Hong Life CalculatorsUnderstanding the TDS calculator — concept and principle
Total dissolved solids (TDS) is the mass of salts and other substances dissolved in water. Measuring it directly means evaporating a sample and weighing the residue, so most meters estimate it from electrical conductivity instead. This calculator corrects a conductivity reading to 25 °C and multiplies it by a conversion factor k to estimate TDS. It is used for drinking water, aquariums, hydroponics and irrigation.
How it is calculated
EC25 = ECt ÷ [1 + α × (t − 25)]
TDS (mg/L) = k × EC25 (µS/cm)
- ECt is the measured conductivity in µS/cm at water temperature t (°C).
- α is the temperature coefficient in % per °C (entered as 2.0 for 2 %).
- k is the conversion factor; published values for natural waters range from 0.54 to 0.96, and 0.64–0.65 is common in irrigation work.
- EC25 is also shown in mS/cm (= dS/m), and TDS is shown for k = 0.54 and 0.96 as a range.
The principle
Pure water barely conducts electricity. Dissolved salts split into charged ions, and those ions carry current, so conductivity rises almost in proportion to how much is dissolved. The exact ratio depends on which ions are present, because each ion carries a different charge per unit mass and moves at a different speed. That is why k is not universal. Ions also move faster in warm water, raising conductivity roughly 2 % per °C, so readings are referred to 25 °C before converting.
Worked example
Default inputs: EC 850 µS/cm at 20 °C, α = 2.0 %/°C, k = 0.65.
- Correction factor = 1 + 0.02 × (20 − 25) = 0.9.
- EC25 = 850 ÷ 0.9 = 944 µS/cm = 0.944 mS/cm.
- TDS = 0.65 × 944.4 = 614 mg/L.
- Range check: k = 0.54 gives 510 mg/L; k = 0.96 gives 907 mg/L.
| Measured conductivity (µS/cm) | Total dissolved solids (mg/L ≈ ppm) |
|---|---|
| 212 | 153 |
| 425 | 307 |
| 638 | 460 |
| 850 | 614 |
| 1,062 | 767 |
| 1,275 | 921 |
| 1,700 | 1,228 |
How to read the result
For drinking water, the WHO notes that water with TDS below about 600 mg/L is generally considered to taste good, and that taste becomes increasingly unacceptable above about 1,000 mg/L. TDS by itself does not tell you whether water is safe: it does not detect bacteria, and harmful substances such as lead or nitrate can be present at low TDS. In irrigation, conductivity in dS/m is used to judge salinity risk for crops.
Common mistakes
- Treating the meter's TDS reading as exact; it is an estimate based on an assumed k.
- Comparing readings taken at different temperatures without correction.
- Using a k value fitted for one water type on a very different water.
- Assuming low TDS means the water is safe to drink.
Frequently asked questions
What is a normal TDS for tap water?
It varies widely by source. Many tap waters fall in the low hundreds of mg/L; check your water supplier's report.
Is ppm the same as mg/L?
For dilute fresh water, 1 mg/L is approximately 1 ppm by mass.
Why do different meters give different TDS for the same water?
They use different built-in k factors, such as 0.5 or 0.7.
Sources: Singh, T. & Kalra, Y. P. (1975), "Specific conductance method for in situ estimation of total dissolved solids", Journal AWWA 67(1): 99–100 · World Health Organization, Guidelines for Drinking-water Quality, "Total dissolved solids" background document