Aquarium volume calculator
Find the true water volume of a rectangular tank in gallons and liters, after you remove substrate and the rim gap.
How the volume is worked out
The calculator starts with the gross volume, which is length times width times height in cubic inches, divided by 231 because there are 231 cubic inches in a US gallon. That gross figure is what the tank would hold if it were filled to the very top edge with no substrate.
Real tanks never run that way. The tool then subtracts your substrate depth and a one inch rim gap from the height to get a water height, and it never lets that fall below zero. Net gallons uses length times width times this water height, so it reflects the water you actually keep. Liters come from multiplying gallons by 3.78541. The net figure is the number you should carry into stocking, dosing and heater choices, since using the label size will overstate what your tank really holds.
Why true volume matters
Almost every fishkeeping decision scales with water volume. Medication and fertilizer doses are set per gallon, so an overstated volume means you underdose and an understated one means you overdose. Stocking guidance depends on how much water dilutes waste. Heater wattage is chosen per gallon. Even the weight your floor carries depends on the water that is really in the tank.
Because substrate and the rim gap can easily remove ten percent or more of a tank's printed size, working from net volume keeps all of those downstream numbers honest. Measure the inside dimensions of the glass rather than the outside, since the frame and glass thickness add up on smaller tanks.
Frequently asked questions
Why is my water volume less than the tank size on the box?
The printed size is the gross volume to the very top rim. Real tanks lose volume to substrate, rocks and the gap you leave below the rim, so the water you actually hold is always less than the label.
How deep should I assume the rim gap is?
This tool assumes a one inch gap below the rim, which is a common safe margin that stops overflow and gives fish surface space. Adjust your height input if your gap is different.
Does substrate really change the volume that much?
Yes. Two inches of substrate across the full footprint of a tank can remove several gallons of water volume, which matters for dosing, stocking and heater sizing.
How do I convert gallons to liters?
Multiply US gallons by 3.78541 to get liters. The calculator shows both so you do not have to.