TDS stands for total dissolved solids, and it measures the combined concentration of all inorganic salts and small amounts of organic matter dissolved in water. The result is expressed in milligrams per liter (mg/L) or parts per million (ppm), which are equivalent units. A TDS reading tells you how much “stuff” is dissolved in your water, but it does not tell you exactly what that stuff is.
What TDS Actually Includes
The dissolved solids in water are primarily mineral salts. The main positively charged particles are calcium, magnesium, sodium, and potassium. The main negatively charged particles are carbonate, bicarbonate, chloride, sulfate, and nitrate. These all enter water naturally as it flows through soil and rock, or they can come from pipes, water treatment processes, and runoff.
TDS also captures small amounts of dissolved organic matter, but in most drinking water the bulk of the reading comes from those mineral salts. Technically, TDS includes any dissolved particle smaller than 2 microns (about 0.0002 cm). Anything larger than that is considered a suspended solid rather than a dissolved one.
How TDS Meters Work
When salts dissolve in water, they split into positively and negatively charged particles called ions. These ions conduct electricity. A handheld TDS meter works by measuring how well your water conducts an electrical current, then using a conversion factor to estimate the total dissolved solids. More ions in the water means higher conductivity, which means a higher TDS reading.
This method is fast, cheap, and easy to do at home, but it’s an estimate. The meter can’t distinguish between calcium, sodium, chloride, or any other ion. It just senses the overall electrical activity and converts it to a number. The lab method is more precise: a water sample is filtered, oven-dried, and the leftover residue is weighed. That gives you the actual mass of dissolved material, but it’s expensive and time-consuming.
What the Numbers Mean for Drinking Water
The EPA sets a secondary standard of 500 mg/L for TDS in drinking water. This is a guideline for taste and appearance, not a legally enforced health limit. Water below 500 mg/L generally tastes clean and neutral. As TDS rises above that threshold, you’re more likely to notice off-flavors, discoloration, or mineral deposits on fixtures.
The specific minerals driving a high TDS reading affect what you’ll taste and experience:
- Sodium and chloride make water taste salty and can corrode pipes over time.
- Iron and manganese produce a metallic taste, stain sinks and toilets, and build up inside pipes and water heaters.
- Sulfur compounds create a bitter or salty taste, reduce detergent effectiveness, and at higher levels can have a laxative effect.
Very low TDS water (under about 50 mg/L) can also taste flat or empty, which is why some people find heavily filtered or distilled water unappealing. Most bottled water falls somewhere between 50 and 300 mg/L.
TDS vs. Water Hardness
These two measurements overlap but aren’t the same thing. Hardness specifically measures calcium and magnesium in your water. TDS measures everything dissolved, including calcium and magnesium plus all the other salts and minerals. All hardness contributes to your TDS number, but not all TDS is hardness.
This distinction matters if you use a water softener. A traditional ion-exchange softener swaps calcium and magnesium for sodium. Your water gets softer, but the TDS reading barely changes because you’ve replaced one set of dissolved minerals with another. If you check your water with a TDS meter after installing a softener and the number looks the same, that’s normal and expected.
What TDS Cannot Tell You
A TDS reading is useful for getting a general snapshot of water quality, but it has real blind spots. It cannot identify specific contaminants. A reading of 150 ppm could mean your water is full of harmless calcium and magnesium, or it could mean something less benign is contributing to that number. The meter treats all dissolved ions the same.
TDS meters do not detect bacteria, viruses, or parasites. They won’t reliably flag lead, arsenic, or pesticides at the trace levels that matter for health, because those contaminants are often present in concentrations too small to meaningfully shift the overall TDS number. Water with a perfectly normal TDS reading can still contain harmful substances, and water with a high TDS reading can be perfectly safe to drink. If you want to know whether your water contains specific harmful contaminants, you need a lab test. Your local water utility publishes annual quality reports, and private labs can test well water for regulated parameters like E. coli, heavy metals, and region-specific pollutants.
TDS Beyond Drinking Water
TDS monitoring shows up in several other contexts where dissolved mineral content directly affects outcomes.
In hydroponics, growers use TDS readings to track nutrient concentration in their water supply. Seedlings and young plants do best around 500 to 800 ppm, while mature, fruiting plants can handle up to 1,500 ppm. Going too high causes nutrient burn at the roots; going too low starves the plant.
Aquarium hobbyists monitor TDS to keep dissolved minerals within the range their fish species need. Freshwater tropical fish generally prefer lower TDS, while African cichlids from mineral-rich lakes thrive at higher levels. Sudden shifts in TDS can stress fish even if the absolute number is within range, so consistency matters as much as hitting a target.
Pool and spa owners track TDS because it climbs over time as water evaporates and chemicals are added. High TDS in a pool can reduce the effectiveness of sanitizers and leave the water looking cloudy, even when chlorine levels test fine. Draining and refilling a portion of the water is the main way to bring it back down.