Water Change Schedule Calculator
Plan when to change your reservoir, how many refills a grow needs and how much water and nutrient that uses.
How the calculation works
The first fill is on day 0 and there is a refill every interval after that, up to the end of the grow:
Refills = ceil(grow days ÷ interval) and water used = refills × reservoir volume
Example: a 60 day grow with a change every 14 days has ceil(60 ÷ 14) = 5 refills, on days 0, 14, 28, 42 and 56. With a 20 L reservoir that is 5 × 20 = 100 L of water, and at a combined dose of 5 ml/L it is 5 × 20 × 5 = 500 ml of concentrate.
Water used between changes for top-offs is on top of this. Estimate it with the reservoir top-off calculator, and plan each week's dose with the DWC nutrient dosing calculator.
Adjust the interval if the water looks cloudy, smells off or the readings drift between checks.
Frequently asked questions
How often should I change a hydroponic reservoir?
A common range is every one to three weeks. Change sooner for a small reservoir, a warm room, a heavy feeder, or if you see cloudy water, odour or readings that keep drifting. Larger reservoirs can often go longer.
Do top-offs count as water changes?
No. Topping off replaces evaporation and uptake but does not remove built-up salts or waste. This tool counts full refills only. Use the top-off calculator to plan the water you add in between.
What do I do with the old solution?
Many growers pour used solution on garden beds or ornamentals, diluted, where local rules allow. Do not pour it into natural waterways, and check what your area allows.
Is the first fill included?
Yes. Day 0 is your first fill, so a 60 day plan with a 14 day change has refills on days 0, 14, 28, 42 and 56.