# TDS Calculator (Total Dissolved Solids)

URL: https://speedycalculator.com/calculators/chemistry/tds/
Last updated: 2026-10-30

> Calculate total dissolved solids (TDS) in water by adding the concentrations of ions from a water analysis – with an anion–cation balance check in meq/L – or estimate TDS from electrical conductivity with a conversion factor.

## Inputs

- Method
- Bicarbonate counted as
- Chloride (Cl⁻) (mg/L, ppm, µg/L, g/L)
- Dihydrogen phosphate (H₂PO₄⁻) (mg/L, ppm, µg/L, g/L)
- Sulfite (SO₃²⁻) (mg/L, ppm, µg/L, g/L)
- Sulfate (SO₄²⁻) (mg/L, ppm, µg/L, g/L)
- Carbonate (CO₃²⁻) (mg/L, ppm, µg/L, g/L)
- Bicarbonate (HCO₃⁻) (mg/L, ppm, µg/L, g/L)
- Fluoride (F⁻) (mg/L, ppm, µg/L, g/L)
- Nitrite (NO₂⁻) (mg/L, ppm, µg/L, g/L)
- Nitrate (NO₃⁻) (mg/L, ppm, µg/L, g/L)
- Silicate (SiO₄⁴⁻) (mg/L, ppm, µg/L, g/L)
- Sodium (Na⁺) (mg/L, ppm, µg/L, g/L)
- Calcium (Ca²⁺) (mg/L, ppm, µg/L, g/L)
- Magnesium (Mg²⁺) (mg/L, ppm, µg/L, g/L)
- Iron (Fe²⁺) (mg/L, ppm, µg/L, g/L)
- Barium (Ba²⁺) (mg/L, ppm, µg/L, g/L)
- Potassium (K⁺) (mg/L, ppm, µg/L, g/L)
- Strontium (Sr²⁺) (mg/L, ppm, µg/L, g/L)
- Manganese (Mn²⁺) (mg/L, ppm, µg/L, g/L)
- Aluminium (Al³⁺) (mg/L, ppm, µg/L, g/L)
- Lithium (Li⁺) (mg/L, ppm, µg/L, g/L)
- Electrical conductivity (at 25 °C) (µS/cm, mS/cm, dS/m, S/m)
- Conversion factor k

## Results

- Anions
- Cations
- Total dissolved solids
- Water type
- Drinking-water taste rating
- Anions
- Cations
- Ion balance error
- Estimated conductivity (k = 0.67)
- Dissolved solids per US gallon

## 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 mass
Balance error = (Σcations − Σanions) ÷ (Σcations + Σanions) × 100
```

## Explanation

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 typebelow 1,000Fresh1,000–10,000Brackish10,000–35,000Salineabove 35,000Brine (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

FieldWhat to enterMethodChoose one: Water analysis (sum of ions), Electrical conductivityBicarbonate counted asChoose one: Full HCO₃⁻ (sum of ions), 0.4917 × HCO₃⁻ (dried residue, Standard Methods)Chloride (Cl⁻)Units: mg/L, ppm, µg/L, g/LDihydrogen phosphate (H₂PO₄⁻)Units: mg/L, ppm, µg/L, g/LSulfite (SO₃²⁻)Units: mg/L, ppm, µg/L, g/LSulfate (SO₄²⁻)Units: mg/L, ppm, µg/L, g/LCarbonate (CO₃²⁻)Units: mg/L, ppm, µg/L, g/LBicarbonate (HCO₃⁻)Units: mg/L, ppm, µg/L, g/LFluoride (F⁻)Units: mg/L, ppm, µg/L, g/LNitrite (NO₂⁻)Units: mg/L, ppm, µg/L, g/LNitrate (NO₃⁻)Units: mg/L, ppm, µg/L, g/LSilicate (SiO₄⁴⁻)Units: mg/L, ppm, µg/L, g/LSodium (Na⁺)Units: mg/L, ppm, µg/L, g/LCalcium (Ca²⁺)Units: mg/L, ppm, µg/L, g/LMagnesium (Mg²⁺)Units: mg/L, ppm, µg/L, g/LIron (Fe²⁺)Units: mg/L, ppm, µg/L, g/LBarium (Ba²⁺)Units: mg/L, ppm, µg/L, g/LPotassium (K⁺)Units: mg/L, ppm, µg/L, g/LStrontium (Sr²⁺)Units: mg/L, ppm, µg/L, g/LManganese (Mn²⁺)Units: mg/L, ppm, µg/L, g/LAluminium (Al³⁺)Units: mg/L, ppm, µg/L, g/LLithium (Li⁺)Units: mg/L, ppm, µg/L, g/LElectrical conductivity (at 25 °C)Units: µS/cm, mS/cm, dS/m, S/mConversion factor kA 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

## Worked 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.

## FAQ

**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).


## Sources

- [Standard Methods for the Examination of Water and Wastewater – 2540 Solids and 1030 E (checking analyses)](https://www.standardmethods.org/)
- [USGS – Water science school](https://www.usgs.gov/special-topics/water-science-school)
- [US EPA – Water research and drinking water](https://www.epa.gov/ground-water-and-drinking-water)