Dissolved-ion sum, charge balance and EC–TDS converter

Sum dissolved-ion concentrations, check the charge balance of a water analysis, or estimate TDS from electrical conductivity using a selected conversion factor.

These are two different calculations. The first sums the listed constituents in your analysis. The second applies an empirical EC–TDS conversion factor; neither replaces a laboratory dissolved-solids measurement.

1. Dissolved-ion sum and charge balance

Enter the concentration of each named ion and select its unit. Blank entries are treated as zero. Enter results expressed as the ion itself: for example, nitrate as NO₃⁻, rather than nitrate as nitrogen (N). Convert hardness or alkalinity reported as CaCO₃ to the relevant ion concentrations before using this table.

Ion	Concentration	Unit
Cl⁻ chloride
NO₂⁻ nitrite
NO₃⁻ nitrate
F⁻ fluoride
HCO₃⁻ bicarbonate
CO₃²⁻ carbonate
SO₄²⁻ sulfate
H₂PO₄⁻ dihydrogen phosphate
Na⁺ sodium
K⁺ potassium
NH₄⁺ ammonium
Mg²⁺ magnesium
Ca²⁺ calcium
Fe²⁺ iron(II)
Mn²⁺ manganese(II)
Al³⁺ aluminium

Use a decimal point or comma, without thousands separators. For dilute water with a density near 1 kg/L, ppm is approximated as mg/L.

Your water-analysis results

Sum of listed ions (mg/L)

Absolute charge imbalance (%)

Cation sum (meq/L)

Anion sum, magnitude (meq/L)

Charge imbalance is 100 × |C − A| / (C + A), where C and A are the positive magnitudes of the cation and anion sums in meq/L. Values above 10% prompt a check in this calculator; this is a screening flag, not a universal laboratory acceptance limit. A balanced analysis can still omit constituents, for example if missing positive and negative charges cancel.

Show ion conversions, molar masses and charges

Ion	Molar mass g/mol	Charge
Cl⁻	35.5	-1
NO₂⁻	46.0	-1
NO₃⁻	62.0	-1
F⁻	19.0	-1
HCO₃⁻	61.0	-1
CO₃²⁻	60.0	-2
SO₄²⁻	96.1	-2
H₂PO₄⁻	97.0	-1
Na⁺	23.0	+1
K⁺	39.1	+1
NH₄⁺	18.0	+1
Mg²⁺	24.3	+2
Ca²⁺	40.1	+2
Fe²⁺	55.8	+2
Mn²⁺	54.9	+2
Al³⁺	27.0	+3

Results are rounded to four significant figures. Calculations retain full internal precision. Molar masses, charges and calculated outputs are fixed to prevent accidental changes.

How the ion sum differs from laboratory TDS

The ion sum includes only the 16 listed species. It does not include dissolved silica, dissolved organic substances or unlisted ions. It is therefore reported as a sum of listed ions, rather than a complete measured TDS value.

Residue-on-drying and sum-of-constituents methods also differ in how bicarbonate is treated. For a residue-equivalent sum, bicarbonate is commonly converted to carbonate mass using a factor of 0.4917. This calculator retains the entered bicarbonate mass in the ion sum; it does not claim to reproduce a dried-residue measurement.

2. EC ↔ estimated TDS converter

Use conductivity normalized to 25 °C, also called specific conductance. Temperature compensation depends on the solution and the measurement method; this converter does not apply temperature compensation.

Conversion direction

Conductivity unit

Input value

For EC → TDS, enter EC in the selected conductivity unit. For TDS → EC, enter TDS in mg/L (approximately ppm for dilute water).

TDS conversion factorCustom factor (used only when selected)

Estimated result

EC → TDS gives mg/L. TDS → EC gives the selected conductivity unit.

TDS (mg/L) ≈ k × EC (µS/cm). In reverse, EC (µS/cm) ≈ TDS (mg/L) / k. Select a factor suited to the water and consistent with the instrument or calibration method.

For 1000 µS/cm = 1 mS/cm, a factor of 0.5 gives an estimated 500 mg/L, while a factor of 0.7 gives 700 mg/L. These are different conversion scales, not universal relationships. The factor depends on water composition; changing a meter's scale does not change the water itself.

A typical EC/TDS meter measures conductivity and estimates TDS using a conversion factor. Conductivity alone does not identify individual nutrient concentrations, and this converter cannot determine nitrate, phosphate or potassium concentrations.

Read more about water conductivity and the definition of conductivity, or use our conductivity unit converter.