EC to PPM Converter for Hydroponic Nutrients
EC (Electrical Conductivity) measures the total dissolved salts in your nutrient solution, expressed in milli-Siemens per centimeter (mS/cm). PPM (Parts Per Million) translates that conductivity into an approximate concentration of dissolved solids. Because EC meters and PPM meters are both widely used in hydroponics, growers must convert between the two using a scale factor: PPM = EC (mS/cm) × Scale. Three industry scales exist: 500 (Hanna/HM Digital), 640 (Eutech/Truncheon), and 700 (Bluelab). A nutrient solution reading EC 1.5 mS/cm equals 750 ppm on the 500 scale, 960 ppm on the 640 scale, or 1050 ppm on the 700 scale — a 40% spread that can cause serious over- or under-feeding if the wrong scale is assumed.
When to use this calculator
- Dialing in vegetative-stage nutrient strength for lettuce or basil (target EC 1.2–2.0 mS/cm / 600–1,000 ppm on scale 500) without overfeeding young root systems.
- Cross-checking a Bluelab Combo meter (scale 700) reading against a Hanna HI-9813 (scale 500) to confirm both meters agree on the same reservoir batch.
- Converting a nutrient manufacturer's feed chart (written in EC) to the PPM scale printed on your home TDS pen so you can follow the schedule exactly.
- Troubleshooting salt build-up in a deep-water culture (DWC) system when runoff PPM is 30%+ higher than input, signaling excessive fertilizer accumulation.
Calculation Example
- EC=1.5, Scale: 500
- ppm = 750
How it works
3 min readHow It's Calculated
EC meters pass a small electrical current through the solution and measure resistance. The conductivity value (mS/cm) is then multiplied by a fixed scale factor to produce PPM. Three scales dominate the industry:
PPM (Scale 500) = EC (mS/cm) × 500
PPM (Scale 640) = EC (mS/cm) × 640
PPM (Scale 700) = EC (mS/cm) × 700Most US and Canadian nutrient brands (General Hydroponics, Advanced Nutrients, Fox Farm) publish schedules in EC (mS/cm). TDS pens sold in North America predominantly use Scale 500. European and Australian brands (Canna, Plagron) and the Bluelab Truncheon use Scale 700. The Eutech TDS meters and some Milwaukee instruments use Scale 640.
> ⚠️ Temperature affects conductivity: EC increases ~2% per °C rise. Quality meters auto-compensate to 25 °C (77 °F). Always calibrate with a fresh 1,413 µS/cm standard solution.
---
Reference Table
| Crop / Growth Stage | Target EC (mS/cm) | PPM (×500) | PPM (×640) | PPM (×700) |
|---|---|---|---|---|
| Seedlings / Clones | 0.4 – 0.8 | 200 – 400 | 256 – 512 | 280 – 560 |
| Leafy greens (lettuce, spinach) | 1.2 – 1.8 | 600 – 900 | 768 – 1,152 | 840 – 1,260 |
| Herbs (basil, mint) | 1.0 – 1.6 | 500 – 800 | 640 – 1,024 | 700 – 1,120 |
| Tomatoes (vegetative) | 2.0 – 3.5 | 1,000 – 1,750 | 1,280 – 2,240 | 1,400 – 2,450 |
| Tomatoes (fruiting) | 3.5 – 5.0 | 1,750 – 2,500 | 2,240 – 3,200 | 2,450 – 3,500 |
| Cucumbers | 1.7 – 2.5 | 850 – 1,250 | 1,088 – 1,600 | 1,190 – 1,750 |
| Cannabis (vegetative) | 1.0 – 2.0 | 500 – 1,000 | 640 – 1,280 | 700 – 1,400 |
| Cannabis (flowering) | 1.5 – 2.5 | 750 – 1,250 | 960 – 1,600 | 1,050 – 1,750 |
| Strawberries | 1.0 – 1.4 | 500 – 700 | 640 – 896 | 700 – 980 |
Source: aggregated from University of Arizona Controlled Environment Agriculture Center (CEAC) crop guidelines and General Hydroponics Drain-to-Waste charts.
---
Typical Examples
Example 1 — Lettuce in DWC, Hanna meter (Scale 500)
Your meter reads EC 1.6 mS/cm.
PPM = 1.6 × 500 = 800 ppmThis sits in the optimal range for mature lettuce (600–900 ppm). No adjustment needed.
Example 2 — Tomatoes fruiting stage, Bluelab Combo (Scale 700)
Target from the nutrient brand's chart: EC 4.0 mS/cm.
PPM = 4.0 × 700 = 2,800 ppmIf you mistakenly used Scale 500: 4.0 × 500 = 2,000 ppm — a 28% under-read, potentially leading you to add extra nutrients and push EC to 5.6 mS/cm, causing salt stress.
Example 3 — Cross-meter check
Bluelab Truncheon shows 1,050 ppm (scale 700). Your Hanna pen should show:
EC = 1,050 ÷ 700 = 1.5 mS/cm
PPM (scale 500) = 1.5 × 500 = 750 ppmIf your Hanna reads 750 ppm, both meters agree. If not, one needs calibration.
