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Quick answer
Total dissolved solids (TDS) is the combined amount of minerals, salts, metals and small amounts of organic matter dissolved in water, usually reported in milligrams per liter (mg/L), which equals parts per million (ppm).
It is a single total, not a list. Calcium, magnesium, sodium, chloride, sulfate and bicarbonate make up most of it. TDS tells you how mineral-heavy water is, but not which substances are in it or whether any one of them is harmful.
Quick facts
The key facts about Total Dissolved Solids (TDS), with sources:
- Unit
- mg/L, the same as ppm; 1 ppt = 1,000 ppm 3
- EPA secondary guideline
- 500 mg/L (aesthetic, not federally enforced) 1
- WHO taste rating
- Under 300 mg/L "excellent"; over 1,200 mg/L "unacceptable" 2
- WHO health value
- None proposed for TDS itself 2
- Natural range
- Under 30 to as much as 6,000 mg/L, depending on geology 2
- Lab method
- Evaporate a 0.1-liter sample and weigh what is left 3
Key takeaways
- TDS is one number for everything dissolved in your water, measured in mg/L or ppm.
- The EPA suggests 500 mg/L as an aesthetic limit for taste, scale and staining; it is not a health limit.
- A handheld TDS meter measures electrical conductivity and converts it with a factor, so its reading is an estimate.
- A high or low reading is a reason to run a lab test for specific contaminants, not a verdict on safety.
- Reverse osmosis and distillation lower TDS; a standard water softener mostly swaps one mineral for another.
What TDS measures, in plain English
Pour a liter of tap water into a clean dish, boil it all away, and a thin crust stays behind. That crust is the dissolved solids.
A lab does the same thing with care: North Dakota State University Extension describes the standard method as evaporating a 0.1-liter sample and weighing the minerals left in the vessel 3.
The World Health Organization defines TDS as the inorganic salts and small amounts of organic matter in solution. The main parts are calcium, magnesium, sodium and potassium on one side, and carbonate, bicarbonate, chloride, sulfate and nitrate on the other 2.
Montana State University’s well program calls TDS a good general indicator of water quality and notes it covers particles smaller than 2 microns, which an ordinary filter cannot catch 4. That is the key point for a homeowner: TDS is dissolved, so it passes straight through a sediment filter.
TDS is not the same as hardness
Hardness counts only calcium and magnesium. TDS counts those plus sodium, chloride, sulfate and the rest. Soft water can still have high TDS, especially after an ion exchange softener adds sodium.
How a TDS meter turns electricity into ppm
A pen-style TDS meter never weighs anything. NDSU Extension and the WHO background paper explain the method 32:
- Ions carry current. Dissolved minerals split into charged ions, and the more ions there are, the better the water conducts electricity. Distilled water conducts poorly 3.
- The probe reads conductivity. Two electrodes measure electrical conductivity (EC), usually in microsiemens per centimeter (µS/cm) 3.
- A factor converts it. The meter multiplies EC by a conversion factor to estimate TDS. That factor is not fixed; in NDSU’s table it rises from 0.50 at low EC to 0.75 at 3,000 µS/cm 3.
- The water type matters. WHO notes the factor varies with the type of water, so the same EC can mean different TDS in different wells 2.
- Calibration keeps it honest. NDSU says to check the meter with a calibration solution before sampling, after a battery change, or after a drop 3.
TDS (mg/L) ≈ EC (µS/cm) × k
EC- Electrical conductivity read by the probe
k- Conversion factor, about 0.50–0.75 in NDSU’s table, rising with EC
TDS- Estimated dissolved solids in mg/L (ppm)
A lab weighs the residue instead, which is why a lab number and a meter number can differ.
Some things the meter misses: uncharged substances such as many pesticides and solvents do not conduct, so they barely move the reading. The NDSU paper treats EC as a proxy, not a full analysis 3.
What makes up TDS, and where it comes from
| Part of TDS | Common source | What you may notice |
|---|---|---|
| Calcium and magnesium | Limestone and other rock the water passed through 2 | Scale on fixtures and in heaters; see water hardness |
| Sodium and chloride | Natural salts, road de-icing salt, softener regeneration 2 | Salty taste at higher levels 1 |
| Sulfate | Natural minerals in some aquifers 2 | Bitter taste; WHO lists it among scale and corrosion drivers 2 |
| Bicarbonate and carbonate | Dissolved rock; the main part of alkalinity 2 | Scale and deposits 2 |
| Nitrate | Fertilizer, septic systems and runoff 2 | Nothing; it has no taste. It needs its own test |
| Small amounts of organic matter | Sewage, urban and farm runoff, industrial wastewater 2 | Varies; a TDS meter barely sees uncharged organics |
Example: reading a meter on a private well
This is a worked illustration using NDSU’s published conversion table and the reverse osmosis rejection math in University of Nebraska–Lincoln Extension guide G1490. The well and its reading are hypothetical.
