If you’ve ever watched two growers argue about whether 800 PPM is “light” or “hot,” you’ve seen why a solid cannabis EC levels chart beats a PPM number every time. Electrical conductivity (EC) is the one measurement that doesn’t lie about how strong your nutrient solution actually is, and once you learn to read it, dialing in feed becomes far less guesswork. This guide breaks down what EC and PPM really measure, why the same water can read as two different PPM numbers, and exactly what to target at each stage of growth.

What EC and PPM Actually Measure (and How They Relate)

EC measures how well your solution conducts electricity, reported in millisiemens per centimeter (mS/cm) or microsiemens (µS/cm). Dissolved mineral salts — the nitrogen, phosphorus, potassium, calcium, and magnesium your plants eat — are what carry that current. More dissolved salts means higher conductivity, so EC is a direct, honest proxy for how concentrated your feed is.

PPM (parts per million) is not a separate measurement. It’s just EC run through a conversion factor so it reads as a “weight” of dissolved solids. That conversion is where confusion creeps in, because there’s more than one factor in common use. EC itself never changes for a given solution; only the PPM label on the screen does. That’s the single most important thing to understand before you trust any number on a meter.

Practical takeaway: if you can, run your meter in EC. It’s universal, it’s unambiguous, and every serious feeding chart can be expressed in it. Use PPM as a secondary readout once you know which scale your meter speaks.

The 500 vs 700 Scale Confusion, Solved

Here’s why one grower’s water reads 500 PPM and another’s identical water reads 700. Meters convert EC to PPM using a factor, and two factors dominate the market:

• 500 scale (NaCl / “US” 0.5 factor): PPM = EC (in µS/cm) × 0.5. Common on Hanna and many US-market meters.
• 700 scale (442 / “Euro” 0.7 factor): PPM = EC × 0.7. Common on Bluelab and many European-market meters.

Run the math on an EC of 1.4 mS/cm (1400 µS/cm): the 500 scale reads 700 PPM, the 700 scale reads 980 PPM. Same water, same strength, two very different numbers. A grower following a “1000 PPM” recipe on the wrong scale could be feeding 40% harder or softer than intended — the fastest route to nutrient burn or a starving plant.

The fix is simple. Check which scale your meter uses (it’s in the manual or a settings menu), and always share EC alongside PPM so nobody has to guess. When in doubt, convert everything back to EC and compare there.

Cannabis EC Levels Chart by Growth Stage

Below is a stage-by-stage cannabis EC levels chart with target ranges. These are general, defensible ranges for a healthy plant in soilless or hydro media — treat them as starting points, not gospel. Genetics, environment, and media all shift the ideal. Note that EC includes your base water: if your source water is already 0.3 EC, that counts toward the total. PPM columns show both common scales so you can read whichever your meter speaks.

A few notes on reading it: young plants want it gentle — high EC scorches tender roots. Peak flower is where demand climbs, but “more” isn’t automatically better; watch the plant, not just the meter. In the final week or two, many growers taper EC down (or flush with plain, pH’d water) to let the plant finish clean.

How to Measure and Calibrate Your Meter

An EC meter is only as trustworthy as its last calibration. Probes drift as the reference solution ages and electrodes foul, so a meter that was perfect last month can quietly lie today. Build a simple habit:

• Calibrate on a schedule. Use fresh calibration solution (commonly 2.77 mS/cm / 1413 µS/cm standard) at least monthly — weekly if you feed daily or run a large room.
• Rinse, don’t wipe. Rinse the probe in distilled water between readings; wiping can leave residue or scratch the electrode.
• Mind temperature. EC readings shift with solution temperature. Use a meter with automatic temperature compensation (ATC), or take readings at a consistent temp.
• Store it wet. Many probes must stay in storage solution, not dry air. A dried-out electrode reads erratically.

Measure your input (the solution you feed) every time you mix, and log it. A two-minute record of “what I fed” turns troubleshooting from guesswork into detective work you can actually win.

Reading Runoff EC to Know If You’re Over- or Under-Feeding

Input EC tells you what you gave the plant. Runoff EC — the solution draining out the bottom of the pot — tells you what the plant didn’t take, and that’s where the real diagnosis lives. Collect a sample of runoff, let it settle to room temperature, and compare it to your input:

• Runoff higher than input: salts are accumulating in the media. The plant isn’t eating everything you’re giving it — you’re over-feeding, or root uptake has stalled. Dial the feed EC back and consider a lighter flush.
• Runoff roughly equal to input: you’re in a healthy balance. Hold your program.
• Runoff lower than input: the plant is stripping nutrients out fast — it’s hungry. You have room to raise EC.

Runoff readings also catch problems your input number can’t: salt buildup, pH drift in the root zone, and uptake issues from heat or overwatering. Check it a couple of times a week in flower and you’ll rarely be blindsided.

Why Inconsistent Hand-Feeding Wrecks EC

Here’s the uncomfortable truth about mixing by hand: every batch is a little different. A slightly heavier pour of base nutrient, water that evaporated and concentrated in the tank overnight, a probe that drifted since last week — and suddenly your “1.8 EC” is really 1.5 on one table and 2.1 on the next. Multiply that across dozens or hundreds of plants and shifting weather, and EC becomes a moving target you’re always chasing. Plants hate that inconsistency far more than they hate a number that’s slightly off; swings are what trigger lockout, burn, and uneven canopies.

The plants that finish most uniformly are the ones that get the same EC, every feed, every day. That’s hard to do with a bucket and a stir stick, but it’s exactly what automated fertigation is built for: continuous monitoring and dosing that holds EC and pH in a tight band without a human re-checking every tank.

If you’re running EC by hand and fighting drift across a commercial room, this is where the right equipment pays for itself. Eden Water Technologies’ precision fertigation platform — the Paragon — measures and corrects EC automatically so every plant on every table gets the same target strength, feed after feed. Dial in your chart once, and let the system hold the line. Talk to Eden about the Paragon and take the guesswork out of your feed.

EC & PPM FAQ