Total Dissolved Solids (TDS) and Electrical Conductivity (EC) describe the same thing from two directions: how much dissolved material is in water, and how well that water carries an electric current. Handheld meters measure EC directly, because it can be read instantly with electrodes. TDS, strictly speaking, is measured by evaporating a sample and weighing what remains — slow, laboratory work. So in practice, TDS is estimated from EC using a conversion factor, and understanding that factor is the key to using either number well.
What TDS Measures
TDS is the total mass of dissolved substances in water — sodium, calcium, magnesium, potassium, chlorides, sulfates, bicarbonates, and smaller amounts of other ions — expressed in parts per million (ppm), which for dilute water is equivalent to milligrams per liter (mg/L). Drinking water typically falls between 50 and 500 ppm, hydroponic nutrient solutions between roughly 700 and 2,000 ppm depending on crop and growth stage, and seawater at about 35,000 ppm.
What EC Measures
EC measures how easily water conducts electricity, in microsiemens per centimeter (µS/cm) or millisiemens per centimeter (mS/cm), where 1 mS/cm equals 1,000 µS/cm. Dissolved ions carry charge, so more dissolved solids generally mean higher conductivity — pure water conducts almost nothing, while a strong salt solution conducts readily. But EC is not a mass measurement: different ions conduct differently at the same mass concentration. A solution rich in sodium chloride conducts more per milligram than one dominated by heavier, slower ions, which is exactly why the TDS-to-EC conversion cannot be a single universal constant.
Why Meters Read EC
Conductivity can be measured in seconds by passing a small current between two electrodes and reading the resistance. That makes EC the practical field measurement — and why almost every handheld "TDS meter" is actually an EC meter that converts and displays ppm.
The Conversion Factors: 0.5, 0.64, and 0.7
The conversion runs EC (µS/cm) = TDS (ppm) ÷ factor, or in reverse, TDS (ppm) = EC (µS/cm) × factor. Three factors dominate in practice:
- 0.5 — for sodium-chloride-dominated water, such as softened water, brackish water, and many hydroponic solutions mixed from single salts.
- 0.64 to 0.65 — for average mixed municipal supplies, where several ion types are present in typical proportions.
- 0.7 — for the "442" natural-water standard (a reference mixture of salts) that many meter manufacturers use for calibration.
The factor varies because it is the empirical TDS-to-EC ratio of a particular ionic mix, not a law of nature. Using the wrong factor is the most common source of conversion error: at 1,000 µS/cm, the 0.5 factor gives 500 ppm while the 0.7 factor gives 700 ppm — a 40% difference from the same reading. Check your meter manual to see which factor it applies, and match it when converting by hand with our TDS to EC calculator or the reverse EC to TDS calculator.
Worked Example
Suppose a hydroponic reservoir reads 1,400 µS/cm on a meter, and the nutrient manufacturer specifies a target of 700 ppm TDS. With the 0.5 factor, 1,400 × 0.5 = 700 ppm — the reservoir is exactly on target. But if the meter was actually applying the 0.7 factor, the true TDS would be 1,400 × 0.7 = 980 ppm, meaning the solution is considerably stronger than intended. Same reading, two very different conclusions — which is why the factor must be known, not guessed.
Where the Conversion Matters
Hydroponics
Grow charts specify nutrient strength as EC (for example, 1.2–2.4 mS/cm depending on crop and stage), while many growers think in TDS. Converting accurately keeps feeding on schedule and avoids both deficiency and nutrient burn.
Aquariums
Freshwater and reef aquarists track TDS to judge water purity for changes and top-offs, but their instruments report EC. A consistent factor matters more than the "right" one here — switching factors mid-record makes trends meaningless.
Drinking water
Utilities and filter makers quote TDS because consumers understand it as "how much is dissolved." Field checks, however, are EC readings, so convert with the local supply's factor for a meaningful comparison.
How Accurate Is the Conversion?
The EC-based TDS estimate is usually within 10–20% for typical waters — good enough for growing, aquarium keeping, and filter checks, but not a substitute for laboratory analysis when precision matters, such as regulatory compliance or research. Disagreements usually trace back to the conversion factor, temperature compensation, or a fouled electrode.
Summary
TDS is mass, EC is conductivity, and the bridge between them is an empirical factor that depends on what is dissolved. Know your water type, pick the matching factor — 0.5, 0.64, or 0.7 — and your conversions will be consistent. Try it with the TDS to EC calculator and the EC to TDS calculator, and keep the factor the same across your records.