Aquarium Volume Calculator: How Many Gallons Is My Fish Tank?
To find how many gallons your fish tank holds, multiply the inside length, inside width and water depth in inches, then divide by 231. For litres, multiply the gallons by 3.785411784. That gives the volume of the space the water fills. The number that matters for dosing is smaller again: subtract what the substrate and the rock take up. In the illustrative example below, a tank sold as 75 gallons ends up holding 58.9 gallons (223 L) of actual water - 79% of the label.
The aquarium volume formula
- US gallons = length x width x depth (inches) / 231
- Litres = gallons x 3.785411784
- Litres from centimetres = length x width x depth (cm) / 1000
None of these numbers is an approximation. A US gallon is defined as exactly 231 cubic inches, and that works out to exactly 3.785411784 litres, because an inch is defined as exactly 2.54 cm. If you measure in centimetres, skip the gallons entirely and divide cubic centimetres by 1000.
Three measurements, and each one has a trap:
- Length and width are the inside of the glass, not the outside. A tape measure across the frame includes two thicknesses of glass.
- Depth is the depth of the water, not the height of the tank. The water line sits below the rim, and the bottom glass takes its share too.
- What is in the water - substrate, rock, wood - displaces its own volume, and none of it is in the box dimensions.
Gallons and litres by footprint and water depth
Use this as an aquarium volume calculator for the common rectangle shapes. Find your inside footprint, read across to your water depth. All 32 cells were computed two ways (see the method section below).
| Inside footprint | 12 in of water | 16 in of water | 20 in of water | 24 in of water |
|---|---|---|---|---|
| 24 x 12 in | 15.0 gal / 57 L | 19.9 gal / 76 L | 24.9 gal / 94 L | 29.9 gal / 113 L |
| 30 x 12 in | 18.7 gal / 71 L | 24.9 gal / 94 L | 31.2 gal / 118 L | 37.4 gal / 142 L |
| 36 x 12 in | 22.4 gal / 85 L | 29.9 gal / 113 L | 37.4 gal / 142 L | 44.9 gal / 170 L |
| 48 x 12 in | 29.9 gal / 113 L | 39.9 gal / 151 L | 49.9 gal / 189 L | 59.8 gal / 227 L |
| 36 x 18 in | 33.7 gal / 127 L | 44.9 gal / 170 L | 56.1 gal / 212 L | 67.3 gal / 255 L |
| 48 x 18 in | 44.9 gal / 170 L | 59.8 gal / 227 L | 74.8 gal / 283 L | 89.8 gal / 340 L |
| 60 x 18 in | 56.1 gal / 212 L | 74.8 gal / 283 L | 93.5 gal / 354 L | 112.2 gal / 425 L |
| 72 x 18 in | 67.3 gal / 255 L | 89.8 gal / 340 L | 112.2 gal / 425 L | 134.6 gal / 510 L |
These are volumes of water space before anything goes into the tank. If your footprint is not here, use the formula - it is one multiplication and one division.
Worked example: from the box to the water
Take a tank sold as 75 gallons whose outside measures 48 x 18 x 21 inches. All the inputs below are illustrative - glass thickness, the gap below the rim, substrate depth and rock volume vary from tank to tank, so measure yours.
The outside box is 48 x 18 x 21 = 18,144 cubic inches, and 18,144 / 231 = 78.5 gallons (297 L). That is more than the label. Now take away what is not water, one layer at a time, using 0.375 in glass on every wall and the bottom, a water line 1.5 in below the top edge, a 2 in substrate bed, and rock and wood that displaced 1.5 gallons when sunk in a measured bucket:
| Step | Dimensions (in) | Gallons | Litres |
|---|---|---|---|
| Outside box | 48 x 18 x 21 | 78.5 | 297 |
| Inside the glass | 47.25 x 17.25 x 20.625 | 72.8 | 275 |
| Filled to the water line | 47.25 x 17.25 x 19.125 | 67.5 | 255 |
| Minus the substrate bed | 47.25 x 17.25 x 2 removed | 60.4 | 229 |
| Minus rock and wood | 1.5 gal measured | 58.9 | 223 |
The real water is 58.9 gallons, 16.1 gallons short of the label. Look at where it went:
- The glass took 5.8 gallons.
- The gap below the rim took 5.3 gallons. With the water line 1.5 in down, the water is 19.125 in deep, not the 21 in on the spec sheet.
- The substrate bed took 7.1 gallons - more than the rock did.
The rock is the obvious thing to subtract, yet in this example it is the smallest of the four deductions. A couple of inches of sand spread across the whole footprint is a large slab.
Why the real volume changes a dose
Anything dosed “per gallon” or “per 10 gallons” - a water conditioner, a fertilizer, a treatment - assumes you know how many gallons are in the tank. This post gives no dosing advice for any product; follow the label of what you are using. The arithmetic is the point: the label tells you the amount per gallon, and your volume decides how many gallons you multiply by.
Dose the example tank as 75 gallons and you add 7.5 “per 10 gallons” measures. The water needs 5.9. The same amount of product in 58.9 gallons instead of 75 is 1.27 times the intended concentration - 27% stronger than the label says - because concentration is amount divided by volume, and the volume was smaller than the one you divided by.
The correction runs the same way for anything you dose: scale the label amount by real volume / assumed volume, here 58.9 / 75 = 79%.
One honest caveat: the calculation treats the substrate bed as solid. Gravel and sand hold some water between the grains, so the water in the system is somewhat more than 58.9 gallons; how much depends on the substrate. If you want one measured number instead of a calculated one, fill the tank once with a bucket of known size, after the substrate and rock are in and before the fish, and count.
Water changes use the same number
A water change described as a percentage is a percentage of the real water. Take out 18.75 gallons - 25% of the label - and from 58.9 gallons that is a 32% change. A true 25% change in this tank is 14.7 gallons.
Changes also do not add. Two 25% changes in a row remove 43.8% of the original water, not double: the second change draws from water that is already partly fresh.
How these were calculated
Every figure on this page is computed by a script, not typed. Volumes come from the same code that builds the product sheet (gallons = cubic inches / 231, the substrate as a slab of the inside footprint, water changes as a straight percentage), and from a second, independent route written for this page that converts inches to centimetres, cubic centimetres to litres, and litres to gallons at 3.785411784. The page builds only if the two agree. The script also asserts every comparison the prose makes - that each step reduces the volume, that the bed costs more than the rock, that a label-based change is a bigger share of the real water. The 75-gallon tank and every input to it are illustrative. This is arithmetic, not veterinary or product advice; follow the instructions on anything you add to a tank.
If you would rather read your tank off a printed sheet - gallons by footprint and depth, what each substrate depth takes back, and a water change table by real volume - this one-page planner does it: