TDS stands for total dissolved solids, a measurement of all the inorganic salts and trace organic matter dissolved in water. It’s expressed in parts per million (ppm) or milligrams per liter (mg/L), and it tells you how much “stuff” is actually in your water beyond the water molecules themselves. The EPA sets a secondary guideline of 500 ppm for drinking water, and most people find water tastes best somewhere between 50 and 300 ppm.
What TDS Actually Measures
When minerals, salts, and tiny amounts of organic matter dissolve in water, they break apart into charged particles called ions. TDS captures the total weight of all these dissolved substances in a given volume of water. The main players are calcium, magnesium, sodium, and potassium on the positive side, and carbonate, chloride, sulfate, and nitrate on the negative side.
These substances enter water naturally as it flows through rock and soil, but they also come from agricultural runoff, industrial discharge, water treatment chemicals, and even the pipes water travels through. Two water sources with identical TDS readings can have very different compositions. A TDS number alone doesn’t tell you which specific minerals or contaminants are present, just the total amount dissolved.
How TDS Is Measured
The handheld TDS meters you can buy for $10 to $30 don’t directly weigh dissolved solids. They measure electrical conductivity: how well your water conducts a small electrical current. Pure distilled water conducts electricity poorly because it has almost no dissolved ions. The more minerals present, the more conductive the water becomes. The meter then multiplies that conductivity reading by a conversion factor to estimate TDS in ppm.
This method is fast and convenient but not perfectly precise. The conversion factor varies depending on water chemistry, so two water samples with different mineral profiles could give slightly different TDS estimates even if their actual dissolved solids are identical. For everyday home testing, these meters work well enough. Calibrate yours regularly with a calibration solution, especially after changing batteries or dropping it. If your reading comes back above 4,500 ppm, send a sample to a lab for detailed analysis rather than relying on the meter alone.
TDS Ranges and How Water Tastes
TDS has a direct effect on how your water tastes, and most people have a clear preference even if they’ve never tested their water:
- 0 to 50 ppm: Extremely pure water that often tastes flat or “empty.” This is what you get from distillation or aggressive filtration. Some people describe it as tasteless.
- 50 to 150 ppm: The sweet spot for most palates. Clean, crisp, and refreshing. Most bottled water brands target this range.
- 150 to 300 ppm: Pleasant with noticeable mineral character. Still perfectly fine for drinking and household use.
- 300 to 500 ppm: Acceptable, though mineral taste becomes more prominent. This is the upper end of the EPA’s guideline.
- 1,000 to 2,000 ppm: Noticeably salty, bitter, or metallic. Water at this level can damage appliances and leave visible scale deposits.
- Above 2,000 ppm: Essentially undrinkable. Harsh and offensive tasting, requiring treatment before any use.
Both the EPA in the United States and Canadian guidelines recommend keeping drinking water below 500 ppm. The EPA classifies TDS as a secondary standard, meaning it’s a voluntary guideline focused on taste and appearance rather than an enforceable safety limit. Some states, however, adopt it as a mandatory standard.
Health Effects of High TDS Water
High TDS water isn’t automatically dangerous, but it can signal the presence of specific minerals at concentrations that cause real problems. Elevated sodium contributes to high blood pressure and cardiovascular strain. High sulfate levels act as a laxative, particularly in children and people not used to the water. Excessive fluoride can cause fluorosis, a condition that damages teeth and bones, while high nitrate levels pose a serious risk to infants through a condition called methemoglobinemia, sometimes called “blue baby syndrome.”
Beyond individual minerals, consistently drinking very high TDS water has been linked to issues with the digestive system, skeletal system, and kidneys. The water also causes practical headaches: staining on dishes and fixtures, cloudy appearance, and mineral scale that builds up inside pipes, water heaters, and appliances, shortening their lifespan.
The critical thing to understand is that TDS is a screening number, not a diagnosis. A reading of 800 ppm dominated by calcium and magnesium is a very different situation from 800 ppm driven by sodium and nitrates. If your TDS is elevated, a full water quality test that identifies specific contaminants is far more useful than the TDS number alone.
Health Risks of Very Low TDS Water
Water that’s been stripped of nearly all minerals carries its own set of concerns, and this is something many people don’t expect. Reverse osmosis systems and distillers can push TDS below 20 ppm, removing the calcium, magnesium, and other trace minerals that your body actually uses.
The bigger issue is cooking. When you boil vegetables, cook grains, or prepare meat in demineralized water, the water aggressively pulls minerals out of your food. Research published in the Medical Journal of Armed Forces India found that cooking with low-mineral water can cause losses of up to 60% for magnesium and calcium, 66% for copper, 70% for manganese, and 86% for cobalt. Since most of your mineral intake comes from food rather than water, this cooking effect can create meaningful nutritional gaps over time.
Studies have also linked long-term consumption of soft, low-mineral water to higher fracture risk in children, certain neurodegenerative diseases, pregnancy complications including preeclampsia, and some types of cancer. Magnesium in drinking water appears particularly protective for heart health. Hard water, which naturally has higher TDS from calcium and magnesium, is associated with somewhat lower cardiovascular disease risk.
If you use an RO system or distiller at home, adding a remineralization filter or mineral drops to your treated water can restore some of that beneficial mineral content.
How to Lower TDS
Reverse osmosis (RO) is the most common home treatment for high TDS water. A standard RO membrane removes 95 to 99% of total dissolved solids, bringing even fairly mineralized water well within the ideal range. These systems fit under a kitchen sink and typically include pre-filters for sediment and chlorine, the RO membrane itself, and a storage tank.
Distillation also eliminates nearly all dissolved solids by boiling water and collecting the steam. It’s highly effective but slow and energy-intensive compared to RO, which is why most households choose reverse osmosis.
Standard carbon filters, like those in pitcher-style systems, improve taste by removing chlorine and some organic compounds but do very little to reduce TDS. If your goal is specifically to lower dissolved mineral content, you need RO, distillation, or a deionization system. Actual rejection rates depend on your water’s temperature, pressure, pH, and specific chemistry, so your results may vary from the manufacturer’s estimates.
What Your TDS Reading Actually Tells You
A TDS meter gives you a useful snapshot, not a complete picture. It can confirm whether your RO filter is working (a sudden jump in output TDS means the membrane needs replacing), flag unusually mineralized tap or well water, and help you compare water sources. It cannot tell you whether your water is safe, because it doesn’t distinguish between harmless calcium and harmful lead. Both contribute to the TDS count equally.
For most homes on municipal water, TDS falls between 100 and 400 ppm and isn’t a concern. Well water is more variable, and regular testing for both TDS and specific contaminants is worth the investment. If your TDS reads above 500 ppm or your water tastes off, a detailed lab analysis will tell you exactly what’s in it and whether treatment is needed.