# EC vs PPM: the 500 and 700 scales, CF, and how to convert

> EC (electrical conductivity, in mS/cm) is what every meter actually measures. 'PPM' or TDS readings are EC multiplied by an assumed factor: 0.5 on the 500 (NaCl) scale, 0.7 on the 700 (442 or KCl-style) scale, sometimes 0.64. So 1.0 mS/cm reads 500 ppm on one meter and 700 ppm on another. Always talk in EC, or state the scale. Feed charts quoting ppm without a scale are ambiguous by 40%.

- Canonical: https://genetixbay.com/knowledge/ph-ec/ec-ppm-scales.html
- Category: pH and EC · Level: Beginner
- Evidence: Established · Status: Draft: awaiting expert review (some figures are being checked)
- Stage: Any stage · Systems: Any system
- Updated: 2026-10-31 · Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay Knowledge Base (genetixbay.com/knowledge)"

## Questions and answers

**How do I convert EC to ppm?** To convert EC to ppm, multiply EC in mS/cm by your meter's factor: 500 for the 500 or NaCl scale, 700 for the 700 scale, or 640 used by some agricultural sources. For example, 1.4 mS/cm equals 700 ppm on the 500 scale and 980 ppm on the 700 scale. To switch scales, multiply 500-scale ppm by 1.4.

**What is the difference between the 500 and 700 ppm scales?** The 500 and 700 ppm scales differ only in the factor a meter multiplies EC by: 0.5 on the 500 scale, based on [sodium](https://genetixbay.com/knowledge/glossary/sodium.md) [chloride](https://genetixbay.com/knowledge/glossary/chloride-nutrient.md) and common in North America, and 0.7 on the 700 scale, common in Europe and Australia. The same solution at 1.0 mS/cm reads 500 or 700 ppm, a 40 % gap. Neither is the true ppm of your nutrients.

**Is my meter 500 or 700 scale?** To tell whether your meter is 500 or 700 scale, check the manual or settings menu, or test it in a 1.413 mS/cm calibration standard: a 500-scale meter reads about 707 ppm and a 700-scale meter about 990 ppm. If the meter can display EC directly, switch to EC and stop using ppm altogether.

**What is CF in hydroponics?** CF, conductivity factor, is another unit for electrical conductivity used in hydroponics, equal to EC in mS/cm multiplied by 10. So 1.0 mS/cm is CF 10, 1.8 mS/cm is CF 18 and 2.2 mS/cm is CF 22. Like ppm, CF describes the same meter measurement; it says nothing about which nutrient ions are present.

**What EC is 1000 ppm?** 1,000 ppm equals 2.0 mS/cm EC on the 500 scale, about 1.56 mS/cm on the 640 scale and about 1.43 mS/cm on the 700 scale. That is why a 1,000 ppm [feed chart](https://genetixbay.com/knowledge/glossary/feed-chart.md) written on the 700 scale overfeeds by about 40 % when followed with a 500-scale meter. Check which scale the chart uses, or work in EC.

**mS/cm vs µS/cm vs dS/m?** mS/cm, µS/cm and dS/m are all units of electrical conductivity. 1 mS/cm equals 1,000 µS/cm, and 1 mS/cm equals exactly 1 dS/m and 1 mmho/cm. So a 1,413 µS/cm calibration standard is 1.413 mS/cm. EC readings should be temperature-corrected to 25 °C, so readings from meters without compensation are not comparable.

**Short answer.** Every "ppm" or TDS meter actually measures electrical conductivity (EC) and then multiplies by an assumed factor [1]. On the **500 scale** (NaCl) the factor is 0.5; on the **700 scale** (often called 442 or KCl-style) it is 0.7; some agricultural sources use 0.64 [1][2]. So the same solution at **1.0 mS/cm reads 500 ppm on one meter and 700 ppm on another.** Talk in EC (mS/cm), or always state the scale.

## What's happening

Dissolved salts conduct electricity; more ions, more conductivity. EC is measured in siemens per distance and corrected to 25 °C. It does not tell you *which* ions are present, only the total charge-carrying load.

"PPM" (TDS, total dissolved solids, mg/L) is an estimate: different salts conduct differently per gram, so no single factor is right for every solution. NDSU notes the true factor for natural waters rises with conductivity, from about 0.65 at 1,000 µS/cm to about 0.82 at 10,000 µS/cm [1]. Meter makers just pick one:

- **500 scale (NaCl, factor 0.5):** common in North America.
- **700 scale (factor 0.7):** common in Europe and Australia; often based on a "442" reference mix of sodium [sulfate](https://genetixbay.com/knowledge/glossary/sulfate.md), sodium bicarbonate and sodium chloride.
- **640:** used in some agricultural and extension material [2].

