Calculate How Much Water to Add to Bring Your Reservoir EC Down to Target
This tool was built by Serge (MSc) using established science and government sources. My academic background covers plant biology, environmental biology, and biogeochemistry. My tools are for educational purposes and give practical guidance, not professional advice.
When a hydroponic reservoir reads too strong, the fix is to add plain water, but guessing how much leads to a solution that is still off.
This calculator works out exactly how much fresh water to add to bring a high EC down to your target.
It uses the standard dilution rule, so the answer is exact for the readings you enter.
Water to add:
New total volume:
Add the water, stir, then re-check the EC, since meters and mixing are never perfect.
How this works
Diluting does not remove nutrients, it spreads the same amount through more water, which lowers the EC.
The rule is current EC times current volume equals target EC times new volume.
That rule is rearranged to find the new volume, then your current volume is subtracted to give the water to add.
Understanding your result
The water figure is how much plain water, ideally low-EC water like rainwater or reverse osmosis water, to add to the reservoir.
Add it, mix well, and read the EC again.
If it is still a touch high, add a little more.
This only lowers EC.
To raise a weak reservoir you add nutrient, not water.
Important note
This is an exact dilution for the readings you enter, but it assumes your source water has a very low EC.
If your tap water has a high EC of its own, adding it will not lower the reservoir as much as the formula predicts, so always re-check with your meter after mixing.
Scientific basis
The role of EC in hydroponic nutrient management comes from university extension research, and the dilution rule is standard solution chemistry:
- Oklahoma State University Extension: Electrical Conductivity and pH Guide for Hydroponics
- Oklahoma State University Extension: EC and pH Guide (full fact sheet PDF)
Keep Planning With These Calculators
I recommend these calculators alongside this one because a reservoir is only balanced when strength, light, and feed all line up.
To compare readings from different meters, the EC to PPM Calculator converts nutrient strength across the 500, 640, and 700 scales.
When mixing a fresh feed from a label, the Fertiliser Dilution Calculator gives the exact dose for a given volume.
Indoor reservoirs also depend on light, and the DLI Calculator shows how much a plant actually receives each day.
Related reading
My guide on hydroponic nutrient solutions covers mixing a feed from scratch.
My guide to hydroponic pH and EC meters explains how to get a reading you can trust.
This calculator is for educational purposes and general guidance only. It is not professional horticultural advice. Always re-check readings with your own calibrated meter.