| Step | Value | Where the number comes from |
|---|---|---|
| Meter reads conductivity | 1,000 µS/cm | Hypothetical reading |
| Conversion factor at that EC | 0.65 | NDSU table 3 |
| Estimated TDS | 650 mg/L | 1,000 × 0.65 3 |
| Compared with EPA guideline | 150 mg/L above 500 mg/L | EPA secondary standard 1 |
| WHO taste band | "Fair" (600–900 mg/L) | WHO panel ratings 2 |
| After an RO unit with 85% rejection | About 98 mg/L | 650 × (1 − 0.85); UNL explains rejection rate this way 5 |
The 650 mg/L result says the water is mineral-heavy and likely to scale a water heater. It does not say whether nitrate, arsenic or lead is present. That needs a certified lab test for each one.
Where you will see a TDS number
On a utility water quality report. Public systems publish a yearly Consumer Confidence Report, and UNL Extension points readers to it as the first place to check what is in city water 5. Many reports list TDS beside the secondary standards.
On a well test. Montana State’s well program reports TDS with pH and alkalinity, and notes these three tend to stay stable over time, which makes them useful baselines 4.
On a reverse osmosis faucet. Some RO systems ship with a TDS meter or a built-in readout so owners can compare tap and filtered water. A rising filtered reading is the usual sign a membrane is wearing.
In a sales pitch. A salesperson may test your tap with a meter. The reading is real, but it cannot identify a single contaminant, so treat it as a starting point for a lab test, not proof that you need a product.
What a TDS reading can and cannot tell you
Useful for
Why low TDS is not automatically better
WHO notes that water with very low TDS can taste flat and is often corrosive to supply systems 2. Pure water is hungry for minerals, so it can pull metal from pipes. That is one reason the pH of drinking water and its mineral content are often judged together.
A clean-looking 50 mg/L reading can still hide a problem. Lead from an old service line or nitrate from a nearby field can sit at harmful levels while hardly moving the total. The only way to know is a test for that substance.
What drives the cost of lowering high TDS
We do not print prices for tests or treatment; fair numbers need dated local quotes.
Cost turns on four things. First, the lab panel: a single TDS result is a small part of a test, while a full well panel with metals and nitrate costs more.
Second, the fix: a point-of-use RO unit under one sink is a smaller job than whole-house treatment.
Third, wastewater: UNL says most household RO units recover only 20 to 30 percent of feed water, so the rest goes down the drain and onto your water bill 5. Fourth, upkeep: prefilters and membranes wear out.
Scale has its own cost. WHO says high TDS shortens the service life of water heaters and appliances 2. Our plumbing cost guide explains how plumbers build quotes for treatment and heater work.
How to test for TDS and act on the result
- Start with the report. On city water, read your Consumer Confidence Report first 5. On a well, testing is your job, and EPA advises private well owners to test every year for total coliform bacteria, nitrates, TDS and pH 6.
- Use a certified lab for decisions. Ask for TDS plus the specific ions that worry you. Montana State and others note TDS alone cannot name a contaminant 4.
- Use a meter for tracking. Calibrate it, rinse the probe, and test water at room temperature for steady readings 3.
- Match treatment to the problem. Scale only? A softener may help. Salt taste or high TDS overall? Reverse osmosis or distillation 4. A health contaminant? Treat for that one.
- Install and retest. UNL advises testing before choosing equipment and checking it works after 5. Under-sink RO is often a homeowner or plumber job; whole-house work usually needs a licensed plumber and, in many places, a permit.
- Read the CCR or test the wellBaseline number
- Lab panelName the ions
- Pick treatmentRO, distillation or a softener
- RetestConfirm the drop
- Track yearlyMeter checks between lab tests
How to keep TDS treatment working
- Check filtered water with a meter every month or two and log the reading next to the tap reading.
- Change RO prefilters on the maker’s schedule; UNL notes sediment filters protect the membrane from fouling 5.
- Replace the RO membrane when its rejection drops; UNL says performance falls as membranes age and are fouled 5.
- Re-calibrate the meter before each important test 3.
- Retest a well each year, as EPA advises for TDS 6, and after floods or nearby land changes, since TDS tends to be stable and a jump means something changed 4.
Which signs point to a TDS problem, and when to call a pro
- Crusty white scale on faucets, shower heads and inside kettles, which WHO links to high TDS 2.
