How do you calculate aquarium volume?
Multiply the inside length, width and height, then convert. In inches, divide by 231 (the cubic inches in a US gallon); in centimeters, divide by 1,000 for liters:
gallons = L × W × H (in) ÷ 231 · liters = L × W × H (cm) ÷ 1,000
A "20 gallon long" measures 30 × 12 × 12 inches: 4,320 in³ ÷ 231 = 18.7 gallons (71 L) — to the rim, with no glass. That last clause is the whole story of this page: the number on the box, the number to the rim, and the number your fish actually swim in are three different numbers, and the calculator above shows all three.
Standard aquarium sizes: what they really hold
The trade names are approximate. Some tanks hold more than their label to the rim (a "75" holds 78.5); none of them hold their label once you fill an inch below the rim over two inches of gravel. Every row is computed by this page's engine from the standard outside dimensions.
| Tank | L × W × H (in) | To the rim | As kept* | Under label |
|---|---|---|---|---|
| 5.5 gallon | 16 × 8 × 10 | 5.5 gal · 21 L | 4.3 gal · 16 L | 21% |
| 10 gallon | 20 × 10 × 12 | 10.4 gal · 39 L | 8.5 gal · 32 L | 15% |
| 15 gallon | 24 × 12 × 12 | 15 gal · 57 L | 12.2 gal · 46 L | 19% |
| 20 gallon long | 30 × 12 × 12 | 18.7 gal · 71 L | 15.3 gal · 58 L | 24% |
| 20 gallon high | 24 × 12 × 16 | 19.9 gal · 76 L | 17.2 gal · 65 L | 14% |
| 29 gallon | 30 × 12 × 18 | 28.1 gal · 106 L | 24.6 gal · 93 L | 15% |
| 37 gallon | 30 × 12 × 22 | 34.3 gal · 130 L | 30.9 gal · 117 L | 17% |
| 40 gallon breeder | 36 × 18 × 16 | 44.9 gal · 170 L | 38.7 gal · 147 L | 3% |
| 55 gallon | 48 × 13 × 21 | 56.7 gal · 215 L | 50.8 gal · 192 L | 8% |
| 65 gallon | 36 × 18 × 24 | 67.3 gal · 255 L | 61.2 gal · 231 L | 6% |
| 75 gallon | 48 × 18 × 21 | 78.5 gal · 297 L | 70.3 gal · 266 L | 6% |
| 90 gallon | 48 × 18 × 24 | 89.8 gal · 340 L | 81.5 gal · 309 L | 9% |
| 120 gallon | 48 × 24 × 24 | 119.7 gal · 453 L | 108.7 gal · 412 L | 9% |
| 125 gallon | 72 × 18 × 21 | 117.8 gal · 446 L | 105.5 gal · 399 L | 16% |
| 150 gallon | 72 × 18 × 28 | 157.1 gal · 595 L | 144.7 gal · 548 L | 4% |
| 180 gallon | 72 × 24 × 25 | 187 gal · 708 L | 170.6 gal · 646 L | 5% |
| 210 gallon | 72 × 24 × 29 | 216.9 gal · 821 L | 200.5 gal · 759 L | 5% |
*As kept: water 1 in below the rim over 2 in of gravel (gravel displaces 60% of its poured volume). No glass allowance and no rock — real tanks hold a little less again. Standard outside dimensions per the major US manufacturers.
The pattern holds across the range: as kept, a standard tank holds 3%–24% less than its name, and the shallow, wide footprints (breeders, longs) lose the most because a two-inch bed is a bigger share of a short tank.
Why doesn't my "75 gallon" hold 75 gallons?
Four subtractions, and they stack. Take the 75 — 48 × 18 × 21 in outside:
| Step | Gallons | Liters |
|---|---|---|
| Outside dimensions to the rim | 78.5 | 297 |
| Inside the ⅜ in glass, water 1 in below the rim | 70.6 | 267 |
| Minus 2 in of gravel (94.2 lb) | 66.3 | 251 |
| Minus a modest 30 lb of rock | 62.4 | 236 |
Computed by this page's engine. Rock at ~1 gal per 10 lb; your bucket test beats the estimate.
So the water in a typical, nicely scaped 75 is about 62 gallons. Dose it as 75 and everything you add is 20% strong. For a water conditioner that is harmless; for a medication, a fertilizer regime or a reef two-part schedule it is the difference between a target and a problem — which is why the dashboard and every dosing tool on this site run on the true volume, and why "Save as my tank" above stores the gravel and rock along with the dimensions.
On a 48 × 18 in footprint, a 2 in bed of gravel is about 94.2 lb (42.7 kg) and takes up 7.1 gal of space — but only displaces 4.2 gal of water, because roughly 40% of a poured gravel bed is water-filled voids. Subtract the whole bed and you'll under-dose; subtract nothing and you'll over-dose. The calculator subtracts the solid fraction.
How to measure a bowfront, hexagon, cylinder or corner tank
Bowfront: measure the length, the depth at the straight ends, and how much further the glass bulges at the center — that extra depth is the "bow". The footprint is the rectangle plus a circular segment. Most online tools use the parabolic shortcut ⅔ × length × bow; the true segment is about 0.6% bigger for a 36 in tank with a 3 in bow, which is why our answer runs a hair higher.
Hexagon: a regular hexagon can be measured across the flats or across the corners, and the two numbers are not the same tank. A "24 inch" hexagon is 52 gallons if that's flat to flat and 39 if it's corner to corner — 25% less. Tell the calculator which one you measured.
Cylinder and half cylinder: diameter and height. A half cylinder (flat side to the wall) is exactly half. Corner tank: the quarter-round kind — measure along one wall from the corner to where the curve starts; that's the radius. Pentagon-front corner tanks are close to a quarter round of the same wall length; the difference is a few percent.
Manufacturers quote outside dimensions and so does every "standard size" chart. If you measured the outside, enter the glass thickness under "what's really in the water" and the calculator subtracts two panes from length and width and one from height. On a 75 with ⅜ in glass that's about 5.8 gallons — small, but it's free precision. If you measured inside, leave it blank.
Aquarium gallons to liters (and the imperial-gallon trap)
One US gallon is 3.785 liters. British and Irish keepers, and older UK-published books, use the imperial gallon — 4.54609 liters, 20% bigger — so a "50 gallon" tank in a UK forum thread is a 60 gallon tank in a US one. Liters avoid the argument entirely; the calculator always shows both.
| US gallons | Liters | Imperial gallons |
|---|---|---|
| 10 | 38 | 8.3 |
| 20 | 76 | 16.7 |
| 29 | 110 | 24.1 |
| 40 | 151 | 33.3 |
| 55 | 208 | 45.8 |
| 75 | 284 | 62.5 |
| 90 | 341 | 74.9 |
| 125 | 473 | 104.1 |
| 180 | 681 | 149.9 |
1 US gal = 3.785 L; 1 imperial gal = 4.54609 L.
Which volume should I use?
True water volume (rim, minus glass, gap, substrate and rock, plus sump) for anything you dose: conditioner, medication, fertilizer, alkalinity, calcium, salt for a water change. That's the number the dashboard keeps. Rim volume for weight — the floor doesn't care that the gravel isn't water. Footprint, not gallons, for stocking; a 40 breeder and a 40 high hold the same water and very different fish. And for a reef, system volume — display plus sump minus rock — is what your two-part consumption is measured against, so a sump you forgot to add is a dose that's silently 20% light.