Total dissolved solids is the total mass of everything dissolved in water – mostly inorganic salts of calcium, magnesium, sodium and potassium with bicarbonate, chloride, sulfate and nitrate – expressed in mg/L (equal to ppm for fresh water). It is a quick indicator of water quality: very low TDS tastes flat, high TDS tastes salty or bitter and can scale pipes and kettles. The US EPA secondary standard for drinking water is 500 mg/L.
There are two ways to get it without evaporating a sample. If you have a laboratory analysis of the major ions, TDS is their sum. Because dried residue loses about half of its bicarbonate as CO₂ and water, Standard Methods counts bicarbonate as 0.4917 × HCO₃⁻ when comparing with a measured residue – you can choose either convention. Converting each ion to milliequivalents also lets you check the analysis: positive and negative charges should balance within a few percent.
Field meters instead measure electrical conductivity and multiply it by a factor k. The factor depends on which ions dominate – about 0.5 for sodium chloride solutions, up to 0.8 for sulfate-rich water – with 0.64–0.67 a common default.
How to use the TDS Calculator
- Method
- Bicarbonate counted as
- Chloride (Cl⁻)
- Dihydrogen phosphate (H₂PO₄⁻)
- Sulfite (SO₃²⁻)
- Sulfate (SO₄²⁻)
- Carbonate (CO₃²⁻)
- Bicarbonate (HCO₃⁻)
- Fluoride (F⁻)
- Nitrite (NO₂⁻)
- Nitrate (NO₃⁻)
- Silicate (SiO₄⁴⁻)
- Sodium (Na⁺)
- Calcium (Ca²⁺)
- Magnesium (Mg²⁺)
- Iron (Fe²⁺)
- Barium (Ba²⁺)
- Potassium (K⁺)
- Strontium (Sr²⁺)
- Manganese (Mn²⁺)
- Aluminium (Al³⁺)
- Lithium (Li⁺)
- Electrical conductivity (at 25 °C)
- Conversion factor k
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
In analysis mode the calculator adds the mg/L of every ion you enter, converts each to meq/L (concentration × charge ÷ molar mass) and reports the anion–cation balance. In conductivity mode it multiplies the conductivity at 25 °C by k. The result is classified as fresh, brackish, saline or brine water.
Worked example
A tap-water analysis gives (mg/L) Cl⁻ 20, SO₄²⁻ 25, HCO₃⁻ 150, NO₃⁻ 5, F⁻ 0.5, Na⁺ 15, Ca²⁺ 45, Mg²⁺ 10 and K⁺ 2. The anions add to 200.5 mg/L and the cations to 72 mg/L, so TDS = 272.5 mg/L (excellent for drinking). In milliequivalents, anions total 3.65 meq/L and cations 3.77 meq/L – a balance error of 1.6%, which is acceptable. Counting bicarbonate as 0.4917 × 150 = 73.8 mg/L instead gives a dried-residue TDS of 196.2 mg/L. By conductivity, a meter reading of 500 µS/cm × 0.67 = 335 mg/L.
TDS classification
| TDS (mg/L) | Water type |
|---|---|
| below 1,000 | Fresh |
| 1,000–10,000 | Brackish |
| 10,000–35,000 | Saline |
| above 35,000 | Brine (hypersaline) |
Tips
- Use the final volume of the solution, not the volume of solvent added, for molarity.
- For dilutions, C₁V₁ = C₂V₂ only holds when both concentrations use the same unit.
- Add acid to water, never water to acid.
- Everything is calculated in your browser.
Common mistakes to avoid
- Mixing w/w, w/v and v/v percentages.
- Forgetting that pH is logarithmic – one unit is a tenfold change in [H⁺].
- Using pKa where Ka is asked for (or the other way round).
