Understanding EC and PPM in Hydroponics
You've mixed your nutrient solution, your plants are in the system — and then you see two numbers on your meter: EC 1.8 and PPM 900. Are those good? Too high? Too low? Understanding EC and PPM is one of the most fundamental skills in hydroponics, and getting them right directly determines your yield and plant health.
Quick Summary
- EC (Electrical Conductivity) measures how many dissolved ions are in your water — a direct proxy for total nutrient concentration.
- PPM (Parts Per Million) expresses the same thing as a weight ratio — but beware: there are two different conversion standards in use.
- EC is more reliable than PPM for hydroponics because it is a universal physical measurement independent of meter brand.
What Is EC and How Is It Measured?
Electrical Conductivity (EC) is a measure of how well a solution conducts electricity. Pure distilled water conducts almost no electricity. The moment you dissolve salts into it — Calcium Nitrate, Potassium Sulfate, Epsom Salt — the dissociated ions become charge carriers, and the solution starts to conduct.
EC is measured in mS/cm (millisiemens per centimetre). A typical hydroponic reservoir runs between EC 1.2 and EC 3.5 depending on the crop and growth stage. Your tap water might already start at EC 0.3–0.8 — always measure your source water before mixing nutrients.
A good EC meter works by passing a tiny electrical current between two probes submerged in your solution and measuring the resistance. Temperature affects conductivity significantly (warmer water conducts better), so always use a meter with automatic temperature compensation (ATC).
What Is PPM — and Why Two Different Numbers?
PPM (Parts Per Million) describes how many milligrams of dissolved solids are present per litre of water (mg/L). It sounds simple, but here is where most beginners get confused: PPM meters do not actually measure PPM directly. They measure EC and then apply a conversion factor.
The problem is that two different conversion standards exist:
500 Scale (Hanna / HM Digital)
PPM = EC × 500
EC 2.0 → 1000 PPM. Common in the US market. Used by most HM Digital and Hanna meters.
700 Scale (Truncheon / Bluelab)
PPM = EC × 700
EC 2.0 → 1400 PPM. Standard in Europe and Australia. Used by Bluelab and Truncheon.
Always state which scale you usewhen sharing nutrient recipes online. A "1200 PPM" recipe on a US forum means EC 2.4 — which translates to 1680 PPM on a European meter. This mismatch causes more burned plants than any other single mistake in hydroponics.
EC Target Values by Crop and Stage
There is no universal "correct" EC. The right target depends on the crop, the growth stage, the system type (DWC, NFT, Coco), and ambient temperature. Here are reliable starting points:
| Crop | Seedling / Clone | Vegetative | Flowering / Fruiting |
|---|---|---|---|
| Lettuce / Leafy Greens | 0.8 – 1.2 | 1.2 – 1.6 | 1.4 – 1.8 |
| Tomatoes | 1.0 – 1.5 | 2.0 – 3.0 | 2.5 – 3.5 |
| Cucumbers | 1.0 – 1.5 | 1.8 – 2.5 | 2.0 – 2.8 |
| Basil / Herbs | 0.6 – 1.0 | 1.0 – 1.6 | 1.4 – 2.0 |
| Peppers | 1.0 – 1.5 | 2.0 – 2.5 | 2.5 – 3.0 |
| Strawberries | 1.0 – 1.2 | 1.4 – 1.8 | 1.6 – 2.2 |
All values in mS/cm (EC). Adjust downward by 10–15% in warm environments (>26°C) as nutrient uptake increases with temperature.
EC Drift: What It Tells You About Your Plants
Once your reservoir is running, EC will change over time — and how it changes is diagnostic information:
EC drops → Plants are eating
Plants are absorbing ions faster than water. Replenish with fresh nutrient solution at your target EC to compensate.
EC rises → Plants are mostly drinking water
In hot or dry conditions, plants transpire heavily and absorb water faster than nutrients. Top up with plain (pH-adjusted) water to dilute the reservoir back to target. This is a heat stress signal.
EC stable but plants look deficient
High overall EC does not mean all individual nutrients are present in the right ratio. This is why ppm-per-element analysis matters — your EC could be dominated by Sodium from tap water, masking a real Calcium or Magnesium deficiency.
How HydroNutrientCalc Shows You More Than Just EC
Most EC meters give you a single number. What HydroNutrientCalc does is fundamentally different: it calculates the predicted EC of your solution from first principles — by summing the individual ionic contributions of every salt you add.
This means you can see not just your total EC, but exactly how much of that EC comes from Nitrogen, Calcium, Potassium, Phosphorus, and each micronutrient. You can verify that your solution is genuinely balanced, not just numerically high.
Try it yourself
Open the Calculator, add a few salts to your recipe, and watch the EC prediction update in real time. The results panel shows your predicted EC alongside the individual ppm contribution of each element — giving you the full picture that a single EC meter never can.