TDS Calculator (Total Dissolved Solids from Conductivity)

Total Dissolved Solids (TDS) measures the combined content of all inorganic and organic substances dissolved in water, in milligrams per liter (mg/L). High TDS can affect taste, and the U.S. EPA sets a non-enforceable secondary guideline of 500 mg/L for palatability — not because it's necessarily unsafe, but because water above that level often tastes noticeably different.

It uses the standard approximation TDS (mg/L) = ke × EC (µS/cm), where EC is the water's electrical conductivity at 25 °C and ke is a conversion factor that depends on which ions dominate the water. This is the same relationship used by water-testing meters that display a 'TDS' reading alongside conductivity.

0.67 is a commonly used default when the water's ionic makeup is unknown, but published guidance places the factor anywhere from about 0.55 to 0.8 for natural waters. Groundwater and typical tap water often fall around 0.65–0.7; water dominated by calcium and sulfate can run as high as 0.8; seawater is closer to 0.75. If you know your water's chemistry or have a lab-calibrated factor, use that instead of the default.

No. EC is a direct physical measurement (how well the water conducts electricity), while TDS is an estimate derived from it. Many handheld 'TDS meters' actually measure conductivity and then apply an internal ke factor to display a TDS number — that's exactly the calculation this tool performs, but with the factor exposed so you can adjust it.

Because different meters are calibrated with different default ke factors (commonly 0.5, 0.64, 0.67, or 0.7) or use different reference solutions (NaCl vs. KCl vs. a 442 blended standard). The same raw conductivity reading can therefore produce noticeably different displayed TDS values depending on the meter's internal assumption.

Not necessarily. The EC-based method only captures dissolved solids that conduct electricity (ions); it misses non-ionic dissolved substances like dissolved organic matter or silica. The gravimetric method — evaporating a water sample and weighing the residue — remains the accurate laboratory reference. This calculator's EC-based estimate is a fast, practical approximation, not a lab substitute.

There's no single enforced number, but common guidance bands are: below 300 mg/L is considered excellent, 300–600 mg/L good, 600–900 mg/L fair, 900–1,200 mg/L poor, and above 1,200 mg/L unacceptable for many palates. The EPA's own guidance references 500 mg/L as its secondary (aesthetic, non-enforceable) threshold.

Conductivity rises with temperature because ions move faster in warmer water, so a reading taken at 30 °C will read higher than the same water at 25 °C — by a widely used rule of thumb, about 2% higher per degree above the 25 °C reference (per the USGS specific-conductance field manual). The TDS = ke × EC formula assumes EC has already been temperature-compensated to that 25 °C reference — most digital meters do this automatically, but manual/analog meters may need a correction applied first.

Yes — the same conductivity-to-TDS relationship applies to any dilute aqueous solution. Aquarists and hydroponic growers commonly use a similar factor (often close to 0.5–0.7 depending on the meter/scale used) to estimate nutrient concentration from an EC reading. Just make sure your EC value is already normalized to 25 °C.

For dilute freshwater, where the solution's density is very close to 1 g/mL, milligrams per liter (mg/L) and parts per million (ppm) are numerically equivalent — 1 mg/L ≈ 1 ppm. This equivalence breaks down at very high concentrations (like seawater), where density deviates more from 1 g/mL.

Because TDS = ke × EC is directly proportional to ke. Doubling ke doubles the calculated TDS for an unchanged conductivity reading. This is why choosing an appropriate ke for your specific water type matters more than the EC reading itself when comparing results across different meters or calculators.

Less accurate than for freshwater. The 0.55–0.8 factor range is derived mainly from dilute natural (fresh) waters; seawater's very high ionic strength and different ion mix (dominated by sodium and chloride) push its effective factor toward roughly 0.75, and the simple linear approximation becomes less reliable as concentration rises into the thousands of mg/L.

The U.S. Geological Survey classifies water by dissolved-solids concentration: freshwater is below 1,000 mg/L, brackish water runs 1,000–10,000 mg/L, a salinity transition zone spans 10,000–35,000 mg/L, and saline water covers 35,000–100,000 mg/L (with true brine above that). This is a different scale from the drinking-water palatability bands above — a reading can be classified 'freshwater' by this salinity scale while still tasting poor on the palatability scale.