| It’s 9pm. Your reservoir reads EC 2.4 but your lettuce wants 1.0. How much water do you add? Or the opposite — your fresh mix is at 1.5 and your tomatoes want 2.5. How much stock do you pour in? This calculator answers both questions instantly, no algebra required.
Built around the standard dilution formula every chemistry student learns and every grower forgets. Works with any reservoir size, any nutrient brand, any target. |
| Why your EC drifts off targetA reservoir’s EC almost never stays where you set it. Plants don’t drink water and nutrients in the same ratio — they pull water faster than they pull dissolved salts, especially in warm weather. So as the water level drops, the remaining nutrients become more concentrated. EC creeps up. By the third or fourth day, a reservoir mixed at 1.4 might be reading 1.8 or higher.
The opposite happens during heavy feeding phases. Fruiting tomatoes can drink down salts faster than water in cool conditions, and EC drops. Either way, you end up needing to either dilute the reservoir back down or top it up with stock to bring it back up.
The maths to do this correctly is the standard dilution equation, which most growers half-remember from school. This calculator just runs it for you with the right unit conversions, so you don’t end up adding too much water and starving your plants, or too much stock and burning them. |
| The dilution formulaFor diluting (adding water):
Water to add = Current volume × (Current EC − Target EC) ÷ (Target EC − Water EC)
For concentrating (adding stock):
Stock to add = Current volume × (Target EC − Current EC) ÷ (Stock EC − Target EC)
Worked example (dilution): 20 L reservoir at EC 2.4. Target is EC 1.4. Tap water EC is 0.3. Water to add = 20 × (2.4 − 1.4) ÷ (1.4 − 0.3) = 20 × 1.0 ÷ 1.1 = 18.2 litres. Yes — it really does take that much water. Most growers underestimate how much dilution costs in volume, which is why fresh starts are sometimes easier than corrections.
Worked example (concentration): 20 L reservoir at EC 1.5. Target 2.5. Stock EC 3.5. Stock to add = 20 × (2.5 − 1.5) ÷ (3.5 − 2.5) = 20 × 1.0 ÷ 1.0 = 20 litres. That’s why most growers keep their stock at 3.5+ rather than full target strength — concentrated stocks need less added volume. |
| Frequently asked questionsMy calculation says I need to add more water than my reservoir holds. What now?
That happens when you’ve drifted very far from target. You have two options: drain part of the reservoir first, then dilute what remains. Or — easier — drain the whole reservoir and start fresh. Beyond about 50% drift, fresh mixes are usually faster.Should I adjust pH before or after dilution?
After. Adding water or stock will change pH on its own. Dilute first, let the solution mix and stabilise for 10 minutes, then test and adjust pH.
What if I don’t know my stock solution’s EC?
Mix a small test batch (1 litre) at the strength you’d normally use for full-strength feeding, then measure it. That number is your stock EC. Note it down for next time — it won’t change unless you change brands or formulas.
Why is my top-up water EC even relevant?
Tap water isn’t pure water. It contains dissolved minerals which add to EC. If your tap is 0.4, you can never dilute below 0.4 no matter how much you add. The calculator factors this in so the maths actually works for your water source.
What if I’m using two-part (A+B) nutrients?
Pre-mix part A and part B together at full strength in equal proportions to make your stock, then measure the EC of that mixture. Treat it as one unified stock for the calculation. Never add A and B separately to a reservoir without dilution — they precipitate at concentrated strength.
When should I just start fresh instead of correcting?
Three triggers: the reservoir is more than a week old, the EC has drifted by more than 50%, or you’ve corrected it three times already. Old reservoirs accumulate root exudates, salts skewed by selective uptake, and sometimes pathogens. Fresh mixes give plants a clean ratio.
Does this work for foliar feeding solutions too?
The maths is identical — it’s just dilution. But foliar EC targets are much lower (typically 0.5–0.8 mS/cm) because leaves can’t tolerate the same strength as roots. Use the calculator the same way, just with foliar-appropriate targets. |
Reservoir management is half of hydroponic growing
Start Your Hydroponics Journey covers EC management, nutrient mixing, top-up routines and reservoir change schedules — the daily practices that separate thriving systems from struggling ones.
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