TDS Calculator

Water Chemistry & Electrical Conductivity to TDS Relationships

Total Dissolved Solids (TDS) represents the total concentration of dissolved inorganic salts (calcium, magnesium, potassium, sodium, bicarbonates, chlorides, sulfates) and small amounts of organic matter dissolved in water, measured in parts per million (ppm) or mg/L.

Primary Water Quality Equations

• TDS Conversion Equation: TDS (ppm) = EC_25 × k_e

• Temperature Compensation Equation (25^ Standard):

EC_25 = (EC_T) / (1 + 0.0191 × (T - 25))

• Reverse Osmosis (RO) Membrane Rejection Rate:

Rejection \% = (Feed TDS - Permeate TDSFeed TDS) × 100\%

Standard Conversion Multipliers (k_e)

• k = 0.50: Common in Hanna instruments.

• k = 0.55 (Municipal Tap Water Standard): Standard for natural groundwater and urban utilities.

• k = 0.64 (442 Natural Water Standard): EPA benchmark for natural river basins.

• k = 0.70: Used in commercial greenhouse nutrient monitoring.

WHO / EPA Drinking Water Palatability Scale

• < 50 ppm: Demineralized / Distilled (flat taste).

• 50 - 300 ppm: Excellent / Ideal Drinking Quality (crisp, refreshing taste).

• 300 - 600 ppm: Good (moderate mineral content).

• 600 - 900 ppm: Fair (hard water, slight scale buildup).

• 900 - 1,200 ppm: Poor (heavy mineral taste).

• > 1,200 ppm: Unacceptable (exceeds EPA secondary aesthetic limits of 500 ppm).

How to Use This Calculator

Enter your Electrical Conductivity (EC in µS/cm) (e.g. 450 in µS/cm), TDS Conversion Factor (k) — choose from 0.55 (Municipal Tap Water & Groundwater Standard), 0.50 (NaCl / Pure Saline Standard), 0.64 (442 Natural Water Standard — EPA Benchmark), Water Sample Temperature (°C) (e.g. 25.0 in °C), and Post-RO Filter Permeate TDS (ppm — optional) (e.g. 25 in ppm). Results update instantly as you type — adjust any value to compare scenarios.

The Formula

Formula Reference: TDS Calculator

Input variables:

electricalConductivityInput — Electrical Conductivity (EC in µS/cm)

tdsFactorSelect — TDS Conversion Factor (k)

waterTempCelsiusInput — Water Sample Temperature (°C)

roPermeateTdsInput — Post-RO Filter Permeate TDS (ppm — optional)

Computed outputs:

calculatedTdsPpm — Total Dissolved Solids (TDS)

temperatureCompensatedEcMicroSiemens — Temperature-Compensated EC (25°C)

roRejectionEfficiencyPercentage — RO Membrane Rejection Rate

whoPalatabilityGrade — WHO Drinking Water Palatability Tier

waterSalinityGramsPerLiter — Estimated Salinity (Salt Content)

Mathematical relationships extracted from calculation logic:

ec25 = safeDiv(rawEc, 1 + tempCoeff * (tempC - 25))

tdsPpm = ec25 * factor

roRejection = Math.max(0, safeDiv(tdsPpm - roPermeate, tdsPpm) * 100)

Reference constants:

tempCoeff = 0.0191

Standard: NIST / ISO mathematical definitions

Worked Example

Testing tap water with an EC meter reading 450 µS/cm yields 247.5 ppm TDS. Passing this water through a RO filter down to 25 ppm indicates an 89.9% contaminant removal rate.

Understanding Your Result

Evaluate your result in the context of your specific use case and relevant industry standards or reference values. A single calculated result is a starting point — test two to three scenarios to understand how sensitive the outcome is to your key input assumptions. For high-stakes decisions, verify your result against a secondary source or subject-matter expert before acting.