None is the "true" ppm of your nutrients. A nutrient solution's ppm-by-element (e.g. N 180 mg/L) is a different thing again.

## Units

| Unit | Equals |
|---|---|
| 1 mS/cm | 1,000 µS/cm |
| 1 mS/cm | 1 dS/m |
| 1 mS/cm | 1 mmho/cm |
| CF (conductivity factor) | EC (mS/cm) × 10 |

## Conversion table

| EC (mS/cm) | CF | ppm 500 scale | ppm 640 | ppm 700 scale |
|---|---|---|---|---|
| 0.2 | 2 | 100 | 128 | 140 |
| 0.4 | 4 | 200 | 256 | 280 |
| 0.6 | 6 | 300 | 384 | 420 |
| 0.8 | 8 | 400 | 512 | 560 |
| 1.0 | 10 | 500 | 640 | 700 |
| 1.2 | 12 | 600 | 768 | 840 |
| 1.4 | 14 | 700 | 896 | 980 |
| 1.6 | 16 | 800 | 1,024 | 1,120 |
| 1.8 | 18 | 900 | 1,152 | 1,260 |
| 2.0 | 20 | 1,000 | 1,280 | 1,400 |
| 2.2 | 22 | 1,100 | 1,408 | 1,540 |
| 2.5 | 25 | 1,250 | 1,600 | 1,750 |
| 3.0 | 30 | 1,500 | 1,920 | 2,100 |

Formulas: ppm₅₀₀ = EC × 500; ppm₇₀₀ = EC × 700; ppm₇₀₀ = ppm₅₀₀ × 1.4; ppm₅₀₀ = ppm₇₀₀ × 0.714.

## How strong should a cannabis feed be?

Typical working ranges are 0.6–1.0 mS/cm for [seedlings](https://genetixbay.com/knowledge/glossary/seedling.md) and clones, 1.2–1.8 in veg and 1.6–2.2 in flower (see [building a feed by stage](https://genetixbay.com/knowledge/nutrients/feed-by-stage.md)); these are *grower practice*. The research supports not going higher: in deep-flow hydro, doubling input from 2 to 4 mS/cm raised solution nutrients but not yield or quality [3]; in [rockwool](https://genetixbay.com/knowledge/glossary/rockwool.md), yield did not differ between 1.8 and 3 mS/cm, while 12 mS/cm cut yield about threefold [4].

## How to tell which scale your meter uses

1. Check the manual or the meter's setting menu (many let you choose).
2. Test a calibration standard: 1.413 mS/cm (1,413 µS/cm) solution should read ~707 ppm on the 500 scale and ~990 ppm on the 700 scale.
3. If the meter can display EC, switch it to EC and forget ppm.

## Common mistakes

- Following a "1,000 ppm" feed chart written on the 700 scale with a 500-scale meter, and [overfeeding](https://genetixbay.com/knowledge/glossary/nutrient-toxicity.md) by 40%.
- Forgetting that [tap water](https://genetixbay.com/knowledge/glossary/tap-water.md) already has EC; feed charts usually mean *total* EC, but some mean "added". Read the chart notes.
- Comparing EC readings taken at different temperatures on a meter without [automatic temperature compensation](https://genetixbay.com/knowledge/glossary/atc.md).
- Thinking EC tells you nutrient balance. It doesn't; sodium chloride has EC too.

## Evidence

- Meters measure EC and convert with an assumed factor; the factor varies with solution: *Established* [1].
- 500, 640 and 700 conventions: *Established* industry and extension usage [1][2].
- No cannabis yield gain above roughly 2–3 mS/cm: *Supported* (two studies [3][4]).

## 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
2. University of California Cooperative Extension, Ventura County. Salinity units and conversions (640 ppm = 1 dS/m). https://ucanr.edu/node/125832
3. Hershkowitz JA, Westmoreland FM, Bugbee B. 2025. Elevated root-zone P and nutrient concentration do not increase yield or cannabinoids in medical cannabis. Frontiers in Plant Science 16:1433985. https://doi.org/10.3389/fpls.2025.1433985
4. Baas R, Wijnen D. 2023. Effects of EC on cannabis in rockwool (1.8 to 12 mS/cm). Acta Horticulturae 1377. https://www.ishs.org/ishs-article/1377_97
5. Sonneveld C, Voogt W. 2009. Plant Nutrition of Greenhouse Crops. Springer. https://doi.org/10.1007/978-90-481-2532-6