Aquarium Volume Calculator
This aquarium volume calculator tells you how many gallons and liters your fish tank holds. Pick the tank shape, rectangle, cube, cylinder, bow-front or hexagon, enter the measurements in inches or centimeters, and you get US gallons, liters and the weight of the water in pounds and kilograms.
Tank volume drives nearly every aquarium decision: stocking levels, filter and heater size, medication and water conditioner doses, and how much water to change each week. The water weight matters too. Even a mid-size tank carries hundreds of pounds of water, so the number helps when you choose a stand and a spot on the floor.
Aquarium Volume Calculator
How to Use the Aquarium Volume Calculator
- 1
Choose the tank shape
Select Rectangle, Cube, Cylinder, Bow-front or Hexagon. The inputs change to match the shape.
- 2
Pick inches or centimeters
Use the unit switch, then measure the tank. Centimeters are converted to inches behind the scenes.
- 3
Enter the dimensions
Type the length, width and height, or the diameter, side length, or bow-front end and center depths. Use the water height if you do not fill to the top.
- 4
Read the results
See the volume in US gallons and liters, and the water weight in pounds and kilograms.
How aquarium volume is calculated
The calculator finds the area of the tank's footprint, multiplies by the height to get cubic inches, then divides by 231, the number of cubic inches in one US gallon. Liters come from cubic inches times 0.016387. Water weight uses 8.34 pounds per US gallon, or about 1 kilogram per liter, for fresh water. Salt water is roughly 2 to 3% heavier.
US gallons = footprint area (sq in) x height (in) / 231- Rectangle: length x width
- Cube: side x side (height is the same side)
- Cylinder: 3.1416 / 4 x diameter squared
- Bow-front: length x end depth, plus 0.785 x length x (center depth - end depth), treating the bow as a half-ellipse
- Hexagon (regular): 2.598 x side length squared
Worked example: a 55 gallon tank
A common 55 gallon tank measures about 48 in long, 13 in front to back and 21 in tall on the outside.
- Cubic inches: 48 x 13 x 21 = 13,104
- Gallons: 13,104 / 231 = 56.7 US gallons
- Liters: 13,104 x 0.016387 = 214.7 liters
- Water weight: 56.7 x 8.34 = about 473 lb (215 kg)
- Now allow for 3/8 in glass and water 1 in below the rim. Entering the inside size of 47.25 x 12.25 x 19.625 in gives 49.2 gallons, about 13% less than the outside measurement.
- Bow-front example: 36 in long, 12 in deep at the ends, 16 in at the center of the bow and 20 in tall holds about 47.2 gallons (178.7 liters).
Common tank sizes and their volume
These are typical outside dimensions for standard rectangular tanks. Exact sizes vary by manufacturer, so measure your own tank. Gallons and weights are calculated from the dimensions shown.
| Tank | L x W x H (in) | Calculated volume | Water weight |
|---|---|---|---|
| 10 gallon | 20 x 10 x 12 | 10.4 gal (39.3 L) | 87 lb |
| 20 gallon long | 30 x 12 x 12 | 18.7 gal (70.8 L) | 156 lb |
| 29 gallon | 30 x 12 x 18 | 28.1 gal (106.2 L) | 234 lb |
| 40 gallon breeder | 36 x 18 x 16 | 44.9 gal (169.9 L) | 374 lb |
| 55 gallon | 48 x 13 x 21 | 56.7 gal (214.7 L) | 473 lb |
| 75 gallon | 48 x 18 x 21 | 78.5 gal (297.3 L) | 655 lb |
| 125 gallon | 72 x 18 x 21 | 117.8 gal (446 L) | 983 lb |
Real volume vs nominal volume
Tank names are nominal. Measuring the outside includes the thickness of the glass or acrylic, and most people fill an inch or so below the rim. Substrate, rock, wood and decor take up even more space, so the actual water in a planted or rock-heavy tank can be 10 to 20% below the label.
For medication and water conditioner doses, the most accurate approach is to measure the inside of the tank at the water line, or track the water you add during a fill. When in doubt, follow the product label and dose for the lower, real volume.
Planning a stand and floor location
- The water weight shown is only part of the load. Add the glass, substrate, rock, equipment and the stand itself.
- A widely used rule of thumb puts a filled tank at around 10 lb per gallon once glass and substrate are included. Treat that as a rough guide, not a rating.
- Use a stand designed for your tank size, and keep it level so the load spreads evenly across the base.
- For very large tanks, especially on upper floors, place them near a load-bearing wall or across floor joists, and ask a contractor or structural engineer if you are unsure.
Features
- Rectangle, cube, cylinder, bow-front and hexagon tank shapes
- Dimensions in inches or centimeters
- Volume in US gallons and liters
- Water weight in pounds and kilograms for stand planning
- Shows the formula for the selected shape
Frequently Asked Questions
How do I calculate how many gallons my fish tank holds?
Multiply length x width x height in inches to get cubic inches, then divide by 231. A tank that is 30 x 12 x 18 inches holds 6,480 cubic inches, or about 28.1 US gallons. For liters, multiply cubic inches by 0.016387, which gives about 106.2 liters for the same tank.
How much does a 55 gallon aquarium weigh when full?
A 48 x 13 x 21 inch tank holds about 56.7 gallons of water by its outside dimensions, and that water alone weighs about 473 pounds (215 kg). Glass, substrate, rock and equipment add more. A widely used rule of thumb of about 10 pounds per gallon puts a nominal 55 gallon tank at roughly 550 pounds in total.
How do I measure a bow-front aquarium?
Measure the length across the back, the front-to-back depth at each end, the depth at the middle of the bow, and the height. The calculator treats the curved front as a half-ellipse added to a rectangle. A 36 inch bow-front, 12 inches deep at the ends, 16 inches at the center and 20 inches tall holds about 47.2 gallons.
Why is my tank's real volume less than its label?
The label is based on outside dimensions. Glass thickness, the gap between the water line and the rim, and substrate and decor all take up space. A 48 x 13 x 21 inch tank measures 56.7 gallons outside, but with 3/8 inch glass and water 1 inch below the rim it holds about 49.2 gallons before any gravel or rock.
How do I convert aquarium gallons to liters?
Multiply US gallons by 3.785 to get liters, or divide liters by 3.785 to get gallons. A 20 gallon tank holds about 75.7 liters, and a 200 liter tank is about 52.8 US gallons. This calculator shows both at once, and you can enter measurements in centimeters if your tank is sold in metric sizes.
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