How Many Gallons Is Your Aquarium — Really?
Measure inside the glass — or pick a standard tank — and get geometric volume in US gallons, imperial gallons, liters and cubic inches, then the true water volume after glass thickness, substrate and air gap, with every figure carrying its community source.
Your tank volume
True water volume
Footprint & weight
Reverse-check from salt
Frequently asked questions
How do I calculate how many gallons my fish tank is?
Measure the inside of the tank — length, width and height in inches, from the inner glass and up to the waterline — multiply them together, and divide by 231. That is the exact US gallon count (1 US gallon = 231 cubic inches, a constant the aquarium community pins hard). A 30×12×12 inch tank, for example, is 4,320 cubic inches or 18.7 US gallons.
Why is my "20 gallon" tank only 18.7 gallons?
Tank names are rounded marketing numbers measured to the outside brim. A 20 gallon long measures 30×12×12 inches inside, which is 18.7 real gallons; a 55 gallon is really about 54. The glass thickness, the substrate and the air gap at the top take away a bit more. This calculator shows the geometric volume and the true water volume side by side so the name never hides the number.
What is the difference between US and UK gallons?
A US gallon is 3.785 liters; a UK (imperial) gallon is 4.546 liters — about 20 percent larger. American tank sizes, heaters rated "per gallon" and dosing rules all assume US gallons. This calculator labels every gallon figure and switches the whole page between US and UK with one control, because mixing the two is one of the oldest mistakes in fishkeeping.
How much water does my aquarium actually hold?
Less than the geometric volume: subtract the glass displacement, the substrate height across the whole base, and the air gap you leave at the top. The community rule of thumb is to expect roughly 20 percent under the nominal size. The calculator breaks that down layer by layer, so you can see exactly where the water went instead of trusting a flat percentage.
How do I measure a bow-front, cylinder or hexagon tank?
Cylinder tanks use the inside diameter and height with the circle area formula. Hexagon tanks are usually sold by the flat-to-flat width: square that, multiply by height and by 0.866. Bow-fronts are a rectangle plus a curved bulge: measure the flat back depth, then the extra bow depth separately. The calculator has each shape built in with a diagram of exactly where to put the tape measure.
How do I calculate the volume of a tank with a cut corner?
Treat it as the full rectangle minus the missing corner: work out the rectangular volume as usual, then subtract the notch length times notch width times height. It is simple arithmetic that is notoriously easy to get wrong by hand, which is why the calculator asks for the five measurements separately and refuses to give a negative number if a notch measurement goes past the tank wall.
How heavy is a full 55 gallon aquarium?
About 625 lb including a standard substrate — glass, stand water and gravel together. A 15 gallon runs near 170 lb, a 40 gallon breeder about 468 lb and a 75 gallon about 850 lb with substrate (the bare manufacturer figure for the 75 is 785 lb). Treat these as estimate-grade numbers and think about the stand and the floor, not just the tank: rimmed tanks can rest on two end supports, and a level stand matters more than a perfect fit.
Can I work out true water volume from the salt mix I added?
Yes — a respected reefkeeping cross-check. If you know how many pounds of salt you dissolved and the salinity you mixed to, dividing the salt weight by the brand mixing ratio gives the water volume. With Instant Ocean at SG 1.022 (about 0.30 lb per gallon), 15 lb of salt implies roughly 50 gallons of actual water. It is an estimate good to about ±10–15 percent, so cross-check it against the dimension method.
Does tank shape matter as much as tank volume?
For stocking, often more. Surface area is the aquarium's lung — gas exchange happens at the water surface — so a long shallow tank supports more fish than a tall narrow one of identical gallons. A cube is not equivalent to a rectangular tank of the same volume either; compare footprints, not gallon labels. The calculator reads out bottom and surface areas and classifies your tank shape for exactly this reason.
When someone says a tank is "100 gallons", does that include the sump?
Usually not — recommended stocking levels refer to the display tank alone. A 100 gallon display with a 30 gallon sump is stocked as a 100 gallon, not a 130. "Total system volume" matters for dilution and stability, but fish load, heater sizing and medication doses follow the display gallons. The calculator labels its numbers accordingly so the two calibers never get mixed.
Sources & rule confidence
Every rule baked into this calculator traces to a community source and carries a confidence grade. A = multiple independent community confirmations; B = consistent direction, edge cases remain; C = direction only, numbers are reference values. We show disagreements instead of hiding them.
- A1 · A 1 US gallon = 231 cubic inches exactly; the 1/231.48 variant is rejected.
- A2 · A 1 US gal = 3.79 L; 1 imperial gal = 4.54 L — always label which gallon.
- A3 · A Rectangular tank volume = L x W x H; divide by 231 for US gallons.
- A4 · A Irregular tank volume = total rectangular volume minus corner/notch volume.
- A5 · B True water volume = geometric volume minus substrate displacement minus air gap.
- A13 · B · reference value Full tank weight (with substrate): 15g≈170 lb / 40B≈468 / 55g≈625 / 75g≈850 — estimate-grade, substrate-inclusive (R3 three-source cross-check <10%).
- D9 · B · reference value Nominal to true water volume: subtract glass thickness, substrate, air gap (~20% under nominal); true volume can be reverse-estimated from salt dose.
- D10 · A Recommended gallon counts refer to the display tank, excluding sump/total system water.
- D11 · A Surface area is the criterion: long tanks > standard > tall; never judge by gallons-per-table lookup.
- D12 · A A cube tank is not equivalent to a rectangular tank of the same volume — compare footprint, not gallons.
- D23 · A Rimmed tanks can be supported at both ends; a level stand matters more than a perfect empty fit — filled tanks flex.
- D27 · A Standard US tank footprints: 20H = 24x12 in, 20L = 30x12 in.
Weight figures are estimate-grade and include a standard substrate load (A13, three-source cross-check under 10%, 2026-10-17); the 75 gal figure also shows the bare manufacturer caliber of 785 lb. Salt reverse-check is anchored to the Instant Ocean mixing ratio and is good to about ±10–15 percent (D9). Locked constants (231 in³ per US gallon, hexagon and bow-front factors) were computed with a calculator, not by hand. Bow-front is the box-plus-bulge estimate (B-grade); the exact circular-arc formula stays out as a single-source C-grade.
Disclaimer
All volumes and weights are estimates. Weight figures assume a standard substrate load and do not judge whether a floor or stand is adequate — when in doubt, consult a professional before filling a large tank. This calculator does not replace water testing or veterinary advice.