Understanding EC and parts per million in hydroponics
EC measures conductivity. parts per million is derived with three different conversion factors, which is why your meter and your nutrient bottle disagree.
Two new growers compare notes. One says they're running 800 parts per million. The other says that's way too low. They're both running the same nutrient strength. They have different meters.
Welcome to the EC/parts per million 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 parts per million 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 parts per million on the 500 scale
- 1024 parts per million on the 640 scale
- 1120 parts per million on the 700 scale
Same water. Three numbers. This is why telling someone your parts per million 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 parts per million, always say which scale. "850 parts per million (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.
Why EC is more useful than parts per million
EC (electrical conductivity) measures the total ionic content of the solution by passing a small electrical current between two probes. More dissolved ions means more current flows, which registers as higher EC. The reading is universal: 2.0 mS/cm means the same thing on every meter worldwide.
PPM (parts per million) is derived from EC by multiplying by a conversion factor. The problem is that there are three different conversion factors in common use:
- 500 scale: parts per million = EC x 500. Used by Hanna Instruments, most US-made meters, and many US nutrient companies.
- 640 scale: parts per million = EC x 640. A middle ground used by some European meters.
- 700 scale: parts per million = EC x 700. Used by Bluelab, Truncheon, and common in European and Australian markets.
The same solution reading EC 2.0 mS/cm shows as 1000 parts per million on a 500-scale meter, 1280 parts per million on a 640-scale meter, and 1400 parts per million on a 700-scale meter. The solution hasn't changed. The number on the screen has.
This causes real confusion when growers follow a nutrient feeding chart that says "1200 parts per million" without specifying the scale. If the chart was written for a 700-scale meter and you're reading on a 500-scale meter, your target is actually 857 parts per million on your meter, not 1200. Getting this wrong means overfeeding by 40%.
EC targets by crop
Leafy greens and herbs (lettuce, basil, cilantro, mint): EC 0.8-1.5 mS/cm. These are light feeders. Push above 1.5 and leaf edges may burn or the plant may bolt prematurely.
Fruiting vegetables (tomatoes, peppers, cucumbers): EC 2.0-3.5 mS/cm during fruiting. Higher EC concentrates sugars and improves flavor in fruit, but above 3.5 the plant struggles with water uptake, especially in warm conditions.
Seedlings and fresh transplants: EC 0.5-0.8 mS/cm. Young roots are sensitive to high salt concentration. Start low and increase over 1-2 weeks as the plant establishes.
Strawberries: EC 1.0-1.8 mS/cm. Moderate feeders that are sensitive to both under and overfeeding.
Meter calibration and care
EC meters drift with use and exposure to nutrient solutions. Calibrate monthly using a known reference solution (1.413 mS/cm is the standard calibration value). The process takes 2 minutes: dip the probe in calibration fluid, press the calibrate button, done.
Between uses, rinse the probe in distilled or deionized water. Don't let nutrient solution dry on the probe; the salt crystals interfere with accurate readings. Some probes require storage in KCl storage solution to maintain the electrode surface.
Cheap EC pens ($10-15) work but drift faster and have less precise temperature compensation than mid-range meters ($30-60). For a serious grow, a Bluelab pen ($80-100) or a Hanna HI98312 ($50-70) provides reliable readings for years with proper care.
Use the EC to parts per million converter to translate between scales and verify your meter readings against a known reference.
Quick conversion table
For mental math when reading forums or nutrient labels:
EC 1.0 mS/cm = 500 parts per million (500 scale) = 640 parts per million (640 scale) = 700 parts per million (700 scale). EC 2.0 mS/cm = 1000 parts per million (500) = 1280 parts per million (640) = 1400 parts per million (700). EC 3.0 mS/cm = 1500 parts per million (500) = 1920 parts per million (640) = 2100 parts per million (700).
When someone posts "my parts per million is 1400" and doesn't mention which scale, you can't know if that's EC 2.0 (700 scale, moderate for lettuce) or EC 2.8 (500 scale, very high for lettuce). Always ask which scale, or request EC directly.