The large floor is why breeders are favoured for fry raising, bottom dwellers, reptiles and reef frag systems. It also means substrate takes a bigger bite out of the water volume, so this calculator includes a substrate option that uses the real footprint.
How to measure a breeder tank
- Width is the front-to-back interior measurement. On breeders it is often 18 inches or more, so double-check which dimension you are entering.
- Breeder tanks are frequently run with a low water line; enter the actual fill height rather than the rim height.
- Framed breeders have a centre brace. Measure the open interior below it.
Always measure inside the glass. Exterior sizes include the panel thickness and any trim, which on larger tanks can remove 5 to 10 percent of the apparent volume. See the measuring guide for the full method.
Breeder volume formula
V = L × W × H
A breeder is a rectangular prism, so the standard box formula applies. The extra front-to-back width is what sets its volume apart from a regular tank of the same length.
With inches, divide the cubic result by 231 to get US gallons and multiply gallons by 3.785 for litres. With centimetres, divide by 1,000 for litres directly.
Worked example
Inputs: Length 36 inWidth (front to back) 18 inHeight 16 in
A 36 × 18 × 16 inch breeder is 10,368 cubic inches, which is 44.9 US gallons or 169.9 litres, noticeably more than its 40-gallon label.
Why breeders trade height for footprint
Surface area governs gas exchange and the amount of territory fish can claim on the floor, while height mostly adds swimming room for mid-water species. A breeder puts its volume where it matters for breeding projects and reef frag racks, and the low profile also makes it easier to reach the back of the tank without ladders or long tools.
Getting to real water volume
The formula gives the geometric volume of the tank to the rim. Three things make the real water volume smaller, and the calculator above lets you account for two of them directly:
- Fill height. Most tanks run an inch or two below the rim. Enter the actual water line and the result scales to match.
- Substrate. Gravel and sand sit in the water column. The substrate option removes the solid fraction of the bed (62 percent of its depth for typical gravel) from the total.
- Rock, wood and equipment. Use the displacement calculator to subtract hardscape by weight and equipment by type.
Water weight
Fresh water weighs about 8.34 pounds per US gallon (1 kg per litre). Reef water at a specific gravity of 1.025 is about 8.55 pounds per gallon. The calculator shows both so you can check what your stand and floor will carry before adding glass, substrate and rock on top.