0.65
325
mg/L
325
ppm
4
2
0.5
mS/cm
0.65
325
mg/L
325
ppm
4
2
0.5
mS/cm
The Conductivity to TDS Calculator converts electrical conductivity (EC) measurements to estimated total dissolved solids (TDS) using a user-adjustable conversion factor. Conductivity meters are widely used as a rapid, inexpensive alternative to the gravimetric TDS method, providing real-time measurements in the field, treatment plants, and industrial processes. The relationship between EC and TDS is approximately linear for most natural waters, but the exact conversion factor depends on the specific ionic composition. Common factors range from 0.5 to 0.7, with 0.5 typical for sodium chloride solutions, 0.55-0.65 for natural waters, and 0.7 for mixed mineral waters. This calculator allows you to apply the appropriate factor for your water type, providing estimated TDS in both mg/L and ppm along with a quality classification.
The conversion uses a simple linear relationship:
$$TDS \,(mg/L) = EC \,(\mu S/cm) \times k$$
Where:
The conversion factor depends on the dominant ions in solution:
EC measures the ability of water to conduct electricity, which depends on ion concentration, ion charge, ion mobility, and temperature. Most meters automatically compensate to 25°C reference temperature. The linear approximation holds well for EC below about 5000 μS/cm; at higher conductivities, ionic interactions cause deviations.
The estimated TDS provides a rapid screening value but should be validated against gravimetric TDS for regulatory purposes. If your conductivity readings consistently differ from gravimetric TDS by a fixed ratio, adjust the conversion factor to match. For routine monitoring where the water composition is stable, a calibrated EC-to-TDS conversion is highly reliable. For unknown water sources or variable compositions, use a factor of 0.65 as a reasonable starting estimate. The quality classification follows standard guidelines: Excellent (<300 mg/L) through Unacceptable (>1200 mg/L).
Inputs
Results
EC of 500 μS/cm with a factor of 0.65 gives estimated TDS of 325 mg/L — Good quality municipal water.
Inputs
Results
High conductivity groundwater at 3200 μS/cm gives estimated TDS of 1760 mg/L — requires desalination or blending before use as drinking water.
Electrical conductivity (EC) measures water's ability to conduct an electrical current. It depends on the concentration, charge, and mobility of dissolved ions. Pure water has very low conductivity (~0.05 μS/cm), while seawater has ~50,000 μS/cm. EC is measured using a conductivity probe with two or four electrodes in a known cell geometry.
Different ions have different equivalent conductances (ability to carry current per unit concentration). For example, H⁺ and OH⁻ have very high mobility, while Ca²⁺ and SO₄²⁻ have lower mobility. NaCl solutions have a factor near 0.5 because Na⁺ and Cl⁻ have moderate, similar mobilities. Mixed-salt waters with less conductive ions need higher factors.
Measure both EC and gravimetric TDS on several samples of your water source. Divide TDS by EC for each sample and average the results. This gives your site-specific conversion factor. For routine monitoring, this calibrated factor will give accurate TDS estimates as long as the water composition remains stable.
Temperature strongly affects conductivity — approximately 2-3% per °C. Most modern meters automatically compensate to a 25°C reference temperature. If using a non-temperature-compensated meter, you must manually correct readings. The conversion factor itself is temperature-independent when using compensated EC values.
For seawater and high-salinity waters (>5000 μS/cm), the linear relationship breaks down due to ionic interactions and ion pairing effects. For seawater, salinity is typically measured using a separate conductivity-salinity relationship (PSS-78 Practical Salinity Scale) rather than a simple TDS conversion factor.
Deionized water: 0.5-3 μS/cm. Distilled water: 0.5-5 μS/cm. Rainwater: 15-100 μS/cm. Tap water: 200-800 μS/cm. Rivers/lakes: 100-2000 μS/cm. Brackish water: 2000-10,000 μS/cm. Seawater: ~50,000 μS/cm. Industrial wastewater can exceed 100,000 μS/cm.
They measure the same thing — both use conductivity probes. A TDS meter simply applies an internal conversion factor to the EC reading and displays the result as TDS. Some meters show both values. The key difference is in how the reading is displayed and the assumed conversion factor, which is often fixed at 0.5 in inexpensive TDS pens.
No, conductivity only measures ionic (charged) dissolved substances. Non-ionic dissolved substances like sugars, alcohols, silica, and many organic compounds do not conduct electricity and are invisible to conductivity measurement. Gravimetric TDS captures both ionic and non-ionic dissolved matter.
With a properly calibrated conversion factor, accuracy of ±10% relative to gravimetric TDS is achievable. Using a generic factor of 0.65, accuracy may be ±20-30%. For regulatory compliance, gravimetric TDS is typically required. EC-based TDS is suitable for routine monitoring, process control, and field screening.
Conductivity is measured in siemens per meter (S/m) in SI units. In water chemistry, microsiemens per centimeter (μS/cm) is most common. 1 mS/cm = 1000 μS/cm. Some older literature uses micromhos per centimeter (μmho/cm), which is numerically identical to μS/cm. EC of 1 mS/cm corresponds roughly to TDS of 500-700 mg/L.
Roboculator Team
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