---
Common Mistakes
1. Assuming all TDS meters use the same scale. A reading of 1,000 ppm means EC 2.0 on a Scale 500 meter but EC 1.43 on a Scale 700 meter — a huge difference that will either starve or burn plants.
2. Ignoring source water EC. Tap water can carry 0.2–0.8 mS/cm baseline. Always subtract source water EC before comparing to crop targets. Add nutrients to reach total target EC, not additional EC.
3. Not temperature-compensating. A nutrient solution at 30 °C (86 °F) reads ~10% higher EC than the same solution at 25 °C. Use meters with Automatic Temperature Compensation (ATC) or apply the correction: EC_corrected = EC_measured / (1 + 0.02 × (T°C − 25)).
4. Confusing EC units (mS/cm vs. µS/cm vs. dS/m). 1 mS/cm = 1,000 µS/cm = 1 dS/m. A TDS pen showing "1500 µS/cm" is EC 1.5 mS/cm, not 1,500 mS/cm.
5. Using worn-out calibration solution. Calibration sachets degrade after opening; an uncalibrated probe can drift ±0.3 mS/cm, translating to ±150–210 ppm error on scale 500/700.
---
Related Calculators
Since no internal related slugs were provided for this calculator, explore other gardening tools on Hacé Cuentas for plant spacing, grow light PPFD calculations, and soil amendment rates.
Frequently asked questions
What is the difference between Scale 500, 640, and 700?
These are the multipliers used to convert EC (mS/cm) to PPM. Scale 500 is standard on most North American TDS pens (Hanna, HM Digital). Scale 640 is used by Eutech and Milwaukee instruments. Scale 700 is used by Bluelab and European brands. For EC 2.0 mS/cm: Scale 500 → 1,000 ppm; Scale 640 → 1,280 ppm; Scale 700 → 1,400 ppm. Always check your meter's manual for its scale.
How do I find out which scale my TDS pen uses?
Check the product manual or the manufacturer's website. As a field test: measure a reference solution with a known EC (e.g., 1.413 mS/cm calibration fluid) and observe the ppm reading. 706 ppm = scale 500; 904 ppm = scale 640; 989 ppm = scale 700. Most pens sold under $30 on Amazon use scale 500 by default.
Should I measure EC or PPM for hydroponics?
EC (mS/cm) is the scientifically preferred unit because it is a direct physical measurement, not a conversion estimate. PPM is derived from EC via a scale factor, meaning it can misrepresent the actual ionic concentration if the wrong scale is used. Most agronomic research and commercial greenhouse guides (including USDA and university CEA programs) publish nutrient targets in EC. Use PPM only when your meter doesn't display EC.
What EC should I target for cannabis in the flowering stage?
Most experienced growers and commercial cannabis guidelines recommend EC 1.5–2.5 mS/cm (750–1,250 ppm on scale 500) during flowering. Late flowering (final 2 weeks before flush) is typically tapered to EC 1.0–1.5 mS/cm to allow the plant to consume stored nutrients. Exceeding EC 3.0 mS/cm during flowering can cause salt stress, tip burn, and nutrient lockout.
Does temperature affect EC and PPM readings?
Yes. EC increases approximately 2% per °C above 25 °C (77 °F), and decreases below it. A nutrient solution at 30 °C reads ~10% higher than the same solution at 25 °C. To manually correct: EC_corrected = EC_raw ÷ (1 + 0.02 × (T − 25)). Meters with Automatic Temperature Compensation (ATC) perform this correction internally, which is why ATC is strongly recommended for accurate readings.
How does source water EC affect my nutrient solution?
Source water (tap, well, or municipal) always carries some dissolved minerals, typically EC 0.1–0.8 mS/cm depending on your region. This baseline contributes to your total PPM and can include calcium, sodium, chlorine, and bicarbonates that interfere with nutrient ratios. Always measure your source water EC first, then add nutrients until you reach the total target EC. Failing to account for source water can result in over-concentration and calcium/magnesium lockout.
What happens if EC is too high or too low for my plants?
EC too high (above crop maximum): causes osmotic stress — roots cannot absorb water effectively, leading to wilting, tip burn, brown leaf margins, and stunted growth. EC too low (below crop minimum): causes nutrient deficiency symptoms — yellowing (nitrogen), purple tints (phosphorus), interveinal chlorosis (magnesium or iron). For most crops, maintaining EC within ±0.3 mS/cm of the target prevents both extremes. Flush the system with pH-balanced water if EC exceeds the upper limit by more than 0.5 mS/cm.
Can I use a soil TDS meter for hydroponics?
No. Soil TDS meters (blunt-probe types inserted into growing media) measure moisture and ionic concentration in the substrate, calibrated for soil salinity, not liquid solutions. For hydroponic reservoirs you need a liquid TDS/EC pen or inline EC probe calibrated with a 1,413 µS/cm or 2,760 µS/cm solution. Using a soil meter in your reservoir will give inaccurate readings and potentially damage the probe electrodes.