# Conversion factor

> The conversion factor is the multiplier a TDS meter applies to EC to report ppm, commonly 0.5, 0.64 or 0.7; it is why ppm readings differ between meters.

- Canonical: https://genetixbay.com/knowledge/glossary/conversion-factor.html
- Also written: EC to ppm conversion, TDS factor, ppm conversion factor, k factor, EC conversion
- 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"

A conversion factor, in this context, is the number used to estimate [total dissolved solids](https://genetixbay.com/knowledge/glossary/tds.md) (ppm or mg/L) from measured [electrical conductivity](https://genetixbay.com/knowledge/glossary/ec.md). True TDS depends on which ions are present, so the factor is an approximation. Meters use fixed values: 0.5 (NaCl, the [500 scale](https://genetixbay.com/knowledge/glossary/ppm500.md)), 0.7 (the 442 natural-water scale) or 0.64 (a common agronomy figure).

## In practice

The factor is the reason two growers with identical reservoirs post different ppm numbers. To compare, convert both back to EC: divide ppm by the factor and then by 1,000 to get [mS/cm](https://genetixbay.com/knowledge/glossary/ms-cm.md).

Example: 900 ppm on a 0.5 meter is 1.8 mS/cm; 900 ppm on a 0.7 meter is 1.29 mS/cm. Following a 700-scale chart on a 500-scale meter therefore feeds about 40% stronger than intended.

Some meters let you set the factor. Choose one, label it, and keep it for the life of your logs.

## Units and formula

- ppm = EC ([µS/cm](https://genetixbay.com/knowledge/glossary/us-cm.md)) × factor
- EC (µS/cm) = ppm ÷ factor
- Common factors: 0.5, 0.64, 0.7

## Don't confuse with

- **CF (conductivity factor)**: an EC unit (EC × 10), not a ppm multiplier.
- **Temperature compensation coefficient**: the about 2% per °C correction for EC.

## Related terms

[ppm500](https://genetixbay.com/knowledge/glossary/ppm500.md), [ppm700](https://genetixbay.com/knowledge/glossary/ppm700.md), [TDS](https://genetixbay.com/knowledge/glossary/tds.md), [EC](https://genetixbay.com/knowledge/glossary/ec.md), [CF](https://genetixbay.com/knowledge/glossary/cf.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)

## Sources

1. University of California Cooperative Extension, Ventura County. Salinity units and conversions (640 ppm = 1 dS/m). https://ucanr.edu/node/125832
2. 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