- A salty, bitter or metallic taste; EPA lists a salty taste among TDS effects 1.
- Stains, deposits or colored water, also on EPA’s list 1.
- A sudden jump in a well’s TDS from its usual reading.
- Filtered RO water that reads close to tap water.
- Very low TDS paired with blue-green stains on fixtures, a sign of corrosive water.
A TDS number cannot tell you water is safe
WHO and extension sources agree that TDS cannot identify harmful substances 24. If you suspect lead, nitrate, arsenic or bacteria, or your utility issues a notice, follow the utility’s advice and get a certified lab test. Do not rely on a meter or a filter salesperson’s reading.
A water treatment specialist or plumber can size a system once you have a lab result. Our water treatment service page explains the options.
Who sets TDS limits, and whether they are enforced
Read the figure below as three yardsticks, not three safety lines. Federal (EPA). Under the Safe Drinking Water Act, EPA lists TDS among the secondary drinking water standards at 500 mg/L, with effects of hardness, deposits, colored water, staining and salty taste.
EPA states it does not enforce these secondary levels; they are guidelines for taste, odor and appearance 1.
Two of the ions inside TDS carry their own secondary levels: chloride and sulfate are each set at 250 mg/L, above which water can taste salty 1.
So a well at 450 mg/L total can pass the TDS line and still fail on chloride if road salt is the main source.
States and utilities. EPA notes state agencies and water systems often choose to monitor and treat for secondary contaminants, though federal rules do not require it 1. Some states write their own limits; check your state drinking water program.
Private wells. EPA’s private well guidance tells owners to test annually for total coliform bacteria, nitrates, total dissolved solids and pH, and points them to local health departments, some of which test for free 6.
International. The WHO background document (WHO/SDE/WSH/03.04/16, 2003, from the 1996 second edition) proposes no health-based guideline for TDS and says water under 1,000 mg/L is usually acceptable to consumers 2.
Equipment. Household RO units are commonly certified to NSF/ANSI 58, the standard for reverse osmosis systems. Check the certification listing for the claims a model has actually passed.
Secondary vs primary standards
Primary standards are enforceable health limits. Secondary standards, including TDS and pH, are aesthetic guidelines EPA does not enforce 1.
How TDS differs from hardness, conductivity, pH and turbidity
| Measurement | What it counts | Typical unit | Main difference from TDS |
|---|---|---|---|
| TDS | All dissolved minerals, salts and some organics 2 | mg/L (ppm) | — |
| Water hardness | Calcium and magnesium only | mg/L or grains per gallon | A subset of TDS |
| Electrical conductivity | How well water carries current 3 | µS/cm | What a meter reads before converting to TDS |
| pH | How acidic or basic water is | pH units (EPA range 6.5–8.5) 1 | Measures acidity, not amount |
| Turbidity | Suspended, undissolved particles | NTU | Cloudiness; a sediment filter removes it, unlike TDS |
Which TDS beliefs are wrong
- Myth A low TDS reading proves water is safe.
- Reality TDS cannot name contaminants; harmful ones can be present at low totals 42.
- Myth Anything over 500 ppm is dangerous.
- Reality 500 mg/L is EPA’s aesthetic guideline, not a health limit 1.
- Myth A water softener lowers TDS.
- Reality A softener trades calcium and magnesium for sodium or potassium; the dissolved total stays similar. RO or distillation lowers it 4.
- Myth Zero TDS water is the healthiest.
- Reality WHO says very low TDS water can taste flat and be corrosive to pipes 2.
Why TDS depends on where you live
TDS is set mostly by geology. WHO reports natural levels from under 30 to as much as 6,000 mg/L depending on how soluble the local minerals are 2.
Water that runs through limestone or salt-bearing rock picks up more. Road salt matters too: WHO cites a tenfold rise in groundwater salinity in Burlington, Massachusetts, between 1955 and 1970, blamed on de-icing 2.
Well owners in cold, salted regions and in dry, mineral-rich basins should test more often. Check your hardness range with our water hardness tool and ask a local lab about common problems in your area.
When TDS should change what you do
TDS matters when you are choosing a filter, fighting scale in a water heater, checking a well, or deciding whether an RO unit still works. It matters less when your utility report is normal and the water tastes fine.
If your reading is above 500 mg/L and you have scale or a salty taste, get a lab panel and look at RO for drinking water. If it is very low and you see blue-green stains, test for corrosion and lead before adding more filtration.
