# ppm500

> ppm500 is the TDS scale that converts EC to ppm with a factor of 0.5 (based on sodium chloride), so 1.0 mS/cm reads 500 ppm.

- Canonical: https://genetixbay.com/knowledge/glossary/ppm500.html
- Also written: 500 scale, ppm 500, NaCl scale, TDS 500, 0.5 conversion, NaCl ppm
- 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"

ppm500 (the 500 scale, NaCl scale) is the most common ppm scale on handheld [TDS](https://genetixbay.com/knowledge/glossary/tds.md) meters. The meter measures [EC](https://genetixbay.com/knowledge/glossary/ec.md) and multiplies by 0.5, a factor that approximates the relationship between conductivity and dissolved mass for a [sodium](https://genetixbay.com/knowledge/glossary/sodium.md) [chloride](https://genetixbay.com/knowledge/glossary/chloride-nutrient.md) solution. It is the usual default on meters sold in North America.

## In practice

A reservoir at 1.6 [mS/cm](https://genetixbay.com/knowledge/glossary/ms-cm.md) reads 800 ppm on a 500-scale meter. The same reservoir reads 1,120 ppm on a 700-scale meter. Neither is wrong; they are different conversions of the same EC. Trouble starts when growers copy a chart written for one scale with a meter set to the other, and over- or under-feed by 40%.

If your meter's manual says "NaCl" or "0.5", you are on ppm500. When in doubt, switch to EC mode and work in mS/cm.

## Units and formula

- ppm500 = EC (mS/cm) × 500
- EC (mS/cm) = ppm500 ÷ 500
- [ppm700](https://genetixbay.com/knowledge/glossary/ppm700.md) = ppm500 × 1.4

## Don't confuse with

- **ppm700**: the 442 scale, which reads 40% higher for the same solution.
- **ppm of an element in a recipe**: a true concentration, not a meter scale.

## Related terms

[ppm700](https://genetixbay.com/knowledge/glossary/ppm700.md), [EC](https://genetixbay.com/knowledge/glossary/ec.md), [TDS](https://genetixbay.com/knowledge/glossary/tds.md), [Conversion factor](https://genetixbay.com/knowledge/glossary/conversion-factor.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. 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