Inputs explained
| Field | What to enter |
|---|---|
| Method | Choose one: Water analysis (sum of ions), Electrical conductivity |
| Bicarbonate counted as | Choose one: Full HCO₃⁻ (sum of ions), 0.4917 × HCO₃⁻ (dried residue, Standard Methods) |
| Chloride (Cl⁻) | Units: mg/L, ppm, µg/L, g/L |
| Dihydrogen phosphate (H₂PO₄⁻) | Units: mg/L, ppm, µg/L, g/L |
| Sulfite (SO₃²⁻) | Units: mg/L, ppm, µg/L, g/L |
| Sulfate (SO₄²⁻) | Units: mg/L, ppm, µg/L, g/L |
| Carbonate (CO₃²⁻) | Units: mg/L, ppm, µg/L, g/L |
| Bicarbonate (HCO₃⁻) | Units: mg/L, ppm, µg/L, g/L |
| Fluoride (F⁻) | Units: mg/L, ppm, µg/L, g/L |
| Nitrite (NO₂⁻) | Units: mg/L, ppm, µg/L, g/L |
| Nitrate (NO₃⁻) | Units: mg/L, ppm, µg/L, g/L |
| Silicate (SiO₄⁴⁻) | Units: mg/L, ppm, µg/L, g/L |
| Sodium (Na⁺) | Units: mg/L, ppm, µg/L, g/L |
| Calcium (Ca²⁺) | Units: mg/L, ppm, µg/L, g/L |
| Magnesium (Mg²⁺) | Units: mg/L, ppm, µg/L, g/L |
| Iron (Fe²⁺) | Units: mg/L, ppm, µg/L, g/L |
| Barium (Ba²⁺) | Units: mg/L, ppm, µg/L, g/L |
| Potassium (K⁺) | Units: mg/L, ppm, µg/L, g/L |
| Strontium (Sr²⁺) | Units: mg/L, ppm, µg/L, g/L |
| Manganese (Mn²⁺) | Units: mg/L, ppm, µg/L, g/L |
| Aluminium (Al³⁺) | Units: mg/L, ppm, µg/L, g/L |
| Lithium (Li⁺) | Units: mg/L, ppm, µg/L, g/L |
| Electrical conductivity (at 25 °C) | Units: µS/cm, mS/cm, dS/m, S/m |
| Conversion factor k | A number |
What each result means
- Cations = [Sodium (Na⁺)] + [Calcium (Ca²⁺)] + [Magnesium (Mg²⁺)] + [Iron (Fe²⁺)] + [Barium (Ba²⁺)] + [Potassium (K⁺)] + [Strontium (Sr²⁺)] + [Manganese (Mn²⁺)] + [Aluminium (Al³⁺)] + [Lithium (Li⁺)]
- Estimated conductivity (k = 0.67) = [Total dissolved solids]÷0.67
- Dissolved solids per US gallon = [Total dissolved solids]×3.785411784÷1000
Formula
TDS (analysis) = Σ anions + Σ cations (mg/L)TDS (conductivity) = k × EC (k ≈ 0.55–0.8, 0.67 typical; EC in µS/cm at 25 °C)meq/L = mg/L × charge ÷ molar massBalance error = (Σcations − Σanions) ÷ (Σcations + Σanions) × 100Worked example
Inputs — Method: Water analysis (sum of ions); Bicarbonate counted as: Full HCO₃⁻ (sum of ions); Chloride (Cl⁻): 20 mg/L; Dihydrogen phosphate (H₂PO₄⁻): 0 mg/L; Sulfite (SO₃²⁻): 0 mg/L; Sulfate (SO₄²⁻): 25 mg/L; Carbonate (CO₃²⁻): 0 mg/L; Bicarbonate (HCO₃⁻): 150 mg/L; Fluoride (F⁻): 0.5 mg/L; Nitrite (NO₂⁻): 0 mg/L; Nitrate (NO₃⁻): 5 mg/L; Silicate (SiO₄⁴⁻): 0 mg/L; Sodium (Na⁺): 15 mg/L; Calcium (Ca²⁺): 45 mg/L; Magnesium (Mg²⁺): 10 mg/L; Iron (Fe²⁺): 0 mg/L; Barium (Ba²⁺): 0 mg/L; Potassium (K⁺): 2 mg/L; Strontium (Sr²⁺): 0 mg/L; Manganese (Mn²⁺): 0 mg/L; Aluminium (Al³⁺): 0 mg/L; Lithium (Li⁺): 0 mg/L.
Results — Anions = 200.5 mg/L; Cations = 72 mg/L; Total dissolved solids = 272.5 mg/L; Water type = Fresh water (below 1,000 mg/L); Drinking-water taste rating = Excellent (below 300 mg/L); Anions = 3.65 meq/L; Cations = 3.772 meq/L; Ion balance error = 1.64 %; Estimated conductivity (k = 0.67) = 407 µS/cm; Dissolved solids per US gallon = 1.032 g.
Key terms
- Total dissolved solids (TDS)
- Total mass of dissolved substances per litre of water.
- Electrical conductivity (EC)
- Ability of water to carry current, rising with dissolved ions; usually µS/cm at 25 °C.
- Milliequivalent (meq)
- Millimoles multiplied by the ion's charge.
- Ion balance
- Comparison of total cation and anion charge in an analysis.
Frequently asked questions
What is a good TDS level for drinking water?
Below about 300 mg/L tastes excellent and up to 600 mg/L good; the US EPA secondary (aesthetic) limit is 500 mg/L and water above 1,200 mg/L is generally unpalatable (WHO).
How do I convert conductivity to TDS?
Multiply by a factor k: TDS (mg/L) ≈ 0.67 × EC (µS/cm). 500 µS/cm gives about 335 mg/L.
Are mg/L and ppm the same for TDS?
Yes for fresh water, whose density is close to 1 kg/L. For seawater and brines, ppm (mg/kg) is slightly lower than mg/L.
Why does my ion balance not close?
An error above about ±5% suggests a missing ion (often potassium, nitrate or carbonate), an analytical error or a high organic content.
Why multiply bicarbonate by 0.4917?
When a sample is dried, 2 HCO₃⁻ → CO₃²⁻ + CO₂ + H₂O, so only 60.008 ÷ (2 × 61.016) = 0.4917 of the bicarbonate mass remains in the residue.
Does TDS show whether water is safe?
No. TDS is mainly an aesthetic and scaling indicator; lead, arsenic, nitrate or bacteria can be harmful at very low TDS.
What is the TDS of seawater?
About 35,000 mg/L, mostly sodium chloride.
What formula does the TDS Calculator use?
TDS (analysis) = Σ anions + Σ cations (mg/L).