A TDS meter reading of 450 mg/L might mean your water has high calcium from limestone, completely harmless, just hard water. Or it might include arsenic at levels above the EPA limit. The number alone doesn’t tell you. This matters because most of the advice around “reducing TDS” treats it as a health problem when it’s usually a taste and scale problem.
What TDS Actually Measures
TDS is total dissolved solids. A TDS meter measures electrical conductivity, charged ions in water conduct electricity, and more ions means higher conductivity. The meter converts that reading to an estimated TDS in mg/L.
What it’s counting: calcium, magnesium, sodium, potassium, bicarbonates, sulfates, chlorides, and anything else dissolved in the water. Minerals from rock formations. Road salt from surface runoff. Naturally occurring arsenic. Nitrates from agricultural runoff. The meter doesn’t distinguish. It just measures the total.
The EPA Secondary Maximum Contaminant Level for TDS is 500 mg/L. Secondary standards are for taste, aesthetics, and corrosion, not health. Water at 600 mg/L may taste slightly salty or mineral-heavy. Water at 1,000 mg/L leaves significant scale in appliances. But water at 800 mg/L that’s all calcium and magnesium is not a health problem.
A lab water test tells you what’s actually in your TDS. A TDS meter just tells you it’s elevated.
When High TDS Matters Practically
Scale in water heaters. Above about 250 mg/L, you’ll start to see mineral deposits on heating elements and inside water heater tanks. Above 500 mg/L, scale accumulates fast enough to meaningfully reduce appliance efficiency over time.
Spots on dishes and glassware. The spots are mineral deposits left when water evaporates. Hard water (high calcium and magnesium) causes this even at moderate TDS levels.
Taste. Above 500 mg/L, many people detect a mineral, salty, or heavy taste. Some people are more sensitive than others. Below 200 mg/L, most people can’t taste the difference.
Reverse osmosis membrane performance. RO systems have reduced efficiency at very high source TDS (above 1,500-2,000 mg/L). Most residential source water is well below that.
Methods That Reduce TDS
Reverse osmosis: removes 90-99% of dissolved solids. The RO membrane has a pore size of about 0.0001 microns, too small for most dissolved ions to pass through. A source water reading of 400 mg/L typically comes out of an RO at 10-40 mg/L. RO also removes PFAS, lead, fluoride, arsenic, and nitrates, the actual health-risk contaminants.
An under-sink RO is the practical choice for most homes. Point-of-use RO treats only the drinking and cooking water, which is fine since shower and laundry water doesn’t require TDS reduction. See the best under-sink RO systems for options.
Distillation: water is boiled, the steam condenses, and the condensed water is collected. Minerals stay in the boiling chamber. Output TDS is near zero. The drawbacks: slow (typically 1 gallon per hour), uses energy, and countertop distillers cost $100-300. Practical for specific uses like CPAP humidifiers or lab equipment.
Deionization (DI): ion exchange resin removes all dissolved ions, producing near-zero TDS water. Used in laboratory and industrial water purification. Not standard for home drinking water. The resin regenerates with acids and bases, which is not practical at household scale. You’ll see DI water listed for car washing or aquariums where even very low mineral content matters.
Methods That Don’t Reduce TDS
Activated carbon filters: remove organic compounds, chlorine, and some VOCs. They don’t remove dissolved minerals. A carbon pitcher, faucet filter, or under-sink carbon cartridge won’t change your TDS reading.
Water softeners: replace calcium and magnesium with sodium. The softener doesn’t remove dissolved solids, it substitutes one ion for another. TDS may actually measure slightly higher after softening in some cases because sodium has dissolved mass. Softeners fix hardness, not TDS.
Boiling: concentrates TDS slightly. When water evaporates, the minerals stay behind. Boiling a half-full pot leaves more concentrated mineral content in the remaining water, not less.
Sediment filters: remove physical particles, not dissolved solids. A 5-micron sediment filter won’t reduce your TDS reading at all.
The Remineralization Question
RO water at 10-20 mg/L tastes flat to many people. The minerals that were in the water contributed to its texture and taste. Some people prefer it. Others find it oddly thin.
Some RO systems include a remineralization stage that adds calcium and magnesium bicarbonate back after filtration. This raises TDS slightly (typically to 50-100 mg/L) and restores taste. It’s a preference choice, not a health requirement either way.
If you’re installing RO specifically to reduce scale and mineral taste, you might not want remineralization. If you’re installing it for contaminant removal and want the water to taste more like what you’re used to, remineralization is worth considering.
Using a TDS Meter to Monitor Your System
Once you have an RO system, a TDS meter is useful as a monitoring tool. Measure your source water TDS and your RO output TDS on the same day. Calculate the rejection rate: (source - output) / source x 100. A new RO membrane should reject 90-99%.
Measure again every 3-6 months. When rejection drops below 80%, the membrane is degrading. A membrane usually lasts 2-5 years depending on source water quality and usage.
A rising TDS reading from RO output over time is one of the clearest signs a membrane needs replacement. See when to replace your water filter for membrane replacement schedules.
The Right Order of Operations
Test your water with a certified lab first. Find out what’s actually in the TDS. If it’s primarily calcium and magnesium and you don’t have scale problems, you may not need to do anything. If it’s arsenic or nitrates, you need RO for health reasons, not just taste. If it’s a hardness issue causing scale, you might address it with a softener rather than RO.
Don’t buy an RO system because your TDS meter read 350. Buy it because you know what’s in that 350 and want it removed.
For help choosing the right treatment approach, see what water filter do I need.