Conductivity Converter (S/m, mS/cm, µS/cm, mho)

Understanding the conductivity converter — concept and principle

Electrical conductivity (EC) measures how easily a solution carries electric current, which rises with the amount of dissolved ions. It is a quick indicator of water purity, salinity and nutrient strength in hydroponics. Meters and reports use different units, such as µS/cm, mS/cm, S/m or the older mho, and this converter switches between them.

Conductivity is measured between plates a set distance apart, hence “per cm” or “per m”.

How it is calculated

All units are converted through a common base and divided by the target unit:

1 mS/cm = 1,000 µS/cm
1 S/m = 10,000 µS/cm (because 1 m = 100 cm and 1 S = 10⁶ µS)
1 mS/m = 10 µS/cm
1 S/cm = 1 mho/cm = 1,000,000 µS/cm

result = value × (size of "from" unit) ÷ (size of "to" unit).

The principle

Conductance (siemens, S) is the reciprocal of resistance in ohms; the old name "mho" is "ohm" spelled backwards and is the same unit. Conductivity divides out the geometry of the measuring cell: it is the conductance of a cube of liquid one unit long, which is why it is expressed per centimeter or per meter. Moving from "per cm" to "per m" changes the number by 100, while prefix changes move it by powers of 1,000.

Worked example

Default: 500 µS/cm to mS/cm.

1 µS/cm = 0.001 mS/cm.
500 × 0.001 = 0.5 mS/cm.
In other units: 0.05 S/m, 50 mS/m and 0.0005 S/cm (mho/cm).

How to read the result

Approximate values at 25 °C: ultrapure water about 0.055 µS/cm; distilled water a few µS/cm; drinking water commonly from about 50 to several hundred µS/cm; seawater about 50,000 µS/cm (50 mS/cm, 5 S/m). Many meters also estimate total dissolved solids (TDS) by multiplying EC in µS/cm by a factor of roughly 0.5 to 0.7 to get mg/L. A sample of 500 µS/cm therefore suggests about 250–350 mg/L of dissolved solids.

Common mistakes

Moving one decimal step too few: µS/cm to S/m divides by 10,000, not 1,000.
Comparing readings taken at different temperatures; conductivity typically rises by about 2% per °C, so meters report values compensated to 25 °C.
Treating a TDS estimate as a lab measurement; the factor depends on which ions are present.

Keep in mind: Conductivity shows the total amount of ions, not which ones. Safe-looking EC does not guarantee safe water, because harmful substances such as many organic pollutants or microbes barely change it.

FAQ

Frequently asked questions

Is mho the same as siemens?

Yes, 1 mho = 1 S, so 1 mho/cm = 1 S/cm.

How do I convert mS/cm to µS/cm?

Multiply by 1,000; for example 1.4 mS/cm = 1,400 µS/cm.

What is EC in hydroponics?

It indicates nutrient strength, usually in mS/cm; growers adjust it to suit the crop and growth stage.

Sources: BIPM, The International System of Units (SI Brochure), 9th edition · NIST Special Publication 811 (2008), Appendix B

Checked 8 Nov 2026. Factors from the SI Brochure (BIPM, 9th ed.) and NIST Special Publication 811 (2008), Appendix B.