TDS stands for total dissolved solids, a measurement of all the dissolved minerals, salts, and small amounts of organic matter in water. It’s expressed in milligrams per liter (mg/L) or parts per million (ppm), and it’s one of the most common numbers you’ll see on water filter packaging, in water quality reports, and on handheld testing meters. The EPA sets a recommended limit of 500 ppm for drinking water, though this is a non-mandatory guideline based on taste and appearance rather than safety.
What TDS Actually Measures
TDS captures everything that’s dissolved in water after you remove the suspended particles. The main contributors are mineral ions: calcium, magnesium, sodium, and potassium on the positive side, and carbonate, chloride, sulfate, and nitrate on the negative side. These come from natural sources like rock and soil that water passes through, but also from pipes, agricultural runoff, and water treatment chemicals.
A glass of water with a TDS of 300 ppm contains roughly 300 milligrams of dissolved material per liter. Some of that is beneficial. Calcium and magnesium, for instance, are the same minerals you’d find in a supplement. The TDS number alone doesn’t tell you which specific minerals are present or in what proportion, just the total amount of everything combined.
How TDS Affects Taste and Quality
The World Health Organization rates water with TDS below 600 ppm as generally good-tasting. Above 1,000 ppm, water becomes noticeably unpalatable, often tasting salty, bitter, or metallic depending on which minerals dominate. Very low TDS water (under 50 ppm) can taste flat or empty because your palate expects some mineral content.
The EPA’s secondary standard of 500 ppm exists specifically because of these taste and appearance issues. At higher levels, dissolved minerals can leave deposits on fixtures, stain laundry, and give water an off-putting color. These are cosmetic problems, not health hazards, which is why the standard is a guideline rather than a legal requirement. Public water systems aren’t required to test for TDS, though most do.
TDS vs. Water Hardness
These two measurements overlap but aren’t the same thing. Water hardness specifically measures calcium and magnesium, the minerals responsible for scale buildup in pipes and that filmy feeling on skin after showering. TDS includes calcium and magnesium but also counts every other dissolved substance: sodium, chloride, sulfate, nitrate, and more.
You can have high-TDS water that isn’t particularly hard if most of the dissolved solids are sodium and chloride rather than calcium and magnesium. And moderately hard water might have a relatively low TDS if calcium and magnesium are the only significant minerals present. A water softener addresses hardness specifically by swapping calcium and magnesium for sodium, which actually keeps TDS roughly the same while solving the scale problem.
How TDS Meters Work (and Their Limits)
The handheld TDS meters sold for $10 to $30 don’t directly measure dissolved solids. They measure electrical conductivity, the ability of water to carry an electrical charge, and then multiply that reading by a conversion factor to estimate TDS. This works because dissolved mineral ions conduct electricity: the more ions present, the higher the conductivity.
The conversion factor isn’t fixed. At lower conductivity levels, the meter uses a factor around 0.50, while at higher levels the factor climbs to 0.80 or above. Most consumer meters use a single average factor, which introduces some imprecision. For typical tap water, the estimate is close enough to be useful.
The real limitation is what these meters miss entirely. Because they rely on electrical conductivity, they only detect charged ions. Uncharged organic contaminants like pesticides, solvents, bacteria, viruses, and pharmaceuticals don’t conduct electricity and won’t register on a TDS meter at all. A reading of 50 ppm looks reassuring, but it says nothing about whether your water contains lead at trace levels, volatile organic compounds, or microbial contamination. TDS is one piece of the water quality picture, not the whole thing.
Which Filters Reduce TDS
Not all water filters are designed to lower TDS, and not all situations call for it. Standard carbon filters, the kind found in pitcher filters and refrigerator cartridges, are excellent at removing chlorine taste, some organic chemicals, and certain heavy metals, but they leave most dissolved minerals untouched. Your TDS reading will barely change after running water through a carbon filter, and that’s fine.
Reverse osmosis (RO) is the most common home filtration method for significantly reducing TDS. RO systems push water through a semipermeable membrane with pores small enough to block dissolved ions. A well-maintained RO system typically removes 90% or more of total dissolved solids, bringing a 400 ppm tap water reading down to 40 ppm or less. The tradeoff is water waste: conventional RO systems send two to four gallons down the drain for every gallon of filtered water produced, though newer models have improved this ratio.
Distillation is another highly effective approach. Distillers heat water to steam and then condense it, leaving dissolved minerals behind. Membrane distillation technology has demonstrated TDS removal rates above 99.99% in research settings, and even simple countertop distillers produce water with very low TDS. The process is slow, typically producing about a gallon every four to six hours, and uses significant electricity.
Deionization uses resin beads that exchange dissolved mineral ions for hydrogen and hydroxide ions, effectively stripping minerals from the water. In one study, deionization reduced TDS from 1,000 ppm to 90 ppm in a single pass. Deionization cartridges are often paired with RO in multi-stage systems: the RO membrane handles the bulk of the work, and the deionization stage polishes the water down to near-zero TDS. This combination is common in aquarium systems and in homes where extremely pure water is the goal.
When Lowering TDS Makes Sense
If your tap water TDS is in the 200 to 500 ppm range and tastes fine, there’s no health-based reason to filter it further. Many of those dissolved minerals are nutritionally beneficial, and water in this range meets both EPA and WHO palatability guidelines.
Lowering TDS becomes more practical when your water exceeds 500 to 600 ppm and tastes noticeably salty or bitter, when you’re seeing heavy mineral deposits on cookware and fixtures, or when you have a specific use that demands low-mineral water, like espresso machines, CPAP humidifiers, or aquariums where fish are sensitive to mineral content.
If your concern is actual contamination rather than mineral content, a TDS meter isn’t the right diagnostic tool. Lead, arsenic, nitrate, and bacterial contamination all require specific testing. A certified lab test or a test kit targeting the contaminant you’re worried about will give you far more actionable information than a TDS reading ever could.