- If Reading under 500 mg/L, water tastes fine
- Then No action; recheck yearly on a well
- If Over 500 mg/L with scale
- Then Lab test, then softener or RO 1
- If Salty or bitter taste
- Then Lab test for sodium, chloride and sulfate, then RO or distillation
- If RO water reads near tap water
- Then Change prefilters or membrane 5
- If Any health concern
- Then Certified lab test for that contaminant
Questions about Total Dissolved Solids (TDS)
What is a good TDS level for drinking water?
Under 500 mg/L meets EPA’s secondary guideline, which is set for taste, scale and staining. WHO taste panels rated under 300 mg/L excellent and 300 to 600 mg/L good. Water above 1,000 mg/L is often unpleasant to drink. None of these numbers is a health limit, so test for specific contaminants if you have concerns.
Is high TDS water harmful?
Not by itself, according to WHO, which proposes no health-based guideline for TDS. The concern is what makes up the total. Nitrate, arsenic or lead can be part of TDS and are harmful at certain levels. A high reading is a reason to run a lab test that names each substance, not proof of harm.
Is ppm the same as mg/L for TDS?
Yes. For water, one milligram per liter equals one part per million, so a meter reading of 300 ppm means 300 mg/L. Some meters also show parts per thousand, where 1 ppt equals 1,000 ppm. NDSU Extension explains the units in its guide to EC and TDS meters.
How accurate are TDS meters?
They give a useful estimate, not an exact figure. A meter reads electrical conductivity and multiplies it by a conversion factor.
NDSU’s table shows that factor rising from about 0.50 to 0.75 as conductivity climbs, and WHO notes it varies by water type.
Calibrate the meter and use a lab test for any decision that matters.
Does boiling water reduce TDS?
No. Boiling drives off water, so the minerals left behind become more concentrated, which is why kettles scale up. Distillation is different because it collects the steam and leaves the solids behind. Montana State lists distillation and reverse osmosis as the treatments for TDS.
Does a water softener reduce TDS?
Not much. An ion exchange softener removes calcium and magnesium but adds sodium or potassium in their place, so the dissolved total stays similar. It solves hardness and scale. To lower TDS itself, reverse osmosis or distillation is the usual household choice.
What should my reverse osmosis water read on a TDS meter?
Much lower than your tap. UNL Extension explains RO performance as a rejection rate; at 85 percent rejection, 650 mg/L tap water would drop to about 98 mg/L. Compare filtered and tap readings over time. When the filtered reading creeps up toward the tap reading, the membrane or prefilters need attention.
Can TDS be too low?
Yes, for taste and pipes. WHO notes water with very low TDS can taste flat and is often corrosive to supply systems. That is not usually a health issue on its own, but corrosive water can pull metals such as lead and copper from plumbing, so test for those if you see blue-green stains.
What causes high TDS in well water?
Mostly the rock the water passes through, plus human sources. WHO lists natural minerals, sewage, urban and farm runoff, industrial wastewater and road de-icing salt.
Natural levels range from under 30 to as much as 6,000 mg/L depending on geology. A sudden rise in a well that is usually steady points to a new source nearby.
Does a carbon filter remove TDS?
Mostly no. Carbon filters reduce chlorine, some tastes and certain organic chemicals, but dissolved salts pass through. UNL notes carbon and sediment filters are paired with RO units to protect the membrane and catch what it misses, while the RO membrane does the work of cutting dissolved minerals.
Sources
- U.S. EPA, Secondary Drinking Water Standards: Guidance for Nuisance Chemicals, retrieved .
- World Health Organization, Total dissolved solids in Drinking-water (WHO/SDE/WSH/03.04/16, 2003), retrieved .
- NDSU Extension, Using Electrical Conductivity and Total Dissolved Solids Meters to Field Test Water Quality (WQ1923, rev. January 2024), retrieved .
- Montana State University Extension Well Educated, Alkalinity, pH and TDS, retrieved .
- University of Nebraska–Lincoln Extension, Drinking Water Treatment: Reverse Osmosis (G1490, rev. 2014), retrieved .
- U.S. EPA, Protect Your Home’s Water (private wells), retrieved .
Expert review
Written by the HyrePlumber Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Held under G-REV until a licensed plumber or water-quality professional signs. The 500 mg/L secondary level and its enforcement status are from EPA’s secondary standards page.
Constituents, taste ratings, natural range and the no-health-value finding are from the WHO background document. Meter method and the EC-to-TDS factor table are from NDSU Extension WQ1923.
Treatment options are from Montana State and UNL Extension; RO recovery and rejection math from UNL G1490. The worked example is a labeled illustration.
NSF/ANSI 58 is named as the RO standard; nsf.org blocked automated retrieval on 2026-10-13, so no claim about its test criteria is made. No prices are given.
Suggest a correction. We fix errors and say what changed.