Right. Let’s talk about the single most common source of unnecessary panic in hydroponics. You’ve searched “EC to PPM,” stared at three different charts, and now you’re wondering if you’ve been slowly poisoning your plants for months. Relax. You haven’t. The confusion isn’t your fault—it’s a legacy of cheap meters, regional quirks, and advice passed down without the crucial context.
After a decade of this, in labs and in my own grow tents, I can give you the one rule that cuts through the noise: EC is the measurement. PPM is just a translation. Get that hierarchy straight in your head, and the whole mess starts to unravel.
Here’s what we’re untangling today:
What EC really measures (and why it’s the only number that truly matters)
How EC to PPM conversion works (it’s just arithmetic, I promise)
The maddening difference between the 500, 640, and 700 scales
When you should—and more importantly, shouldn’t—bother with an EC to PPM calculator
The classic mistakes I see every week (so you can skip them)
Let’s clear the air. And the water.
What Does EC Measure in Hydroponics?
EC stands for Electrical Conductivity. Strip away the jargon, and it’s brilliantly simple: it measures how well your nutrient solution conducts electricity.
Why should you care? Because the dissolved mineral salts in your fertilizer—the nitrates, the potassium, the calcium—are what carry that current. More salts, better conductivity. A higher EC.
So, EC tells you, in a universal language, the strength of your soup. It’s your direct line to knowing if you’re underfeeding or heading for a case of nutrient burn.
And here’s the key bit everyone misses: EC is a standard. An EC of 1.8 mS/cm is 1.8 mS/cm in Tokyo, Toronto, or my basement. PPM? Not so much.
Why Is EC Measured First, Then Converted to PPM?
This is the core of the confusion. Your meter isn’t some oracle. Here’s what’s actually happening inside that little plastic case:
It takes a direct, physical reading of the EC.
Then, it runs that number through a pre-programmed formula—a conversion factor—to spit out a PPM value for the display.
See the hierarchy?
EC = The raw, unfiltered data.
PPM = A calculated estimate based on which conversion factor the manufacturer picked.
This is why two growers can look at the same nutrient solution, see different PPM readings, and launch into a forum war. Their plants, meanwhile, are perfectly content because the EC is identical.
The takeaway? Pros talk EC. It’s the only metric that doesn’t lie.
What Is EC to PPM Conversion?
It’s just multiplication. The formula is child’s play:
PPM = EC × Conversion Factor
The problem, of course, is the “Conversion Factor.” It’s not a constant. It’s a choice. And that choice gives us the three scales that haunt your Google searches.
The Three EC to PPM Scales Explained
1. The 500 Scale (NaCl / Hanna)
Conversion: 1.0 EC = 500 PPM
The deal: It’s based on a sodium chloride solution. Common in North America, on older charts, and on a lot of entry-level meters. The numbers are nice and round, which is its main appeal. The downside? Your nutrient solution isn’t pure sodium chloride, so it’s a bit of an approximation.
2. The 640 Scale (Truncheon / EC × 640)
Conversion: 1.0 EC = 640 PPM
The deal: This one’s better. It’s tuned to reflect the conductivity of actual, mixed hydroponic fertilizer salts. It’s the standard in Australia and with commercial growers for a reason—it’s more realistic. If your meter uses this, you’re probably holding a decent piece of kit.
3. The 700 Scale (The 442 Scale)
Conversion: 1.0 EC = 700 PPM
The deal: Based on a specific blend of other salts. You’ll see it in some industrial water testing. For hydroponics? It’s the least accurate of the three and causes the most head-scratching when someone accidentally uses a 700-scale meter with a 500-scale chart.
Why EC Matters More Than PPM (Always)
Let me be blunt: slavishly chasing a PPM number is a great way to stress yourself out—and your plants.
PPM is fragile. It changes with your meter’s whims. A feeding chart rarely shouts which scale it’s using. EC is robust. It’s the bedrock. This isn’t a matter of opinion; it’s a matter of physics. The professionals use EC because it removes the guesswork.
When Should You Use an EC to PPM Calculator?
Only when you’re forced to speak someone else’s language. Use a calculator when:
You’re deciphering a feeding chart that only lists PPM.
You’re comparing notes with a grower who uses a different scale.
You’ve just switched meters and need to translate your old numbers.
The tool is there to prevent scale-mismatch errors. Use our EC ↔ PPM Converter to do the math instantly.
Common EC to PPM Mistakes (And How to Avoid Them)
The Universal PPM Fallacy: Assuming a PPM of 1100 is 1100 everywhere. Always ask, “What scale?”
Number Chasing: Obsessing over hitting a PPM target exactly. Your plants respond to EC ranges, not integers.
The Reservoir-Only Mindset: Not checking the EC of your runoff. What’s in the tank isn’t always what’s at the roots.
Chart Confusion: Blindly following a 500-scale chart with a 700-scale meter. This is how you get nutrient burn.
How EC to PPM Fits Into a Smart Feeding Strategy
Here’s my dead-simple, no-drama workflow:
Measure the EC of your plain water.
Add your nutrients slowly.
Stop when you hit your target EC for the plant’s stage.
Use a calculator to convert that EC to PPM only if you need to reference a chart.
Watch the plant, not just the meter. The numbers are a guide, not the gospel.
EC First, Always
If you take one thing from this, let it be this: EC is the signal. PPM is just noise.
Master that distinction, and you replace anxiety with control. You stop worrying about conflicting advice and start understanding what your plants are actually telling you. That’s the goal here—less guesswork, more growing.
Want to go deeper?
Use the EC ↔ PPM Calculator to lock in your conversions.
Shop Smart for your next meter—prioritize one that displays EC clearly and tells you which PPM scale it’s using. The flashy ones aren’t always the honest ones.
