About the Salinity to Resistivity Calculator
This tool estimates water electrical conductivity and resistivity from a salinity or TDS value. It is useful for quick checks in labs, aquariums, hydroponics, process water, and general water-quality screening where an approximate conversion is acceptable.
How to use this calculator
- Enter the salinity or TDS value.
- Select the input unit: mg/L or ppm, ppt or g/L, or percent.
- Choose the TDS conversion factor that best matches the water or salt mixture.
- Enter the sample temperature in °C and the temperature coefficient in % per °C.
- Click Calculate Resistivity to view conductivity, resistivity, and 25°C-corrected values.
- Click Reset to restore the default example values.
How it works
The calculator first converts the entered salinity to ppm TDS. Values in mg/L or ppm are used directly, ppt or g/L are multiplied by 1,000, and percent values are multiplied by 10,000.
It then estimates electrical conductivity using the approximation TDS ppm = factor × EC in µS/cm. Rearranged, EC = TDS ppm ÷ factor. Resistivity at the sample temperature is calculated as ρ = 10,000 ÷ EC, giving the result in ohm-meters; ohm-centimeters are found by multiplying ohm-meters by 100.
For the optional 25°C correction, the calculator uses EC25 = ECt ÷ (1 + α × (T − 25)), where α is the entered temperature coefficient converted from percent to a decimal. The corrected resistivity is then 10,000 ÷ EC25. These results are approximate because actual conductivity depends on ion composition, meter calibration, and measurement method.
Example calculation
For 500 ppm TDS, a 0.64 conversion factor, 25°C sample temperature, and a 2.0%/°C coefficient: EC = 500 ÷ 0.64 = 781.25 µS/cm. Resistivity = 10,000 ÷ 781.25 = 12.8 Ω·m, or 1,280 Ω·cm. Since the sample is already at 25°C, the corrected EC and resistivity are the same.
Frequently asked questions
Is ppm the same as mg/L for this calculator?
Yes. For dilute water-quality calculations, the calculator treats mg/L and ppm as equivalent TDS units.
Which TDS conversion factor should I use?
Use 0.50 for sodium-chloride-like solutions, 0.64 for a common natural water estimate, and 0.70 for mixed salts or higher-factor estimates. The correct factor depends on ion composition.
Why does resistivity decrease when salinity increases?
More dissolved ions allow water to conduct electricity more easily, increasing conductivity. Since resistivity is the inverse of conductivity, it decreases as salinity rises.
What does the 25°C correction mean?
Conductivity changes with temperature, so the calculator estimates what the conductivity and resistivity would be at the common reference temperature of 25°C.
Can this replace a conductivity meter?
No. It provides an educational estimate from TDS and assumptions. Use a calibrated conductivity or resistivity meter for critical lab, process, or compliance measurements.