# µS/cm

> µS/cm (microsiemens per centimetre) is a unit of EC one thousandth of mS/cm, used mostly for low-conductivity water such as RO and rainwater.

- Canonical: https://genetixbay.com/knowledge/glossary/us-cm.html
- Also written: microsiemens per centimetre, microsiemens per centimeter, uS/cm, µS, microsiemens
- Glossary category: pH, EC and water · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

µS/cm (microsiemens per centimetre) is a unit of [electrical conductivity](https://genetixbay.com/knowledge/glossary/ec.md). 1,000 µS/cm = 1 [mS/cm](https://genetixbay.com/knowledge/glossary/ms-cm.md). Water-quality reports and lab analyses often use it because source waters have low conductivity and whole numbers are easier to read. Many meters switch automatically between µS/cm and mS/cm, which causes misreadings if you miss the unit on the display.

## In practice

Typical values: distilled water about 0.5–3 µS/cm (rises as it absorbs CO2 from air), good [RO water](https://genetixbay.com/knowledge/glossary/ro-water.md) below about 20–50 µS/cm, rainwater roughly 5–50 µS/cm, many [tap waters](https://genetixbay.com/knowledge/glossary/tap-water.md) 200–800 µS/cm, and a flowering nutrient solution 1,400–2,200 µS/cm (1.4–2.2 mS/cm).

A rising RO output reading is the first sign of worn membranes: if your RO water creeps from 15 to 60 µS/cm, it is time to service the system.

The standard [calibration solution](https://genetixbay.com/knowledge/glossary/calibration-solution.md) for EC meters is 1,413 µS/cm (1.413 mS/cm), a potassium [chloride](https://genetixbay.com/knowledge/glossary/chloride-nutrient.md) solution at 25 °C.

## Units and formula

µS/cm ÷ 1,000 = mS/cm. On a [500 scale](https://genetixbay.com/knowledge/glossary/ppm500.md), ppm ≈ µS/cm × 0.5; on a [700 scale](https://genetixbay.com/knowledge/glossary/ppm700.md), ppm ≈ µS/cm × 0.7.

## Don't confuse with

- **mS/cm**: a thousand times larger; "1.4" and "1,400" can be the same solution.

## Related terms

[EC](https://genetixbay.com/knowledge/glossary/ec.md), [mS/cm](https://genetixbay.com/knowledge/glossary/ms-cm.md), [RO water](https://genetixbay.com/knowledge/glossary/ro-water.md), [TDS](https://genetixbay.com/knowledge/glossary/tds.md)

## Read more

- [EC vs PPM: the 500 and 700 scales, CF, and how to convert](https://genetixbay.com/knowledge/ph-ec/ec-ppm-scales.md)
- [Water quality for cannabis: RO vs tap, alkalinity, bicarbonates and chloramine](https://genetixbay.com/knowledge/ph-ec/water-quality.md)

## Sources

1. Prasad L, Meehan M. Using electrical conductivity and total dissolved solids meters to field test water quality. NDSU Extension WQ1923 (rev. 2025). https://www.ndsu.edu/agriculture/extension/publications/using-electrical-conductivity-and-total-dissolved-solids-meters-field-test-water-quality
2. University of California Cooperative Extension, Ventura County. Salinity units and conversions (640 ppm = 1 dS/m). https://ucanr.edu/node/125832