Let’s settle this. You’ve read the forum threads, seen the wildly different numbers, and now you’re standing in your grow room holding a meter, wondering who to trust. The intensity of the debate online would suggest we’re arguing about something profound—like quantum physics or the best way to brew coffee.
We’re not. We’re arguing about a rounding error in a translation.
After a decade of cutting through horticultural hype, here’s the unvarnished truth:
- None of these PPM scales are “wrong” in a mathematical sense.
- But asking which one is “correct” is like asking whether translating “love” to “amour” or “liebe” is more accurate. It misses the point entirely.
The real issue isn’t the number. It’s that most growers are speaking a language their meter doesn’t understand, using a dictionary they’ve never seen.
Let’s fix that.
What Is a PPM Scale, Really?
First, we need to kill a sacred cow. Your TDS meter—that little digital oracle you trust—is lying to you. Well, not lying. Let’s say it’s creatively interpreting.
Your meter does not measure PPM.
What it actually does is measure Electrical Conductivity (EC)—a direct, physical property of your solution. Then, it takes that EC reading and runs it through a tiny, pre-programmed algorithm: the PPM scale. It’s doing math on the fly to give you a number it thinks you want to see.
So when we talk about a 500 vs 640 vs 700 PPM scale, we’re not talking about measurement. We’re talking about interpretation. It’s the difference between measuring the temperature (EC) and then displaying it in Fahrenheit, Celsius, or Kelvin (PPM). The actual warmth is the same. The number is not.
That’s why the same reservoir can read 1000 on one device and 1400 on another, and both plants and growers end up confused.
The Three Main PPM Scales Explained
1. The 500 Scale (The Nostalgia Pick)
The Math: 1.0 EC = 500 PPM
The Origin Story: It’s based on the conductivity of plain old sodium chloride—table salt. It’s the default for a huge chunk of North American gear and the legacy charts that won’t die.
The Reality: It’s simple. It’s round. And it’s a pretty poor fit for the complex cocktail of calcium nitrate, potassium sulfate, and magnesium that actually makes up your nutrient mix. It persists not because it’s best, but because it’s first. A lesson in inertia.
2. The 640 Scale (The Grown-Up’s Choice)
The Math: 1.0 EC = 640 PPM
The Origin Story: This one’s calibrated against a more realistic blend of fertilizer salts. It’s the standard in commercial ops from Australia to the Netherlands for a reason.
The Reality: This is the scale most articles gloss over, usually because the writer hasn’t spent time in a real commercial facility. It’s less elegant, but it’s closer to the truth. If your meter uses a 640 PPM scale, someone along the line was thinking about actual plant science, not just neat numbers.
3. The 700 Scale (The Red Herring)
The Math: 1.0 EC = 700 PPM
The Origin Story: Based on a “442” blend of other salts, it’s rooted in general water hardness testing.
The Reality: For hydroponics? It’s mostly a nuisance. It inflates the PPM reading dramatically, which often scares growers into underfeeding. If you’re following a 500-scale chart with a 700-scale meter, you’re accidentally speaking a different language—and your plants are paying for the translation error.
PPM Scale Comparison Chart
| EC | 500 Scale | 640 Scale | 700 Scale |
|---|---|---|---|
| 1.0 | 500 | 640 | 700 |
| 1.5 | 750 | 960 | 1050 |
| 2.0 | 1000 | 1280 | 1400 |
| 2.5 | 1250 | 1600 | 1750 |
So… Which PPM Scale Is Correct?
Here’s my take, and I won’t sugarcoat it:
The 500 scale is the comfortable old myth.
The 640 scale is the inconvenient truth.
The 700 scale is the wrong tool for the job.
Once you internalize that, the entire debate evaporates. You measure EC. You track your plants’ response. You only convert to PPM when you absolutely must—like when you’re stubbornly using an old chart. (We have a tool for that, by the way—our EC ↔ PPM Hydroponics Converter. Use it to end the guesswork.)
Why EC Removes the Argument Entirely
This is where we mature as growers. EC cuts through this whole tedious debate. It is the source data. It doesn’t require conversion, interpretation, or a manual lookup. 1.8 mS/cm is 1.8 mS/cm on a $20 pen or a $2,000 lab monitor.
This is why, when you walk into a professional facility, you’ll hear them talking EC. Research papers cite EC. It’s the lingua franca. PPM is the regional dialect that causes all the arguments at the border.
The Smart Grower’s Takeaway
My workflow, stripped of all nonsense:
Measure in EC. Full stop. This is your North Star.
Watch the plant, not the meter. The numbers inform; the plant decides.
Use a PPM converter only when forced. When you’re decrypting an old chart or a forum post, use a tool that lets you pick the scale.
Never, ever mix scales and charts. It’s the fastest path to nutrient burn or starvation.
At Grow With Hydroponics, this mindset shift alone has saved growers countless crops from nutrient burn and deficiency. Shop Smart here means buying a meter that displays EC prominently and tells you—clearly—which PPM scale it’s using in the fine print. If it doesn’t, it’s not a tool; it’s a toy.
Stop Chasing PPM Numbers
The entire 500 vs 640 vs 700 PPM scale debate is a distraction. A pantomime we perform because we’ve been told to watch the translated subtitles instead of the film itself.
Shift your mindset to EC-first, and you don’t just settle an argument—you end it. Your feeding becomes calm, consistent, and based on something real. And frankly, that’s when the real growing begins.
