How to use this EC ↔ PPM tool
Set your options in the calculator above, read the result, then verify with a calibrated meter when the decision matters. Bookmark this page for reservoir day — consistent units beat copying numbers from charts that never name their scale or bottle rate.
Formula reference
PPM ≈ EC (mS/cm) × scale factor (500, 640, or 700); EC = PPM ÷ scale
EC is the physical measurement; PPM is a translation
Electrical conductivity measures how easily current moves through your nutrient solution. Dissolved fertiliser salts increase conductivity, so EC rises with nutrient strength. That EC value is consistent between calibrated meters when expressed in mS/cm or µS/cm.
Parts per million displayed on a TDS meter is not a true chemical assay of every ion. The meter measures conductivity and multiplies by a scale factor. Without knowing whether your device uses 500, 640, or 700, a PPM number from a forum post can be misleading by 40%.
What the 500, 640, and 700 scales mean
The 500 scale (NaCl factor) is common on many Hanna-style and budget pens: PPM ≈ EC(mS/cm) × 500. The 700 scale (KCl factor) is common on some Bluelab and Australian/NZ charts: PPM ≈ EC × 700. A 640 scale appears on certain Truncheon and regional meters as a middle factor.
Example: 1.4 mS/cm is about 700 PPM on the 500 scale, 896 PPM on 640, and 980 PPM on 700. Same water, three PPM readings. When in doubt, convert everything back to EC before comparing recipes or troubleshooting deficiencies.
CF and µS/cm in the same family
Conductivity Factor (CF) is simply EC in mS/cm multiplied by 10, so 1.6 mS/cm equals CF 16. MicroSiemens per centimetre (µS/cm) equals EC × 1000, so 1.6 mS/cm equals 1600 µS/cm. These units describe the same conductivity without a TDS scale argument.
Logging EC or CF in your grow journal makes week-to-week comparisons reliable even if you replace a meter. If you prefer PPM for daily checks, note the scale next to every entry so future you — or a helper — can convert correctly.
Using conversions when mixing and diagnosing
After mixing nutrients, compare your reading to the crop’s target EC range first. Convert to PPM only if your bottle chart or meter speaks PPM. If plants show tip burn at a “safe” PPM, check whether you and the chart used different scales before cutting the dose drastically.
Rising EC between reservoir changes often means plants are taking up water faster than nutrients, concentrating the solution. Falling EC can mean heavy feeding or a leak diluting the tank. Converting readings to one unit helps you spot those trends early.
Frequently asked questions
Which TDS scale should I use?
Use the scale built into your meter so the display matches reality, or ignore PPM and work in EC. When following a chart, match that chart’s stated scale or convert its numbers to EC.
Why does my new meter disagree with my old one?
Different scales, calibration age, temperature compensation, and dirty probes all cause gaps. Calibrate both meters with the same solution and compare in EC if possible.
Is PPM the same as mg/L of fertiliser?
Not exactly. Meter PPM is a conductivity-derived estimate, not a lab analysis of each nutrient ion. It is useful for day-to-day strength checks, not for calculating exact N-P-K milligrams.
Related calculators
- Nutrient Mixing Calculator — Calculate exact nutrient amounts for your reservoir based on volume and concentration ratios.
- pH Adjustment Calculator — Estimate how much pH Up or pH Down solution is needed to reach your target pH.
- Reservoir Volume Calculator — Calculate the volume of your reservoir in litres or gallons based on container dimensions.
Keep learning
Pair this calculator with our pH & EC guide, crop targets in plant guides, and meter picks in product reviews.