Total Dissolved Solids (TDS

Hydrochemistry, Water Quality & Total Dissolved Solids Metrology

Total Dissolved Solids (TDS) measures the combined content of all inorganic minerals and organic matter in water:

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• Temperature Compensation & TDS Conversion Formulas (EPA Standards)**:

EC_25 = EC_T1 + α(T - 25^)

TDS = EC_25 × k_e

Salinity (PPT) = TDS1,000, Hardness as CaCO_3 ≈ TDS × 0.60

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• Industry Standard Conversion Factors (k_e)**:

• k_e = 0.50: Pure Sodium Chloride (NaCl) calibration (used in many hydroponic TDS pens).

• k_e = 0.65: Standard freshwater / municipal drinking water (recommended general average).

• k_e = 0.70: '442 Natural Water' standard (40\% Na_2SO_4, 40\% NaHCO_3, 20\% NaCl).

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• WHO Drinking Water Palatability Ratings**:

• < 300 mg/L: Excellent

• 300 - 600 mg/L: Good (Standard high quality municipal tap water)

• 600 - 900 mg/L: Fair (Noticeable mineralized groundwater)

• 900 - 1,200 mg/L: Poor (Heavy mineral aftertaste)

• > 1,200 mg/L: Unacceptable

How to Use This Calculator

Enter your Measured Electrical Conductivity (EC in μS/cm) (e.g. 500 in μS/cm), Water Sample Temperature (°C) (e.g. 20.0 in °C), TDS Conversion Factor (k_e) — choose from k_e = 0.50 (Pure NaCl Solution Standard - Hydroponics EC:TDS), k_e = 0.65 (Standard Freshwater / Municipal Tap Water - Recommended), k_e = 0.70 (442 Natural Water Standard: 40% Na₂SO₄, 40% NaHCO₃, 20% NaCl), and Water Source Quality Benchmark — choose from Custom Water Sample, Municipal Tap Water (500 μS/cm @ 20°C, ke=0.65) → EC₂₅ = 555.6 μS/cm | TDS = 361.1 mg/L (Good Palatability), Reverse Osmosis (RO) Purified (30 μS/cm @ 25°C, ke=0.50) → TDS = 15.0 mg/L (Ultra-Pure). Verify your units match the formula standard (SI vs imperial) before entering values.

The Formula

Temperature compensation to 25°C standard (alpha = 0.020 / °C = 2.0% per °C standard for freshwater)

Salinity in PPT (parts per thousand) = TDS in mg/L / 1000

Worked Example

A homeowner testing their well water measures an electrical conductivity of 500 μS/cm at 20°C, discovering that after 25°C temperature compensation, the true TDS is 361.1 mg/L—well within the EPA 500 mg/L drinking water aesthetic standard.

Understanding Your Result

Scientific calculations require careful attention to significant figures and units. Your result is only as precise as your least precise input — expressing the result with more significant figures than the inputs implies false precision. For laboratory or regulatory work, verify which measurement standard applies (SI, ASTM, ISO) and whether temperature, pressure, or concentration conditions affect the formula. Cross-reference results against published reference values for your specific material or compound.