Hydroponic Reservoir Volume Calculator
Every nutrient dose depends on knowing your true water volume. Measure your tank — rectangular or round, inches or centimeters — set a practical fill level, and get the working capacity in gallons and liters.
Measure the interior, and use the depth of water rather than the full tank height if they differ.
Leave headroom for pumps, air stones, and drain-back — around 90% is typical.
Formula & how it works
Rectangular volume is length × width × depth; round is π × (diameter ÷ 2)² × depth. Cubic inches convert with ÷ 231 for gallons; centimeters give milliliters directly, so ÷ 1000 for liters.
Working volume is the brim-full figure times your fill percentage.
Worked example
A 24 × 18 × 14 inch tank is 6,048 cubic inches, or about 26.2 gallons brim-full. At a practical 90% fill that is 23.6 gallons — roughly 89 liters — leaving about 2.6 gallons of headroom for drain-back.
Sizing a reservoir
Volume drives every dose
Nutrient concentrations, pH adjustments, and additives are all specified per gallon or per liter, so an inaccurate volume quietly skews everything downstream. Measuring the tank once and noting the working volume saves repeated guesswork every time you mix.
Bigger buffers better
Large reservoirs are more forgiving: temperature swings slower, and a dosing mistake is diluted across more water. Small tanks drift faster in EC and pH as plants drink, which means more frequent checking and topping off. If you have the space, err larger.
Account for what sits in the water
Pumps, heaters, air stones, and especially net pots and media displace real volume. The geometric number is your starting point; the honest figure is what you actually pour in. Filling from a marked container the first time is a quick way to confirm.