TDS Calculator

Estimate total dissolved solids from water analysis ions or an EC reading.

Supports mg/L, ppm, ppb, g/L and g/mL plus five conductivity units.

Updated September 11, 2026

CreatorFrank Zhao

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What is TDS?

Total dissolved solids (TDS) measures the combined mass of every inorganic salt dissolved in a water sample, expressed in milligrams per litre (mg/L). The number tells you how "mineral-heavy" a water source is — from very soft rainwater at under 50 mg/L to brackish groundwater that can exceed 10 000 mg/L.

Water analysis mode

Enter up to 10 anions and 10 cations individually. The calculator sums them to give TDS and also shows the anion–cation balance so you can check whether the lab results are internally consistent.

Electrical conductivity mode

Enter a conversion factor and an electrical conductivity (EC) reading from a handheld meter. The calculator converts them to TDS and lets you back-solve for any of the three values.

How to use this calculator

Method A — Water analysis

Pick "Water analysis"

The radio selector at the top should show Water analysis (selected by default).

Enter ion concentrations

Type each ion value in mg/L (or switch units if your lab report uses g/L, ppm, etc.). Leave unknown ions at 0.

Read the TDS result

The Total dissolved solids (TDS) field updates automatically. You can also type a known TDS value directly.

Worked example

A tap-water sample has chloride at 35.5 mg/L, bicarbonate at 61 mg/L, sodium at 23 mg/L, calcium at 40.1 mg/L and potassium at 39.1 mg/L. All other ions are 0.

\mathrm{TDS} = 35.5 + 61 + 23 + 40.1 + 39.1

=

198.7\ \mathrm{mg/L}

The calculator also shows the anion sum (96.5 mg/L), cation sum (102.2 mg/L), the mEq/L and mmol/L equivalents, and the anion–cation balance. A balance under 10% means the analysis is likely consistent.

Method B — Electrical conductivity

Switch to "Electrical conductivity"

Click the Electrical conductivity radio option.

Enter the conversion factor (k)

The default is 0.67, a common approximation when the actual factor is unknown. Range: 0.55–0.8.

Enter the EC reading

Type the value measured by your EC or TDS meter, in µS/cm (or switch to mS/cm, dS/m, etc.).

Read or back-solve TDS

The TDS result appears automatically. You can also type a known TDS to back-solve for k or EC.

Worked example

A portable meter reads 1000 µS/cm and you use the default factor of 0.67.

\mathrm{TDS} = 0.67 \times 1000

=

670\ \mathrm{mg/L}

Because the EC mode is bidirectional, you can also enter 670 mg/L as TDS to recover the original EC or the conversion factor.

Formulas and variables

TDS from ion concentrations

\mathrm{TDS} = \sum_{i=1}^{10} \mathrm{A}_i \;+\; \sum_{j=1}^{10} \mathrm{C}_j

$\mathrm{A}_i$ and $\mathrm{C}_j$ are the anion and cation concentrations in mg/L. Every value you enter flows straight into the sum — there is no rounding or adjustment applied to individual ions.

TDS from electrical conductivity

\mathrm{TDS} = k \times \mathrm{EC}

$k$ is the conversion factor (dimensionless, typically 0.55–0.8) and $\mathrm{EC}$ is the electrical conductivity in µS/cm. The relation is fully reversible: given any two of the three values, the third is calculated automatically.

Anion–cation balance

\Delta\,[\%] = 100 \times \frac{\lvert E_c - E_a \rvert}{E_c + E_a}

$E_a$ and $E_c$ are the anion and cation sums in milliequivalents per liter (mEq/L). A balance under 10% is normal for a properly conducted analysis. Values above 10% are flagged in the calculator as a quality check — they may indicate missing ions, measurement error, or unreported species.

Equivalent and molar sums

E = \sum_i \frac{1000\,c_i}{M_i / z_i}

,\qquad

n = \sum_i \frac{1000\,c_i}{M_i}

$c_i$ is the stored concentration in g/L, so the factor of 1000 converts to mg/L. Dividing by the equivalent weight $M_i / z_i$ gives mEq/L, and dividing by the molar mass $M_i$ gives mmol/L. Both sums use the rounded molar masses and integer charges from the calculator's built-in ion table.

Tips and best practices

Choose the right k factor

The conversion factor varies with water composition. Use 0.67 as a first approximation for tap water. For more accurate results, calibrate k against a known TDS sample from the same source, or check the meter manufacturer's recommendation for your water type.

Check the balance first

Before relying on the TDS number, glance at the anion–cation balance. A balance above 10% means the lab data may be incomplete or inaccurate — it is worth reviewing the input values before drawing conclusions.

Use consistent units

The calculator handles unit conversions automatically, but entering all ions in the same unit (mg/L is most common) reduces the chance of a decimal-place error. Double-check that your lab report units match what you enter.

EC mode is bidirectional

In the electrical conductivity mode, any two of the three values (k, EC, TDS) determine the third. Type a known TDS to back-solve for the conversion factor, or type a known k and TDS to recover the EC reading.

Limitations

Approximate molar masses. The calculator uses rounded molar masses and integer charges (e.g. chloride = 35.5 g/mol, charge = 1). The mEq/L and mmol/L sums are therefore approximate — suitable for screening and comparison but not for high-precision stoichiometric work.
k factor variability. The EC-to-TDS conversion factor is not universal. Different water compositions produce different k values. Using the default 0.67 without calibration can introduce 10–30% error for some water types.
Not a drinking-water compliance tool. TDS is one indicator of water quality, but regulatory limits (e.g. WHO recommends below 600 mg/L for palatability, while some standards allow up to 1000 mg/L) depend on local regulations and the specific contaminants present. This calculator is for estimation and educational purposes — always verify with certified laboratory testing and applicable local standards.
Twenty ions only. The water analysis mode covers the 20 most common inorganic ions. If your sample contains significant amounts of other species (e.g. organic acids, heavy metals not listed), the TDS result will be lower than the true value.

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