The Root of the Confusion
There is no single universal PPM conversion standard for measuring nutrient solution concentration. Three competing standards emerged from different manufacturers:
Conversion Formulas
PPM (500) = EC × 500PPM (640) = EC × 640PPM (700) = EC × 700EC (mS/cm) = PPM ÷ scale_factorCross-Scale Reference Table
| EC (mS/cm) | PPM 500 | PPM 640 | PPM 700 |
|---|---|---|---|
| 0.4 | 200 | 256 | 280 |
| 0.8 | 400 | 512 | 560 |
| 1.0 | 500 | 640 | 700 |
| 1.2 | 600 | 768 | 840 |
| 1.6 | 800 | 1024 | 1120 |
| 2.0 | 1000 | 1280 | 1400 |
| 2.4 | 1200 | 1536 | 1680 |
| 3.0 | 1500 | 1920 | 2100 |
Best Practice: Always Communicate in EC
When sharing nutrient readings with other growers — on Reddit, Discord, or forums — always include your EC value. PPM without a scale qualifier is ambiguous. Saying "my solution is at 1400 PPM" tells another grower nothing unless they know you are using a 700-scale meter.
EC 2.0 mS/cm is universally understood. It is the scientific standard, does not depend on any manufacturer convention, and is directly comparable across meters and countries.
Frequently Asked Questions
Why do different PPM meters give different readings?
PPM meters convert EC (electrical conductivity) to PPM using a fixed multiplier that varies by manufacturer. Hanna uses 500, Milwaukee uses 640, and Bluelab/Truncheon uses 700. The same nutrient solution at EC 2.0 reads as 1000, 1280, or 1400 PPM depending on which meter you use.
Which PPM scale is most accurate?
No scale is more accurate than another — they are different conversion conventions, not different measurements. EC itself is the universal standard. If accuracy and comparability matter, measure and communicate in EC (mS/cm) rather than PPM.
How do I know which PPM scale my meter uses?
Check your meter's manual or manufacturer website. Hanna Instruments meters (HI series) use the 500 scale. Milwaukee meters use 640. Bluelab and Truncheon meters use 700. When in doubt, measure a known EC calibration solution and see which multiplier matches.
Can I use the 700 scale targets with a Hanna meter?
No — or at least, not directly. If a guide says "target 1400 PPM" and uses the 700 scale, that is EC 2.0. On your Hanna (500 scale) meter, EC 2.0 reads as 1000 PPM. Always convert to EC first, then to the PPM reading your meter will show.
What is the conversion between 500 and 700 scale PPM?
To convert 500-scale PPM to 700-scale PPM: multiply by 1.4. Example: 1000 PPM on the 500 scale = 1400 PPM on the 700 scale. To go from 700 to 500: multiply by 0.714. The safest practice is to always convert both to EC and compare at that level.