A TDS meter does not measure dissolved solids — it measures how well the water conducts electricity and multiplies by a factor. Knowing which factor your meter uses, and converting between EC and TDS properly, is the difference between water reports that agree and hydroponic recipes that mysteriously differ by 40 percent. This TDS calculator converts in both directions across the three standard scales (NaCl, 442 and KCl), so an EC from any source becomes the ppm your chart expects.
The Conversion and Its Factors
The whole computation is one multiplication: TDS (ppm) = EC (µS/cm) × factor. The science hides in the factor, because conductivity counts charge-carrying ions while TDS weighs everything dissolved, and different salts conduct differently per gram. The industry settled on reference salts: the NaCl scale (0.50) fitted to sodium chloride solutions and used by most inexpensive pens; the 442 scale (0.64), a blend (40 percent sodium sulfate, 40 percent sodium bicarbonate, 20 percent sodium chloride) chosen to mimic natural fresh waters, standard in agriculture and aquaculture; the KCl scale (0.55); and the 0.70 scale used by several European meter lines. The page’s default — 800 µS/cm on the 442 scale — gives 512 ppm; the same water reads 400 ppm on a NaCl pen and 560 on a 0.70 meter.
EC Units Without Tears
Conductivity arrives in two unit sizes a thousand apart: microsiemens per centimetre (µS/cm) for drinking water, hydroponics and aquariums (tens to a few thousand), and millisiemens per centimetre (mS/cm) in agronomy and oceanography (seawater is about 50 mS/cm). Divide µS/cm by 1,000 for mS/cm — the calculator prints both. Every TDS factor in the table assumes EC in µS/cm; feeding mS/cm into the multiplication without converting inflates the answer a thousandfold, the single most common conversion accident in grow forums. Temperature also matters: conductivity rises about 2 percent per degree Celsius, and honest meters temperature-compensate to 25 °C automatically.
Why EC Is a Proxy, Not a Measurement
Only ionised substances conduct. Dissolved sugars, silica, most organics and un-ionised gases add real mass — real TDS by the laboratory’s evaporate-and-weigh method — while contributing almost nothing to EC. Conversely, a water dominated by highly mobile ions (hydrogen and hydroxide are champions) conducts more per gram than the factor assumes. So EC-derived TDS is an estimate whose accuracy depends on the water’s ionic recipe resembling the reference salt’s: within about 10 percent for typical fresh waters on the right scale, and badly off for juices, syrups, some industrial effluents and very pure waters. When a regulation or a warranty says “TDS,” it usually means the gravimetric number; the meter is the field estimate.
Where the Number Earns Its Keep
Hydroponics runs on it: nutrient strength is dosed and charted in EC (or in ppm on a stated scale — a recipe quoting “1,200 ppm” without its scale is underdetermined by 40 percent). Aquariums track it for mineral creep and water-change discipline. Drinking-water quality uses TDS as a taste and scaling indicator (palatability fades and kettles fur as it climbs past a few hundred ppm; secondary standards commonly cite 500 ppm as an aesthetic ceiling). Boiler and cooling-tower operators blow down on conductivity to cap dissolved solids before scale forms. In every case the workflow is the same: measure EC, convert on the scale your standard assumes, compare with the target for your water type.
Calibrating and Trusting a Meter
Pens drift; calibration solutions (typically 342 ppm NaCl or 1413 µS/cm KCl) exist to pin them. Calibrate on the standard nearest your working range, rinse between sample and standard (carryover is a silent error at these dilutions), and read in a clean cup rather than the reservoir when precision matters. If two meters disagree on your water, do not average them — identify each one’s scale from its manual and convert; nine times in ten they agree in EC and differ only in the factor they silently applied.
Common Mistakes
Comparing ppm values recorded on different scales as if they were one unit. Feeding mS/cm into a µS/cm formula. Expecting a TDS pen to detect non-ionic contamination (it cannot see sugar, alcohol, most pesticides or bacteria at all — a “low TDS” reading is not a safety certificate). Measuring hot samples without temperature compensation. And calibrating a 0.50-scale pen with a 442-scale standard, building a permanent 28 percent lie into every subsequent reading.
Related Tools
The water bench continues in the Chemistry Calculators hub — the Vapor Pressure of Water Calculator covers water’s other great table, and the Concentration Calculator handles ppm from the mass side.
Frequently Asked Questions
How do you convert EC to TDS?
Multiply EC in µS/cm by your scale’s factor: 0.50 for NaCl, 0.64 for 442, 0.55 for KCl, 0.70 on Hanna-style meters. An EC of 800 µS/cm is 512 ppm on the 442 scale and 400 ppm on the NaCl scale.
Why do two TDS meters give different readings on the same water?
They apply different conversion factors. Both measure the same EC; one multiplies by 0.5 and the other by 0.64 or 0.7. Check each meter’s scale in its manual and convert before comparing.
What TDS should drinking water be?
Taste and scaling guidance commonly treats up to about 500 ppm as acceptable, with the best palatability below roughly 300 ppm. TDS alone does not certify safety — it says nothing about bacteria or non-ionic contaminants.
What is the 442 scale?
A reference blend (sodium sulfate, sodium bicarbonate and sodium chloride) formulated to imitate natural fresh water, with a conversion factor of about 0.64. It is the standard scale in agriculture and aquaculture.
Does TDS measure everything dissolved in water?
A meter does not — it infers TDS from conductivity, which only ions produce. Dissolved sugars, silica and most organics are invisible to it. The laboratory TDS (evaporating and weighing the residue) counts them all.
How do I convert TDS back to EC?
Divide by the same factor you would multiply by: 512 ppm on the 442 scale is 512 ÷ 0.64 = 800 µS/cm (0.800 mS/cm). This calculator’s reverse direction does it directly.