How to calculate aquarium volume
Measure inside the glass, in inches, and multiply the floor area by the height. A US gallon is 231 cubic inches, so:
- Rectangle: length × width × height ÷ 231.
- Cylinder: π ÷ 4 × diameter² × height ÷ 231.
- Bow front: (length × end depth + ⅔ × length × bulge) × height ÷ 231, where the bulge is the depth at the middle of the bow minus the depth at the ends. This treats the bow as a smooth curve. Real bows differ slightly, so take the answer as close rather than exact.
- Hexagon: for a regular hexagon, 2.598 × side² × height ÷ 231, where the side is the inside width of one glass panel.
In centimeters, the same products divided by 1,000 give liters.
Standard aquarium sizes chart
Common US glass tanks and their usual outside dimensions. The volumes are computed from those dimensions, so they come out a little above what the tank really holds. Dimensions differ slightly between brands, so measure yours.
| Tank | Common size (L × W × H) | Computed volume | Liters | Rough set-up weight |
|---|---|---|---|---|
| 10 gallon | 20 × 10 × 12 in | 10.4 gal | 39 L | 104 lb |
| 20 high | 24 × 12 × 16 in | 19.9 gal | 76 L | 199 lb |
| 20 long | 30 × 12 × 12 in | 18.7 gal | 71 L | 187 lb |
| 29 gallon | 30 × 12 × 18 in | 28.1 gal | 106 L | 281 lb |
| 40 breeder | 36 × 18 × 16 in | 44.9 gal | 170 L | 449 lb |
| 55 gallon | 48 × 13 × 21 in | 56.7 gal | 215 L | 567 lb |
| 75 gallon | 48 × 18 × 21 in | 78.5 gal | 297 L | 785 lb |
| 125 gallon | 72 × 18 × 21 in | 117.8 gal | 446 L | 1,178 lb |
The weight column uses the rule of thumb of about 10 pounds per gallon of tank size for water, glass, substrate and decor. It is an estimate, not a rating; check what your stand and floor are built for.
Other shapes, using inside measurements:
| Tank | Measurements | Volume |
|---|---|---|
| Bow front | 36 in along the back, 12 in deep at the ends, 15 in at the middle, 20 in tall | 43.6 gal (165 L) |
| Cylinder | 12 in diameter, 24 in tall | 11.8 gal (44 L) |
| Hexagon | 10 in panels, 20 in tall | 22.5 gal (85 L) |
Example: how much water is really in a 55
A 55-gallon tank measures about 48 × 13 × 21 inches outside. Inside the glass it is closer to 47.5 × 12.5 inches, and the water stands 20 inches deep with the lid on.
- Volume of the water space: 47.5 × 12.5 × 20 = 11,875 cubic inches.
- Gallons: 11,875 ÷ 231 = 51.4 gallons (194.6 liters).
- Two inches of gravel and some rock take about 15 percent: 51.4 × 0.85 = 43.7 gallons of water (165 liters).
- That water weighs 43.7 × 8.34 = about 364 pounds.
So a "55" really holds about 44 gallons of water once it is set up. Use the lower figure for water changes and medication doses; follow the medication's label for the dose itself.
Tips
- Measure inside, at the water line. Outside dimensions include the glass and the frame.
- Dose by real water volume. Medications and fertilizers are dosed per gallon, so use the corrected figure and follow the product's label.
- Weight adds up fast. Use a stand made for the tank's size, and set large tanks near a load-bearing wall, across the floor joists rather than between them. If you are unsure about the floor, ask a builder.
- Check level before you fill. A tank that is out of level puts uneven stress on the seams and shows in the water line.
Frequently asked questions
How do you calculate how many gallons a fish tank holds?
Multiply length × width × height in inches and divide by 231, the number of cubic inches in a US gallon. A 30 × 12 × 18 inch tank is 6,480 cubic inches, or about 28.1 gallons. In centimeters, multiply the three and divide by 1,000 for liters.
How much does a 55-gallon aquarium weigh when full?
The water alone weighs about 460 pounds (55 × 8.34). A common rule of thumb for the whole set-up, with glass, gravel and decor, is about 10 pounds per gallon of tank size, so plan for roughly 550 pounds, and more with a lot of rock.
What is the difference between a 20 long and a 20 high tank?
A 20 long is usually 30 × 12 × 12 inches and a 20 high 24 × 12 × 16 inches. They hold about the same water, but the long tank has more floor and surface area (360 against 288 square inches), which suits active swimmers, bottom dwellers and the surface-area stocking rule.
Why does my tank hold less water than its label says?
Labels are nominal and usually based on outside dimensions. The glass takes up some room, you fill to an inch or so below the rim, and substrate, rock and wood push water out. Allowing 10 to 20 percent less is common; the calculator lets you set the figure.
How many liters are in a gallon?
A US gallon is 3.785 liters. A UK (imperial) gallon is 4.546 liters, about 1.2 US gallons, so tank sizes in British sources look smaller in gallons than the same tank sold in the US.
How do I measure a bow front aquarium?
Measure the length along the straight back, the front-to-back depth at the ends, and the depth at the middle of the bow, all inside the glass. The calculator treats the bow as a smooth curve and adds two thirds of the bulge times the length to the rectangle.
Last updated October 17, 2026.