TDS Calculator

What TDS Actually Measures

Total Dissolved Solids (TDS) is the combined mass of all dissolved minerals, salts, and other charged substances in water, reported in milligrams per liter (mg/L), the same unit as parts per million (ppm) for dilute aqueous solutions. Wikipedia's total dissolved solids entry notes TDS is a bulk measure, not an identity test, a water sample and a soft drink can share a similar TDS number while containing entirely different dissolved substances. This calculator covers the two ways TDS is actually determined from raw data, the gravimetric reference method and the sum-of-ions method, plus a quick lookup to assess whether an existing reading falls in a safe, palatable range.

The Gravimetric Method: TDS by Evaporation

The reference-standard method filters a known volume of water, evaporates it in a pre-weighed dish, and weighs the dried residue left behind, using TDS (mg/L) = (Final Weight − Initial Weight) × 1,000 ÷ Sample Volume (L). Northern Arizona University's gravimetric TDS determination protocol specifies a typical 100 to 250 mL sample size and an oven-drying step at a controlled temperature before the final weighing, since incomplete drying leaves residual moisture that inflates the apparent residue weight. This is the method water treatment labs use to validate a field meter's estimate, since it measures the residue directly rather than inferring it from an electrical property.

The Sum of Ions Method

Where a full ion panel already exists, TDS can also be calculated by simply adding up the measured concentration of each major dissolved ion, calcium, magnesium, sodium, chloride, sulfate, and bicarbonate typically account for the large majority of dissolved solids in natural water. Omni Calculator's TDS tool confirms this sum-of-ions approach as one of the standard determination methods alongside gravimetric and conductivity-based estimates, useful whenever a lab report already breaks a sample down ion by ion rather than reporting a single bulk figure.

TDS vs Conductivity-Based Estimates

Most handheld "TDS meters" don't measure dissolved solids directly at all, they measure electrical conductivity and multiply by a conversion factor to estimate TDS, since dissolved ions are what carry the electrical current. Lenntech's conductivity-to-TDS converter explains why that estimate shifts depending on which conversion factor a meter uses and which ions dominate a given sample, meaning two meters can show different TDS numbers for the identical water. If you specifically have an EC (conductivity) reading in mS/cm or μS/cm and need that conversion, our EC to PPM Converter handles that estimate directly, this calculator instead covers the gravimetric and sum-of-ions methods that don't depend on a conductivity reading at all.

Is My TDS Reading Safe? WHO Palatability Ratings

The World Health Organization's Guidelines for Drinking-water Quality classify TDS by taste and consumer acceptance rather than by a single hard safety cutoff. WHO's official TDS background document sets out the palatability scale this calculator's assessment mode uses: under 300 mg/L is rated excellent, 300 to 600 good, 600 to 900 fair, 900 to 1,200 poor, and above 1,200 mg/L unacceptable. The U.S. EPA separately sets a 500 mg/L secondary (aesthetic, non-enforceable) standard, and WHO also notes water with extremely low TDS can taste flat and insipid, TDS palatability isn't simply "lower is always better."

What a High or Low Reading Might Actually Mean

A high TDS reading by itself doesn't tell you what's dissolved, and that ambiguity comes up constantly in real questions about tap water results. This Quora discussion on a 459 mg/L tap water reading makes the key point directly, a reading in that range often reflects naturally occurring calcium and magnesium hardness rather than contamination, and a bulk TDS number can't distinguish the two without a fuller mineral or contaminant-specific test. Our General PPM Calculator is useful if you need to work out a specific solute's own ppm concentration rather than the bulk TDS figure.

Common Mistakes in TDS Testing

In the gravimetric method, the most common error is under-drying the residue, moisture left in the dish adds weight that isn't actually dissolved solid, inflating the result. Health Canada's guidance document on dissolved solids also notes that comparing TDS readings across different meters or methods without accounting for their different conversion factors and reference temperatures is a frequent source of apparently contradictory results between two tests of the same water. Always let a gravimetric sample cool in a desiccator before the final weighing, since a warm dish continues to lose weight to evaporation while sitting on the scale.