CF to EC is a straight divide by 10: the Commonwealth meter multiplies conductivity by 10, so 1.8 mS/cm reads CF 18 and 1,800 µS/cm. Reverse the math — CF ÷ 10 = EC in mS/cm — and a UK pen reading CF 24 is feeding 2.4 mS/cm the moment you convert it. The same converter reconciles PPM on the 500, 640 and 700 scales, so a chart printed in mS/cm, CF or PPM finally lands in one language before you mix.
Key takeaways
- Punch in EC and it returns PPM on the 500, 640 and 700 scales, plus CF and µS/cm, so any meter and any nutrient chart finally agree.
- A 1.8 mS/cm solution reads 900 ppm on the 500 scale, 1260 ppm on the 700 scale, and both are correct.
- EC drifts with solution temperature (~2%/°C).
- Calibrate in 1413 µS/cm buffer and let the reservoir settle before judging a feed.
How EC / PPM / CF Converter Hub Helps You Master This
- Enter the meter reading as EC in mS/cm — the only number you should actually trust from a conductivity pen.
- Select the TDS scale printed on your meter: 500 NaCl, 640 KCl or 700 (USA 442). The hub converts all three every time.
- Read PPM on your chosen scale plus CF and µS/cm simultaneously so any two pieces of gear can be reconciled.
- Compare any two nutrient formulas element-by-element at the same EC to see where one pulls ahead.
- For pH, enter current and target pH plus tank volume; the hub suggests the pH Down / pH Up dose for the medium.
- Retest 30 minutes after any pH or feed change — the dose is a starting point, never the final word.
The Formula Behind the Result
TDS on the USA 442 scale → text{PPM}_{700} = text{EC} ,(text{mS/cm}) times 700NaCl scale — typical of £/American hand-held meters → text{PPM}_{500} = text{EC} times 500EC Factor — the Commonwealth grower scale → text{CF} = text{EC} times 10Conductivity without the milli prefix → text{µS/cm} = text{mS/cm} times 1000pH up/down dose for the tank volume and buffering factor → text{mL} = V,(text{L}) times |Delta pH| times k_{medium}Elemental delta between two formulas at the running EC → Delta e_{i} = k_{i}^{A},text{EC} - k_{i}^{B},text{EC}
Target Ranges: What the Numbers Should Be
- PPM is a calculation the meter makes, not a physical measurement. Only EC is truly measured; the multiplier is a style choice your meter’s firmware baked in.
- Always match the scale to your nutrient charts. A 1.8 mS/cm solution reads 900 ppm on the 500 scale, 1260 ppm on the 700 scale, and both are correct.
- EC drifts with solution temperature (~2%/°C). Calibrate in 1413 µS/cm buffer and let the reservoir settle before judging a feed.
- pH Down liquid should be diluted 1:10 before dosing small tanks, and always distributed around the reservoir, never dumped in one spot.
Use the free Hydroponic System Selector Quiz — Kratky, NFT & Vertical Tower Blueprint to run the numbers on your own setup.
Use the free Reservoir Change Timer — EC Drift & Flush Schedule Blueprint to run the numbers on your own setup.
Use the free Nutrient Lab Pro — Elemental PPM Formulator to run the numbers on your own setup.
Compare every free calculator side-by-side on the MistCulture tools hub — each one keeps your workspace in sync.
Related guides
Worked example: CF 18 and CF 24 converted to EC and µS/cm
A UK grower’s pen reads CF 18 while the feed chart is written in mS/cm. Convert with CF ÷ 10: 18 ÷ 10 = 1.8 mS/cm, or 1.8 × 1000 = 1,800 µS/cm on the conductivity scale. A meter reading CF 24 means the tank is at 24 ÷ 10 = 2.4 mS/cm — a fruiting-strength feed roughly a third stronger than the 1.8 mS/cm vegetative baseline, and the comparison only makes sense after the conversion. Reversed, EC 2.4 mS/cm on a chart reads as CF 24 on the pen (2.4 × 10). CF never needs a scale debate the way PPM does — it is EC times 10, full stop — so the only mixing risk is a chart that quotes PPM on the 500, 640 or 700 scale without saying which, or a temperature swing shifting the reading ~2% per °C.
- CF 18 = 1.8 mS/cm = 1,800 µS/cm (CF ÷ 10).
- CF 24 = 2.4 mS/cm — fruiting-strength vs 1.8 mS/cm vegetative.
- EC 2.4 mS/cm reads CF 24 on the pen (EC × 10).
- CF is EC × 10 with no scale split; PPM requires the scale named.
Frequently Asked Questions
References & Science
- USDA National Agricultural Library
- HortScience — hydroponic nutrient studies
- Cornell CALS — hydroponic research manual
Frequently asked questions
How do I convert CF to EC on a UK meter?
CF to EC is a straight divide by 10: a pen reading CF 18 means 1.8 mS/cm, and CF 24 means 2.4 mS/cm. For µS/cm multiply the EC by 1,000 — 1.8 mS/cm is 1,800 µS/cm. There is no scale ambiguity because CF is always EC × 10.
What is the biggest mistake UK growers make with CF readings?
Treating a CF reading and a PPM chart as the same number. CF is EC × 10, but PPM varies by meter scale — 500, 640 or 700 — so a 1.8 mS/cm solution is CF 18 yet reads anywhere from 900 ppm to 1,260 ppm. Convert CF to mS/cm first, then to the PPM scale the chart uses.
Which tool helps me convert CF to EC?
The EC / PPM / CF converter on this page takes a single conductivity input and returns CF, µS/cm and all three PPM scales, so a chart printed in any unit can be checked against the pen before you mix. Calibrate in 1413 µS/cm buffer and let the tank settle first.
References
- Hoagland, D. R. & Arnon, D. I. (1950) — The Water-Culture Method for Growing Plants Without Soil
- Sonnenveld, C. & Voogt, W. — Plant Nutrition of Greenhouse Crops, Springer
- Resh, H. M. — Hydroponic Food Production (7th ed.), CRC Press
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