TDS stands for total dissolved solids, a measurement of everything dissolved in water, including minerals, salts, and metals. It’s expressed in milligrams per liter (mg/L) or parts per million (ppm), which are essentially interchangeable for drinking water. The EPA sets a recommended limit of 500 mg/L for drinking water, though this is a guideline for taste and appearance rather than an enforceable safety standard.
What TDS Actually Measures
TDS captures any dissolved material in water smaller than 2 microns. That includes naturally occurring minerals like calcium, magnesium, sodium, and chloride, as well as metals like iron and manganese. Sulfur compounds, nitrates, and trace amounts of other elements all contribute to the total number. Even organic matter and residual chemicals from agriculture or industry can show up in a TDS reading.
The specific composition matters more than the raw number. Water at 300 mg/L dominated by calcium and magnesium (what makes water “hard”) is very different from water at 300 mg/L with elevated levels of lead or arsenic. A TDS meter won’t tell you what’s dissolved, only how much. That’s why TDS works as a quick screening tool but not as a complete water quality test.
How TDS Is Measured
Most handheld TDS meters don’t measure dissolved solids directly. They measure electrical conductivity (EC), which is how easily water conducts an electrical current, and then multiply that reading by a conversion factor to estimate TDS. Pure water conducts almost no electricity; the more dissolved ions present, the higher the conductivity.
The conversion factor isn’t fixed. It changes depending on the concentration of dissolved material. At low conductivity (around 100 microsiemens per centimeter), the factor is about 0.50. At higher conductivity (10,000 microsiemens), it climbs to roughly 0.82. Cheap meters typically use a single fixed factor, which means their readings become less accurate at the extremes. The most precise method is gravimetric: you evaporate a known volume of water and weigh what’s left behind. That’s the lab standard, but it’s not practical for home testing.
What Different TDS Levels Mean for Taste
TDS has a direct effect on how water tastes. Research on taste preferences found that water with very low TDS (under 100 mg/L) scored lowest for “freshness,” tasting flat and empty. The sweet spot for taste was between 200 and 400 mg/L, where water had enough mineral character to taste clean and satisfying.
Above that range, taste satisfaction drops steadily. For every 100 mg/L increase in TDS, taste-liking scores fell by about 0.23 points on a 10-point scale. That decline adds up. By the time you reach the EPA’s 500 mg/L guideline, most people notice a slight mineral or salty flavor. Water well above 1,000 mg/L often tastes distinctly salty or bitter, depending on which dissolved solids are present. Sodium and chloride create a salty taste. Iron and manganese give water a metallic flavor and can stain sinks and fixtures. Sulfur compounds produce bitterness.
Health Effects of High TDS
The EPA’s 500 mg/L secondary standard exists primarily for aesthetic reasons: taste, odor, and color. It’s a guideline for states, not a legally enforceable limit. At considerably higher concentrations, though, health concerns can emerge, particularly if the dissolved material includes contaminants like arsenic, lead, copper, or nitrates from agricultural runoff.
Hard water, which is high in calcium and magnesium and often the biggest contributor to elevated TDS in household water, has no known adverse health effects. The World Health Organization has stated that hard water may actually provide a meaningful supplementary source of calcium and magnesium in your diet. So a high TDS reading driven by these minerals is generally not a health concern, even though it can cause scale buildup on fixtures and appliances.
The Problem With Very Low TDS Water
Water that’s been heavily filtered or purified, such as through reverse osmosis (RO) or distillation, can have extremely low TDS, sometimes under 25 mg/L. This isn’t just a taste issue. Research shows that drinking very low mineral water over time can pull minerals from your body and increase their excretion through urine. Sodium, potassium, chloride, calcium, and magnesium losses all increase, with serum sodium concentrations rising by roughly 20% on average as the body compensates.
The downstream effects are concerning. Reduced calcium intake from water, combined with increased mineral excretion, can accelerate bone loss and raise the risk of osteoporosis. A narrative review published in Cureus found that chronic consumption of RO water was associated with declining bone mineral density and may contribute to stunted growth in children. Low-mineral water also reduces the teeth’s natural ability to remineralize, which can worsen cavities over time. Fluoride deficiency from demineralized water further compounds dental risks.
This doesn’t mean purified water is dangerous in moderation, but if it’s your primary drinking source, remineralization (adding minerals back) is worth considering.
How to Lower or Adjust TDS
Standard carbon filters, like the ones in pitcher-style filters, are designed to remove chlorine, some organic compounds, and certain contaminants. They do very little to change TDS because dissolved minerals and salts pass right through activated carbon.
Reverse osmosis systems are the most common home solution for reducing TDS. They force water through a semipermeable membrane that blocks most dissolved solids. Typical RO systems reduce TDS significantly, though the exact percentage depends on membrane quality, water pressure, and the starting TDS level. Distillation is even more effective, achieving up to 99.9% TDS reduction by boiling water and collecting the condensed steam. It’s thorough but slow and energy-intensive for household use.
If your goal is better-tasting water rather than medical-grade purity, the research suggests you want to land in the 200 to 400 mg/L range. Many RO systems include a remineralization stage that adds calcium and magnesium back into the filtered water, which improves both taste and nutritional value. If yours doesn’t, mineral drops or cartridge add-ons are inexpensive options.
When TDS Matters and When It Doesn’t
A TDS reading is useful as a baseline snapshot. If your home water suddenly jumps from 250 to 600 mg/L, something has changed, and it’s worth investigating. If you’re evaluating a water filter, testing TDS before and after tells you whether the system is actually removing dissolved material. And if you’re on well water, periodic TDS checks help you spot shifts in groundwater quality.
What TDS can’t do is tell you whether your water is safe. A reading of 150 mg/L could be perfectly clean mineral water, or it could contain harmful levels of a specific contaminant that happens to be present in small quantities. If you’re concerned about safety, a full water quality panel that tests for individual contaminants is far more informative than a single TDS number.