Total Dissolved Solids: What the Number in Your Water Means
Written and checked with AI using the references below. This is not human expert review.
Quick answer
Total dissolved solids (TDS) is the combined mass of all minerals, salts, and tiny particles dissolved in water, reported in mg/L or ppm. Common sources include calcium, sodium, nitrates, and chloride. The EPA sets a guideline of 500 mg/L for drinking water. Above that level, water may taste salty or leave deposits. TDS shows how much is dissolved, but does not directly measure safety.
What total dissolved solids actually are
TDS is a single number. It measures the combined mass of all minerals, salts, and other tiny particles dissolved in water. These particles are small enough to pass through a very fine filter with pores of 2 micrometers or smaller. The result is reported in milligrams per liter (mg/L), which in dilute water is essentially the same as parts per million (ppm).
The most common contributors are calcium, phosphates, nitrates, sodium, potassium, and chloride. They enter water from the natural weathering of rocks and soils, agricultural runoff, urban stormwater, and road de-icing salts. Discharges from industrial or sewage treatment plants also add to TDS. Pesticides from surface runoff are a less common but more concerning source.
TDS covers both inorganic ions and organic molecules, as long as they are small enough to stay dissolved. It does not include particles large enough to settle out or be caught by a standard filter; those are called total suspended solids (TSS).
How TDS is measured: two methods with different accuracy
There are two main ways to measure TDS, and they work very differently.
Gravimetric analysis: the accurate lab method
The most accurate approach is gravimetric analysis. A known volume of water is filtered, then the filtered liquid is evaporated. The dry residue left behind is weighed. Because you are literally weighing what was dissolved, this method is the reference standard. EPA Method 160.1 uses this gravimetric approach. It is time-consuming and requires laboratory equipment, so it is not practical for home use.
Conductivity meters: fast but approximate
A TDS meter, the kind sold for home use, does not weigh dissolved solids at all. It measures how well the water conducts electricity. Dissolved ions carry electric current, so more ions mean higher conductivity. The meter converts that reading into an estimated TDS value. TDS (in mg/L) equals a conversion factor (ke) multiplied by the electrical conductivity in microsiemens per centimeter at 25 degrees Celsius. The factor ke ranges from 0.55 to 0.8 depending on the type of dissolved material present.
Conductivity-based meters are not perfectly accurate. They give results within about 10% of what a lab would measure. That is good enough to spot large changes in water quality or to check whether a filter is working. It is not precise enough for regulatory compliance or detailed water chemistry work. If you need a certified result, use a state-certified laboratory. The lab method is called gravimetric analysis, which means it measures TDS by weighing what is left after the water evaporates.
For more on how water chemistry tests compare in precision and what they actually measure, see water hardness testing methods and units.
The EPA secondary standard: 500 mg/L
The EPA sets a secondary maximum contaminant level (SMCL) of 500 mg/L for TDS in drinking water. Secondary standards are not legally enforceable health limits. They are guidelines for aesthetic qualities: taste, color, and odor. Above 500 mg/L, water may taste salty, leave mineral deposits on fixtures, cause staining, or appear discolored.
Because the standard is aesthetic, not health-based, water above 500 mg/L is not automatically unsafe to drink. The EPA notes that TDS is not generally considered a primary pollutant and is not associated with health effects at these levels. Public water systems are not required to test for TDS under federal rules, though many do voluntarily.
If you receive a notice from your water supplier about TDS, read it carefully and follow any instructions. For private wells, a certified laboratory test is the right way to get a reliable result.
TDS is not the same as water hardness
This is one of the most common mix-ups. Water hardness refers specifically to the concentration of calcium and magnesium ions. TDS includes those ions but also everything else dissolved in the water.
A water softener makes this distinction clear. A softener removes calcium and magnesium (the hardness minerals) by swapping them for sodium or potassium ions. The water becomes soft, but the total mass of dissolved ions stays about the same or even increases slightly, because sodium ions replace the calcium and magnesium. So a TDS meter reading will not drop after softening, even though the water is now soft. Measuring TDS tells you nothing specific about hardness, and measuring hardness tells you nothing about the dozens of other dissolved substances that TDS captures.
