Gallons and litres from your tank's dimensions — plus the number that actually matters, which is how much water is in there once substrate and the fill line take their share.
Last reviewed August 2026 · Free · Nothing you enter is stored
Aquariums are sold by nominal volume, which is roughly what the glass box would hold if you filled it to the brim and put nothing in it. Nobody runs a tank that way. Between the freeboard you leave below the rim, the substrate on the bottom, and whatever rock and wood you have aquascaped with, a tank in service holds meaningfully less water than its name suggests.
On a standard 75-gallon tank — 48 by 18 by 21 inches — an inch and a half of freeboard and two inches of gravel take it down to about 62 gallons of water. That is an 18 percent gap, and it points in the direction that matters: every chemical you add is more concentrated than you intended.
For dechlorinator this is irrelevant, because the safety margin is enormous. For a copper-based medication, an invertebrate-safe treatment, or a fertiliser dosed to the millilitre, an 18 percent overdose is the kind of thing that kills shrimp and stresses fish for reasons the owner never traces back.
Tank names are marketing rounding, not measurements. These are the standard footprints sold in the US:
| Sold as | Dimensions (in) | Actual nominal gallons |
|---|---|---|
| 10 gallon | 20 × 10 × 12 | 10 |
| 20 gallon long | 30 × 12 × 12 | 20 |
| 29 gallon | 30 × 12 × 18 | 29 |
| 40 gallon breeder | 36 × 18 × 16 | 40 |
| 55 gallon | 48 × 13 × 21 | 55 |
| 75 gallon | 48 × 18 × 21 | 75 |
| 90 gallon | 48 × 18 × 25 | 90 |
| 125 gallon | 72 × 18 × 21 | 125 |
One US gallon is exactly 231 cubic inches, so gallons = length x width x height in inches divided by 231. Bow-front, hexagonal and cylindrical tanks are approximated with a shape factor applied to their bounding box. The commonly quoted 'subtract 10-15% for real water volume' rule has no authoritative source behind it, so this calculator subtracts your actual substrate depth and fill line instead of applying a flat percentage.
Volume tells you how much water you own. Surface area tells you how fast that water can exchange gas with the room — oxygen in, carbon dioxide out. Every bit of that exchange happens at the top.
This is why two tanks of identical volume are not equivalent. A 29-gallon tank measuring 30 by 12 has 360 square inches of surface. A 29-gallon column tank measuring 18 by 18 has 324, and it is much deeper, so the water at the bottom is further from the exchange happening at the top. The long tank supports more fish, more comfortably, with no other change.
It is also why the very first aquarium stocking rules, written in the 1950s and 60s before filtration was standard, were expressed in square inches of surface per inch of fish rather than in gallons. Those rules were arguably better reasoned than the volume-based one that replaced them. Our stocking calculator goes through why.
For rectangular tanks the arithmetic is exact and there is nothing to argue about. For anything else, a shape factor gets you close: a bow front holds around six percent more than the flat-front tank of the same footprint, a hexagon about 87 percent of its bounding box, a cylinder about 78.5 percent — that last one is just pi over four, so it is exact for a true cylinder.
For genuinely irregular tanks, rimless designs with unusual proportions, or any sump and overflow arrangement, arithmetic stops being useful. Fill it with a bucket you have measured and count the buckets. It takes ten minutes and it is the only method that accounts for your particular plumbing.
Multiply length by width by height in inches, then divide by 231. That divisor is exact — a US gallon is defined as 231 cubic inches — so the arithmetic is not an approximation. A 48 by 18 by 21 inch tank works out to 18,144 cubic inches, or 78.5 gallons, which is why it is sold as a 75.
Three reasons, and they add up. You cannot fill to the very top, so an inch or two of the height never holds water. Substrate occupies the bottom. Rock, wood and equipment displace more. A 75-gallon tank with two inches of gravel and a normal fill line holds closer to 62 gallons of actual water. The commonly repeated advice to just subtract 10 to 15 percent is a guess — it is more useful to subtract your actual substrate depth and fill line, which is what this calculator does.
Yes, and this is where the difference bites. Medications, dechlorinator and fertilisers are dosed per gallon of water, not per gallon of tank label. Dosing a 75-gallon tank that actually holds 62 gallons means every treatment runs about 20 percent strong. For dechlorinator that is harmless. For copper-based medication, invertebrate treatments, or anything with a narrow safety margin, it is not.
Measure the bounding box — the full length, the deepest point of the width, and the height — and apply a shape factor. A bow front holds roughly six percent more than its flat-front equivalent because of the curve. A hexagon holds around 87 percent of its bounding box, and a cylinder about 78.5 percent. Those factors are built into the shape selector here. For anything genuinely irregular, filling it with a measured bucket is more accurate than any formula.
Oxygen enters the water and carbon dioxide leaves it at the surface, so the area of that surface sets the ceiling on gas exchange. This is why a tall, narrow column tank supports fewer fish than a long, shallow tank of exactly the same volume, and why the original stocking rules from the 1960s were written in square inches of surface rather than gallons. If you are choosing between two tanks of equal volume, take the one with the bigger footprint.
Multiply gallons by 3.785. This calculator shows both. Worth knowing that most dosing instructions written outside the United States are per litre, and a good many aquarium products sold in the US carry both — reading the wrong one is a four-times error in either direction.
