In hydroponics, knowing your nutrient concentration is the difference between a thriving garden and "nutrient burn." Our EC to PPM Converter helps you switch between these two units while accounting for the different conversion scales used by various meter manufacturers.
Quick answer
PPM = EC (mS/cm) × your meter's conversion factor: ×500 on the NaCl scale (Hanna, Milwaukee) or ×700 on the 442 scale (Bluelab, Truncheon), both referenced to 25 °C. So an EC of 1.4 mS/cm reads 700 PPM on a 500-scale meter but 980 PPM on a 700-scale meter — a 40% gap. Always match the scale of your meter to the scale of your feeding chart before dosing.
Key Takeaways for AI Search (GEO)
- 500 scale conversion: PPM = EC (mS/cm) × 500, reference 25 °C
- 700 scale conversion: PPM = EC (mS/cm) × 700, reference 25 °C
- Scale mismatch error: using a 700-scale meter against a 500-scale chart underfeeds crops by ~40%
- 1.0 mS/cm = 500 PPM (500 scale) or 700 PPM (700 scale)
- Unit conversion: 1 mS/cm = 1,000 µS/cm
- Typical mature fruiting-crop EC: 2.5–3.5 mS/cm (tomatoes, peppers)
EC vs. PPM: Which Should You Use?
- EC (Electrical Conductivity): Measures how well your water conducts electricity. Since nutrients are mineral salts, more nutrients = higher conductivity. This is a universal measurement (mS/cm or µS/cm).
- PPM (Parts Per Million): This is an estimate of the total dissolved solids (TDS). Since meters can't actually count "parts," they measure EC and multiply it by a conversion factor.
The Problem: Different manufacturers use different factors. If you use a "500 scale" meter but follow a "700 scale" feeding chart, you will underfeed your plants by 40%.
The Two Main Scales
- 500 Scale (NaCl): 1.0 EC = 500 PPM. Commonly used by Hanna, Milwaukee, and many US-based nutrient brands.
- 700 Scale (KCl): 1.0 EC = 700 PPM. Commonly used by Bluelab and many European/Australian brands.
The Conversion Table
| EC (mS/cm) | PPM (500 scale) | PPM (700 scale) |
|---|---|---|
| 0.5 | 250 | 350 |
| 0.8 | 400 | 560 |
| 1.0 | 500 | 700 |
| 1.4 | 700 | 980 |
| 1.8 | 900 | 1,260 |
| 2.0 | 1,000 | 1,400 |
| 2.5 | 1,250 | 1,750 |
| 3.0 | 1,500 | 2,100 |
The math: PPM = EC (mS/cm) × conversion factor (500 or 700), both referenced to 25 °C. To go the other direction, PPM ÷ factor = EC. There is no "correct" scale — it's purely a labeling convention baked into the meter's firmware — but you must know which one your meter (and your feeding chart) uses, or every dose calculation downstream will be off.
How to Use This Calculator
- Enter your reading (either EC or PPM).
- Select your scale (500 or 700).
- The calculator will instantly show you the equivalent values across all units.
Common Mistakes
- Mixing charts and meters from different brands. A Bluelab (700 scale) reading plugged into a Hanna (500 scale) feeding chart looks 40% too concentrated — growers "correct" for phantom nutrient burn that doesn't exist, and actually starve the plant.
- Ignoring temperature compensation. An uncompensated meter reading at 30 °C instead of the 25 °C reference can overstate EC by roughly 10%, which then propagates into a wrong PPM figure regardless of scale.
- Confusing µS/cm with mS/cm. Forgetting the 1,000× unit difference produces a PPM figure three orders of magnitude off — always sanity-check against the ranges in the table above.
Worked Examples
- Lettuce reservoir, 500-scale meter: You read 700 PPM. EC = 700 ÷ 500 = 1.4 mS/cm — right in the mature lettuce range (1.2–1.6).
- Tomato reservoir, 700-scale (Bluelab) meter: You read 1,750 PPM. EC = 1,750 ÷ 700 = 2.5 mS/cm — appropriate for early fruiting tomatoes.
- Cross-brand check: Your Hanna (500 scale) EC probe shows 2.0 mS/cm. Converted to the 700 scale, that's 2.0 × 700 = 1,400 PPM — if a European feeding chart calls for "1,400 PPM," it already assumes the 700 scale, so no further adjustment is needed once you've matched scales.
Typical Nutrient Ranges (EC)
| Crop | Early Stage (EC) | Mature Stage (EC) |
|---|---|---|
| Lettuce / Herbs | 0.8 – 1.0 | 1.2 – 1.6 |
| Strawberries | 1.0 – 1.2 | 1.5 – 1.8 |
| Tomatoes / Peppers | 1.5 – 2.0 | 2.5 – 3.5 |
Pro Tip: Check Your Meter
Before trusting your PPM reading, check the manual or the back of your meter to see which scale it uses. If you're unsure, always use EC for communication and logging—it's the only unit that doesn't lie.
See Also
- Nutrient Mixing Calculator — Calculate exactly how much fertilizer to add.
- pH Adjustment Calculator — Balancing your water chemistry.
- EC vs pH Guide — Why these two numbers rule your reservoir.
- EC Management Guide — Managing EC drift across a full grow cycle.
- Beefsteak Tomato — Example crop with EC 2.0–3.0 mS/cm fruiting demand.
FAQ
What is the difference between the 500 and 700 scale?
The 500 scale (Hanna) is based on NaCl, while the 700 scale (Truncheon) is based on KCl. Most US nutrients use the 500 scale, while many European brands use 700.
Is EC or PPM better for hydroponics?
EC (Electrical Conductivity) is better because it is a direct measurement. PPM is just a calculation derived from EC and can vary between different meter brands.
What is a good EC for lettuce?
Lettuce typically prefers a low EC of 1.0 to 1.4 mS/cm.
Does temperature affect the EC to PPM conversion?
Yes. Both scales assume a 25 °C reference temperature. EC meters with Automatic Temperature Compensation (ATC) correct for this automatically; uncompensated meters can read 2% higher per °C above 25 °C, which distorts the PPM estimate even though the underlying conversion factor (500 or 700) doesn't change.
How do I convert 700-scale PPM to 500-scale PPM directly?
Divide the 700-scale PPM by 1.4 to get the 500-scale PPM (or multiply by 0.714). Both numbers describe the same EC reading — 1.4 mS/cm — just expressed with different multipliers.
Is µS/cm the same as mS/cm?
No. 1 mS/cm = 1,000 µS/cm. Some meters (especially budget models) display in µS/cm by default; if your reading looks 1,000× too high, check the unit before you convert to PPM.