# ppm700

> ppm700 is the TDS scale that converts EC to ppm with a factor of 0.7 (the 442 natural-water standard), so 1.0 mS/cm reads 700 ppm.

- Canonical: https://genetixbay.com/knowledge/glossary/ppm700.html
- Also written: 700 scale, ppm 700, 442 scale, TDS 700, 0.7 conversion, natural water scale
- 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"

ppm700 (the 700 scale) converts [EC](https://genetixbay.com/knowledge/glossary/ec.md) to ppm by multiplying by about 0.7. It is based on the "442" natural-water reference, a mixture of 40% [sodium](https://genetixbay.com/knowledge/glossary/sodium.md) [sulfate](https://genetixbay.com/knowledge/glossary/sulfate.md), 40% sodium bicarbonate and 20% sodium [chloride](https://genetixbay.com/knowledge/glossary/chloride-nutrient.md) meant to mimic the ions in fresh water. It is common on meters sold in Europe and Australia and in some nutrient charts.

## In practice

A solution at 1.6 [mS/cm](https://genetixbay.com/knowledge/glossary/ms-cm.md) reads 1,120 ppm on a 700-scale meter and 800 ppm on a 500-scale meter. If a [feeding chart](https://genetixbay.com/knowledge/glossary/feed-chart.md) says "1,000 ppm in flower" and does not say which scale, it could mean 1.43 mS/cm (700 scale) or 2.0 mS/cm ([500 scale](https://genetixbay.com/knowledge/glossary/ppm500.md)). That is a big gap at the plant.

Check your meter's manual or settings for "442" or "0.7". When sharing numbers, give [EC in](https://genetixbay.com/knowledge/glossary/input-ec.md) mS/cm, or at least label ppm as ppm500 or ppm700.

## Units and formula

- ppm700 = EC (mS/cm) × 700
- EC (mS/cm) = ppm700 ÷ 700
- ppm500 = ppm700 ÷ 1.4

## Don't confuse with

- **ppm500**: the NaCl scale; 40% lower readings for the same solution.
- **ppm640**: a factor used in some agronomy references (640 ppm per dS/m).

## Related terms

[ppm500](https://genetixbay.com/knowledge/glossary/ppm500.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