How this instrument works
Total dissolved solids (TDS) is the combined mass of all minerals, salts, and other dissolved inorganic and organic material in a water sample, expressed in milligrams per liter (mg/L). It's a common general indicator of water quality, but it's slow to measure directly (evaporate a known volume and weigh the residue). Electrical conductivity (EC), by contrast, is fast and easy to measure with a simple probe — and because dissolved ions are what actually carry electrical current through water, conductivity correlates closely with TDS, which is why conductivity-based TDS meters exist at all.
The relationship between the two is empirical, not a fixed physical law: TDS = k x EC, where EC is conductivity in microsiemens per centimeter (uS/cm) and k is a conversion factor that depends on exactly which dissolved ions are present, since different ions conduct electricity with different efficiency per unit of dissolved mass. k typically falls somewhere between about 0.5 and 0.8 for natural waters; a commonly used default around 0.64-0.67 is a reasonable middle-ground estimate when the water's specific ionic makeup isn't known.
Because k varies with the water's actual composition, this method is an approximation — typically accurate to within roughly 10% against a true, lab-measured TDS value for typical waters, and less reliable for water with an unusual ionic makeup (very hard water, or water dominated by an ion that conducts unusually well or poorly per unit mass). For a precise TDS figure, a lab gravimetric measurement (evaporation and weighing) remains the reference method; this calculator is built for the fast field-estimate case a handheld conductivity meter is actually used for.
- Enter the water's measured electrical conductivity into Electrical conductivity, EC (uS/cm).
- Enter the conversion factor into Conversion factor, k — use your meter's specified factor if it has one, or a typical default in the 0.5-0.8 range (commonly around 0.64-0.67) if not.
- Read Total dissolved solids, TDS (mg/L) below the inputs — it recalculates instantly as either value changes.
- Treat the result as an estimate; a lab gravimetric TDS measurement is the reference method if precision matters more than a quick field reading.
Worked example — a typical tap-water conductivity reading
Enter 500 into Electrical conductivity, EC (uS/cm) — a common municipal tap-water reading — and 0.67 into Conversion factor, k, a commonly used default value. Total dissolved solids, TDS (mg/L) reads 335 mg/L: 500 x 0.67 = 335.
That 335 mg/L result falls squarely within the normal 300-500 mg/L range typically reported for municipal tap water, which is a reasonable sanity check on both the conductivity reading and the chosen k factor. The EPA's secondary (aesthetic, non-health-based) guideline for TDS in drinking water is 500 mg/L, above which water can start to taste noticeably salty or mineral-heavy — this example sits comfortably under that threshold.
Questions
Why isn't there one fixed conversion factor between conductivity and TDS?
Because different dissolved ions conduct electricity with different efficiency relative to their mass — a water sample dominated by sodium and chloride conducts differently, per milligram of dissolved solid, than one dominated by calcium and bicarbonate (hard water) or by sulfate. Since k folds all of that ion-specific behavior into a single number, its correct value genuinely depends on which ions actually dominate a given water sample, which is why no single k works precisely for every water source.
What conversion factor should I use if I don't know my water's exact composition?
A value in the middle of the commonly cited 0.5-0.8 range, often 0.64-0.67, is a reasonable general-purpose default for typical freshwater. If your specific TDS meter or test kit specifies its own factor (many handheld meters use a fixed internal k, often labeled on the device or in its manual), use that instead, since it's calibrated for how that particular meter's readings correlate with TDS for common water types.
How accurate is this compared to actually measuring TDS in a lab?
This empirical conductivity-based method is generally accurate to within roughly 10% of a true gravimetric TDS measurement (evaporating a known water volume and weighing the residue) for typical waters, but that accuracy degrades for waters with an unusual ionic makeup. For most everyday water-quality checks — drinking water, aquariums, hydroponics, pool or spa water — that level of accuracy is more than sufficient; a certified lab test is the reference standard when precision genuinely matters.
What TDS level is considered high for drinking water?
The US EPA's secondary (non-health-based, aesthetic) guideline for TDS in drinking water is 500 mg/L, above which water may develop noticeable hardness, staining, or a salty or mineral taste. TDS by itself doesn't identify which specific dissolved substances are present, so a high TDS reading indicates general mineral content rather than any specific contaminant or health concern.
Does temperature affect this calculation?
Conductivity itself is temperature-sensitive (warmer water generally conducts better), which is why most conductivity meters automatically apply temperature compensation, normalizing readings to a standard reference temperature (commonly 25 degC) before displaying a value. This calculator assumes the EC figure you enter is already a temperature-compensated reading, as most meters output by default, rather than a raw, uncompensated measurement.