflects, and what it does not
TDS does not measure hardness, bacteria, lead, or nitrate directly, even though all of them can be present regardless of the TDS reading.
Published figure A published standard, regulatory limit, unit conversion or manufacturer specification. It does not change because somebody disagrees with it.
| Question | Does TDS answer it? | Why |
|---|---|---|
| Is my water hard? | No | Hardness is calcium and magnesium specifically. TDS lumps every dissolved substance together and cannot isolate hardness from the rest |
| Is there bacteria in my water? | No | Bacteria are living organisms, not dissolved solids, and contribute nothing to a conductivity reading either way |
| Is there lead or nitrate in my water? | No | Both can be present at concentrations that matter for health while changing overall TDS by an amount too small to notice |
| Is my reverse osmosis membrane still rejecting? | Yes | Comparing feed TDS to permeate TDS is a genuine, direct check of the membrane's performance |
Bacteria, nitrate, lead, arsenic and PFAS are not do it yourself topics, and a low or normal TDS reading is not evidence any of them are absent. Confirming any of them needs a certified laboratory test, not a TDS meter, and a positive result belongs with your local health department or a licensed water treatment professional.
The one genuine use: confirming a membrane is still rejecting
Reverse osmosis membranes are rated by percentage salt rejection under the standard test condition, and a TDS meter is the right tool to confirm that rejection is holding up in service. A FilmTec BW60-1812-75 membrane publishes a stabilized rejection rate of 98 percent, with a minimum of 96 percent; against the 250 ppm test feed water, that works out to a permeate reading of roughly 5 to 10 ppm when the membrane is performing as rated. A budget priced membrane certified to the same standard, such as the Membrane Solutions 75 GPD element at a published 97 percent rejection, lands in the same range.
A permeate TDS reading climbing well above that expected range over time, with feed TDS unchanged, is the genuine, practical signal that a membrane needs replacing. That comparison, feed against permeate, is what TDS is actually good at. It is not a substitute for a certified laboratory test on anything the reverse osmosis system is being relied on to remove.
A membrane rejecting at its published 96 to 98 percent rate turns 250 ppm feed water into roughly 5 to 10 ppm permeate.
Published figure A published standard, regulatory limit, unit conversion or manufacturer specification. It does not change because somebody disagrees with it.
| Feed TDS | Published rejection rate | Expected permeate TDS |
|---|---|---|
| 250 ppm | 98% (stabilized) | about 5 ppm |
| 250 ppm | 96% (minimum) | about 10 ppm |
| 250 ppm | 97% (typical certified) | about 7.5 ppm |
These figures use the manufacturer's own published stabilized and minimum rejection rates against the standard 250 ppm test feed. Real feed water TDS varies by source, so use your own feed reading as the comparison baseline rather than the 250 ppm test figure directly.
Frequently asked questions
What is a good TDS level for drinking water?
There is no single "good" number, since TDS measures how much is dissolved rather than what. The EPA secondary guideline sets 500 mg/L as the point where taste and scale typically become noticeable, but that is an aesthetic threshold, not a safety cutoff. Water above or below it can be perfectly fine or genuinely problematic depending on what is actually dissolved.
Does TDS measure water hardness?
No. Hardness is specifically calcium and magnesium content, while TDS lumps every dissolved substance together into one conductivity based number. Two waters can share an identical TDS reading with very different hardness levels. A hardness test strip or a certified laboratory panel measures hardness directly; a TDS meter does not.
Can a low TDS reading mean my water has no contaminants?
No. Bacteria, nitrate, lead, arsenic and PFAS can all be present at concentrations that matter for health while contributing an amount too small to meaningfully change a TDS reading. TDS is not a screening test for any of these, and confirming their absence needs a certified laboratory test rather than a TDS meter.
How is TDS actually measured?
A TDS meter measures the electrical conductivity of a water sample and converts that reading into an estimated total dissolved solids figure using a standard conversion factor. It is measuring conductivity, a proxy, rather than weighing dissolved solids directly, which is why it cannot distinguish one dissolved substance from another.
What is the best use for a TDS meter at home?
Confirming a reverse osmosis membrane is still rejecting. Comparing feed water TDS against permeate TDS gives a direct, genuine read on membrane performance, since a properly working membrane should reject the large majority of dissolved solids consistently. A permeate reading climbing over time with feed TDS unchanged is a real signal the membrane needs replacing.
Is high TDS always a sign of a water quality problem?
Not automatically. Some naturally mineral rich water carries a higher TDS reading without any associated health issue, while other high TDS water genuinely does warrant investigation. The number alone cannot distinguish the two cases; a certified laboratory test identifying what is actually contributing to the reading is what actually answers the question.
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Researched, not professional advice. This page is compiled from published standards, regulatory limits, manufacturer specifications and owner-review consensus, not hands-on testing. Nothing here diagnoses a water problem or says any product makes water safe to drink. Figures described as a rule of thumb are water treatment trade convention rather than published standards, and they are labeled that way wherever they appear. Test your water before you buy equipment: on city water start with the Consumer Confidence Report your utility publishes each year, and on a private well only a certified laboratory test tells you what you have. Bacteria, nitrate, lead, arsenic and PFAS are not do it yourself topics, they are invisible and tasteless, and they need a laboratory and usually a licensed professional rather than a product. Remember that a certification covers a specific claim under a specific standard, so "NSF certified" with no number attached tells you very little.