Aquarium volume calculator

How the aquarium volume calculator works

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.

Here is a worked example. A standard 55 gallon tank measures about 48 by 13 by 21 inches. Filled to the brim with no substrate that is roughly 56.7 gross gallons, already less than some rounded box figures. Add two inches of substrate and a one inch rim gap and the water height drops to 18 inches, giving about 48.6 net gallons, or 184 liters. That is about eight gallons less than the gross figure and around six gallons less than the name on the tank, which is exactly why the net number is the one worth planning around.

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.

When you buy equipment, size it to the net volume with a little room to spare. A heater or filter chosen for a slightly larger tank copes better on cold nights and heavy stocking than one that runs at its limit. Dosing is the opposite case: measure carefully and dose to the real water volume, because a small tank punishes an overdose far faster than a large one does.

How to measure your tank correctly

Accurate dimensions matter more than the formula. Measure the inside of the glass, not the outside, because the frame, trim and glass thickness can add half an inch or more on each side. On a small tank that easily changes the result by a gallon or two. Use a rigid tape or ruler and measure to the nearest quarter inch.

For length and width, measure across the inside floor of the tank. For height, measure from the inside bottom up to where the water line will sit rather than to the very top rim. If you already leave a rim gap for surface movement and to stop spills, measure to that line and leave the rim gap in this calculator at its default. Take each reading twice, and if the tank is not perfectly square, use the average of the two measurements.

Volume for other tank shapes

The calculator above is built for rectangular tanks, the most common shape. For other shapes, work the volume out in cubic inches with the formula below, divide by 231 to get US gallons, then multiply gallons by 3.78541 for liters. Subtract substrate and a rim gap the same way, using the water height rather than the full height.

• Cube: side times side times side.
• Cylinder: 3.1416 times the radius squared times the height, where the radius is half the diameter.
• Half or quarter cylinder: work out the full cylinder, then divide by 2 or by 4.
• Regular hexagon: 2.598 times the side length squared times the height.
• Bow front: treat the flat body as a rectangle and add the small curved bulge at the front, or use the average front to back depth.

For an unusual shape, split the tank into simple parts, find the volume of each, and add them together.

Rocks, substrate and equipment displacement

Substrate is the biggest thing that lowers your water volume, but it is not the only one. Rocks, driftwood, large ornaments and the submerged parts of a heater or filter intake all take up space that would otherwise hold water. A big rockscape or a thick layer of lava rock under the substrate can remove a real share of the volume in a small or medium tank.

The calculator handles substrate directly through the depth input. For heavy hardscape you can estimate the displaced volume and subtract it, or measure it the simple way: note the water level, add the rocks, and see how far the level rises, then convert that rise into extra gallons. For most lightly decorated tanks the effect is small, but for heavily scaped or rock filled tanks it is worth allowing for when you dose or stock.

Gallons, liters and imperial gallons

This calculator shows US gallons first with liters in brackets, which suits most hobbyists. It still helps to know how the units relate. One US gallon is 3.78541 liters and 231 cubic inches, and one liter is about 0.264 US gallons. If a product or a forum uses imperial gallons, common in the United Kingdom, remember that one imperial gallon is about 1.2 US gallons, so an imperial figure is always a little larger for the same tank.

When you follow a dosing instruction, check which gallon it means. Mixing up US and imperial gallons, or gallons and liters, is a common cause of accidental over or underdosing. When in doubt, convert everything to liters first, since liters leave no room for confusion.

Common aquarium sizes

These are typical nominal sizes and outside dimensions in inches, listed as length by width by height. The real net volume is a little less once you remove substrate and the rim gap, so measure the inside of the glass and use the calculator for your own tank.

• 10 gallon: 20 x 10 x 12
• 20 gallon high: 24 x 12 x 16
• 20 gallon long: 30 x 12 x 12
• 29 gallon: 30 x 12 x 18
• 40 gallon breeder: 36 x 18 x 16
• 55 gallon: 48 x 13 x 21
• 75 gallon: 48 x 18 x 21
• 125 gallon: 72 x 18 x 21