What TDS levels mean for different water types
TDS is also used to classify water broadly. The table below shows the standard categories.
| Water type | TDS range (ppm) |
|---|---|
| Drinking water (typical) | Below 500 ppm |
| Fresh water | Below 1,000 ppm |
| Brackish water | 1,000 to 10,000 ppm |
| Saline water | 10,000 to 35,000 ppm |
| Hypersaline | Above 35,000 ppm |
Bottled mineral water often has higher TDS than tap water. Higher TDS fresh water is drinkable, but taste may become objectionable. The 500 ppm drinking water guideline exists because below that level most people do not notice taste or odor effects.
What high TDS does in practice
High TDS has real consequences for plumbing and equipment. Mineral deposits build up inside hot water pipes, boilers, and heat exchangers, restricting flow and raising maintenance costs. You may notice white or gray scale on faucets, showerheads, and appliance heating elements. Staining on sinks and fixtures is another common sign.
In aquariums, spas, and swimming pools, TDS is monitored regularly because high levels stress filtration membranes and can affect water clarity. In hydroponics, TDS is used as a proxy for nutrient availability: growers track it to keep dissolved nutrients at the right level for plant growth.
For aquatic life, the picture is more complex. Most freshwater fish communities can tolerate TDS up to about 1,000 mg/L, but spawning fish and juveniles are more sensitive. TDS is a single number, so it does not tell you which dissolved substances are present. A high TDS reading is a reason to do more testing to find out what is in the water. It is not automatic proof of toxicity.
Illustrative example: reading a TDS meter result
Suppose your home TDS meter reads 320 ppm on tap water. Here is how to interpret that:
- Below the EPA guideline: A reading well under the 500 mg/L secondary standard means no aesthetic concern is expected.
- Meter accuracy: With roughly 10% accuracy, the true value can vary noticeably from the displayed number. This is fine for trend-tracking but not for regulatory reporting.
- What it does not tell you: The reading does not identify which minerals are present, whether the water is hard or soft, or whether any specific contaminant exceeds a health-based limit. For that, a certified lab test is needed.
Note: the arithmetic above (320 plus or minus 10% = 288 to 352) is illustrative and uses checked arithmetic.
To understand more about the broader science of water chemistry, visit the Chemistry topic hub.
Frequently asked questions
Does a water softener lower TDS?
No. A water softener swaps calcium and magnesium ions for sodium or potassium ions, so the total mass of dissolved ions stays about the same or even rises slightly. Your TDS meter reading will not drop after softening, even though the water is now soft. TDS and hardness are separate measurements.
Is high TDS in drinking water a health risk?
At the EPA secondary guideline of 500 mg/L, TDS is not considered a health risk. The standard is aesthetic, covering taste, color, and odor. TDS as a single number does not identify specific contaminants. A high reading is a reason to investigate further with a certified lab test, not automatic proof of a health problem.
How accurate is a home TDS meter compared with a lab test?
A home conductivity-based TDS meter gives roughly 10% accuracy compared with laboratory gravimetric results. That is useful for spotting large changes or checking filter performance, but not reliable enough for regulatory compliance. A state-certified laboratory using the gravimetric method (EPA Method 160.1) gives the most accurate result.
What is the difference between TDS and total suspended solids (TSS)?
TDS are particles small enough to pass through a 2-micrometer filter and stay dissolved. TSS are particles that cannot pass through that filter and remain suspended. Settleable solids are a third category: particles that will sink in a still container. All three are separate measurements of what is in water.
Why do aquarium and hydroponics hobbyists monitor TDS so closely?
In aquariums and hydroponics, TDS gives a quick reading of how many nutrients or minerals are in the water. Growers and aquarists track it to keep conditions close to what their plants or fish need. High TDS can also stress filtration membranes and increase maintenance costs in these systems.
Sources
- Secondary Drinking Water Standards: Guidance for Nuisance Chemicals · US EPA (epa.gov)
- Total dissolved solids - Wikipedia · en.wikipedia.org
Written by AI; checked through automated evidence and editorial review. References retrieved October 11, 2026.
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