Reservoir Top-Off Calculator
Your reservoir level has dropped. See how much water to add to refill it and what your ppm will be afterwards.
How the calculation works
The water you add is the gap between the full and current volume. The new concentration is a volume-weighted average:
Water to add = full − current and new ppm = (current ppm × current volume + top-off ppm × water added) ÷ full volume
Example: a 40 L reservoir has dropped to 28 L at 900 ppm. You add 40 − 28 = 12 L of 0 ppm water, so the new ppm is 900 × 28 ÷ 40 = 630 ppm.
If you also give a target, the tool tells you whether the top-off lands above, below or on it. To bring a reading to a target by adding water or stronger solution, use the hydroponic PPM calculator.
Mix well and re-measure after topping off. Keep an eye on pH too, because it can shift as the volume changes.
Frequently asked questions
Why does my PPM go up as the water level drops?
Plants and evaporation remove water faster than they remove dissolved nutrients, so what is left becomes more concentrated. Topping off with plain water brings the concentration back down.
Should I top off with water or with nutrient solution?
Usually plain water, because the reservoir is already concentrated. If EC drops below your target instead, top off with a nutrient solution mixed at a lower strength than the reservoir.
Does the calculator include tap water ppm?
Yes. Tap water is not 0 ppm. Enter its reading in the top-off ppm field to see the effect. For reverse osmosis or distilled water, leave it at 0.
Can I use EC instead of ppm?
Yes. Dilution is linear, so use the same unit (EC or ppm) in the current and top-off fields.