TDS (total dissolved solids) is the total mass of dissolved substances in water, properly measured in a lab by evaporating a filtered sample and weighing the residue, in mg/L (ppm). Handheld "TDS meters" do not do this. They measure EC and multiply it by a conversion factor chosen by the maker, usually 0.5 or 0.7, sometimes 0.64.
In practice
Because different ions conduct differently, no single factor is accurate for every solution. A nutrient mix rich in nitrate and potassium and a tap water rich in bicarbonate can have the same EC but very different true TDS. That is why two meters can read "800 ppm" and "1,120 ppm" for the same reservoir, and why ppm figures from someone else's chart are meaningless without the scale.
TDS meters are still useful for checking RO output, comparing your water to its usual baseline, and tracking reservoir changes over time with one meter. For sharing or following nutrient charts, use EC in mS/cm.
- TDS (ppm) ≈ EC (µS/cm) × conversion factor
- 500 scale: × 0.5; 700 scale: × 0.7; some agronomy sources use 640 ppm per dS/m
Don't confuse with
- EC: the real measurement; TDS on a meter is derived from it.
- ppm of a single element: e.g. 150 ppm N in a recipe; unrelated to meter TDS.
EC (electrical conductivity) measures how well a solution conducts electricity, a proxy for total dissolved salts; growers read it in mS/cm to gauge nutrient strength.The conversion factor is the multiplier a TDS meter applies to EC to report ppm, commonly 0.5, 0.64 or 0.7; it is why ppm readings differ between meters.Nitrate (NO3-) is the main form of nitrogen in hydroponic and soilless feeds; its uptake tends to push root-zone pH upward.Tap water is municipal or well water used for irrigation; its EC, alkalinity, hardness, sodium and disinfectant vary widely, so it should be tested before use.RO water is water stripped of most dissolved salts by reverse osmosis; it has near-zero EC and little buffering, so pH moves easily and Ca and Mg must be added.Input EC is the electrical conductivity of the nutrient solution going into the pot or system, in mS/cm, including the base EC of the source water.ppm500 is the TDS scale that converts EC to ppm with a factor of 0.5 (based on sodium chloride), so 1.0 mS/cm reads 500 ppm.ppm700 is the TDS scale that converts EC to ppm with a factor of 0.7 (the 442 natural-water standard), so 1.0 mS/cm reads 700 ppm.mS/cm (millisiemens per centimetre) is the standard unit of EC for nutrient solutions; 1 mS/cm equals 1 dS/m and 1,000 µS/cm.
