Conductivity to TDS Calculator

Every conductivity meter can be told to display TDS, and every one of them needs a conversion factor to do it. Get that factor wrong and the number on the display is wrong — silently, because the meter has no way to check it. This calculator shows the arithmetic and, more usefully, which factor belongs to which application.

Why the same water reads two different TDS values

Conductivity measures how well water carries an electric current, which depends on dissolved ions. TDS estimates total dissolved mass from that conductivity using a multiplier. The multiplier is not a physical constant — different salts carry current differently, so the same water can justify 0.5 or 0.7 depending on what is dissolved in it.

The practical consequence: a conductivity meter set to TDS is a calibrated guess. It is fine for trend watching and screening, and wrong for compliance reporting. Where a permit limit exists, report conductivity in µS/cm and let the limit be written in conductivity terms.

Conversion factors by application

Reading TDS on your own instrument

Most panel meters expose a factor setting, usually labelled K or TDS factor, with selectable values like 0.5, 0.6 and 0.7. The instrument multiplies the conductivity reading by that number and displays the product as mg/L. The default is often 0.5 because NaCl is the calibration standard, not because 0.5 is right for your water.

If your process is mostly a single salt — brine, a rinse stream, a specific fertiliser mix — you can determine your own factor in one afternoon: measure conductivity, take a gravimetric TDS on the same sample, and divide. That site-specific factor beats any default.

Temperature and compensation

Conductivity changes about 2% per °C in most aqueous solutions. An uncompensated meter reading 1000µS/cm at 15°C will read roughly 1200µS/cm at 25°C. Automatic temperature compensation normalises to a reference temperature, usually 25°C, so measurements stay comparable across seasons. For TDS derived from conductivity, this matters just as much as the reading itself.

Instruments for this measurement

Models shown are representative families, not a complete catalogue. Send us your application, water analysis and installation constraints and we will match instruments to it.

Frequently asked

This page provides calculation guidance and a screening cross-check. It is not a substitute for the measurement method named in your permit, and values shown are typical ranges rather than guarantees for a particular instrument.