Understanding EC and PPM in hydroponics
Two new growers compare notes. One says they're running 800 PPM. The other says that's way too low. They're both running the same nutrient strength. They have different meters.
Welcome to the EC/PPM mess.
What you're actually measuring
A pure water sample doesn't conduct electricity well. Dissolved salts (fertilizer is mostly salts) carry charge, so more dissolved nutrients means more conductivity.
EC is the actual physical measurement: electrical conductivity, in millisiemens per centimeter (mS/cm). Sometimes shown as microsiemens (µS/cm), which is just 1000x larger. EC 1.6 mS/cm = 1600 µS/cm.
EC is the honest number. It's what the probe in your meter directly measures.
Where PPM comes from
PPM (parts per million) is calculated from EC. Meter manufacturers picked a conversion factor and printed it on the display, but they didn't all pick the same factor.
Three common scales:
| Scale | Factor | Where it's used |
|---|---|---|
| 500 | EC × 500 | Most US hobby meters (Bluelab, HM Digital, BlueDot) |
| 640 | EC × 640 | European meters, some commercial |
| 700 | EC × 700 | Truncheon meters, some Eutech |
So an EC of 1.6 reads as:
- 800 PPM on the 500 scale
- 1024 PPM on the 640 scale
- 1120 PPM on the 700 scale
Same water. Three numbers. This is why telling someone your PPM is meaningless without telling them which scale.
What to actually report
EC. Just report EC. It's the underlying measurement and there's only one way to read it.
If you must use PPM, always say which scale. "850 PPM (500 scale)" is fine.
Typical ranges
These are EC values, safe across all scales:
- Seedlings, cuttings: 0.4 to 0.8
- Leafy greens (lettuce, herbs): 0.8 to 1.4
- Tomatoes, peppers, cucumbers (veg): 1.6 to 2.0
- Tomatoes, peppers (fruiting): 2.0 to 3.5
- Cannabis veg: 1.2 to 1.6
- Cannabis bloom: 1.8 to 2.4
Your tap water already has an EC of 0.2 to 0.6 depending on where you live. You subtract that from your target when mixing.
Why your plants care
EC is roughly a stand-in for nutrient strength. Too low and the plant is starving. Too high and you've created an osmotic gradient that draws water out of the roots, which is what salt-burn actually is.
EC doesn't tell you anything about ratios. Two reservoirs at EC 1.8 can have completely different N:P:K ratios. The meter just sees dissolved ions, not which ones.
A note on truncheons
The classic Bluelab truncheon is one of the most reliable meters in the hobby. It uses the 700 scale by default, which trips up people moving from a cheap pen meter on 500. Same readings, different number on the display.
There's no universal "right" scale. Pick one and stick with it.