Aquarium Volume Calculator: How Many Gallons Is My Tank?
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The number printed on the box is a nominal size. Your heater, filter, medication doses and stocking all work better from the volume you actually have. Measure the inside of the tank, pick the shape, and the calculator does the rest. Product details below are from the listings; we haven't tested them.
Aquarium volume calculator
How to measure your tank
Measure the inside of the glass, not the outside edges, and use the height of the water, not the height of the tank. Glass thickness and the gap below the rim both shrink real volume, and gravel, rock and decor displace more. The calculator returns the volume of the space you measure, so measuring to the water line gives the most useful number.
- Bow front: measure the depth at one end (side depth) and the depth at the middle of the bow (center depth). The calculator treats the bow as a circular arc.
- Cylinder: measure straight across the widest point for the diameter.
- Hexagon: measure one of the six sides. The calculator assumes all six are equal.
The formulas
| Shape | Volume (cubic inches) |
|---|---|
| Rectangle | length x width x height |
| Cylinder | 3.1416 x (diameter / 2)² x height |
| Hexagon | 2.598 x side² x height |
| Bow front | (length x side depth + bow segment area) x height |
Then convert: gallons = cubic inches / 231, and liters = cubic inches x 0.016387. Both come from NIST's conversion tables, which define the US gallon as 231 cubic inches and a cubic inch as 16.387064 cubic centimeters.
Common tank sizes, worked out
| Dimensions (L x W x H, in.) | Calculated gallons | Water weight |
|---|---|---|
| 20 x 10 x 12 | 10.4 | 87 lb |
| 24 x 12 x 16 | 19.9 | 166 lb |
| 30 x 12 x 18 | 28.1 | 234 lb |
| 36 x 12 x 16 | 29.9 | 249 lb |
| 48 x 13 x 21 | 56.7 | 473 lb |
Outside dimensions overstate volume slightly. The Tetra 20 gallon kit lists 24 x 12 x 16 in. and the Tetra 29 gallon kit lists 30 x 12 x 18 in., per their listings.
What the number is for
Heater size. Aqueon's general rule is 5 watts per gallon for aquariums of 55 gallons or less, so a 20 gallon tank calls for about 100W. Fluval and EHEIM publish their own ratings; our heater guide compares the charts.
Aqueon Pro 100W Heater
- For tanks up to 30 gallons (title)
- Adjustable 68-88°F, accurate to ±1°, per the listing
- Shatterproof; auto shut-off if it overheats
Fluval M 150W Heater
- For tanks up to 45 gallons, per the listing
- Borosilicate glass; ceramic heat sink
Stocking. Gallons alone don't tell you how many fish fit. The CDC calls one inch of fish per gallon "a good standard," but that old shortcut ignores adult size, body bulk, waste, activity and tank shape. Start from the species instead; see how many fish in a tank for its limits.
Water treatments. Conditioner and medication doses are given per gallon on the bottle, so use your real volume.
Stand and floor. At about 8.33 pounds per gallon, water alone in a 55 gallon tank weighs over 450 pounds. Use a stand rated for the tank.
Starting fresh?
Kits list their dimensions, so you can check volume before you buy.
Tetra 20 Gallon Aquarium Kit
- 24 x 12 x 16 in. glass tank with hinged LED hood, per the listing
- Internal filter with carbon cartridge
- Heater and thermometer included
Tetra 29 Gallon Aquarium Kit
- 30 x 12 x 18 in. glass tank, LED hood, per the listing
- Whisper PF 30 filter
- 200W heater and thermometer included
FAQ
How do I calculate aquarium gallons?
Multiply length x width x height in inches to get cubic inches, then divide by 231, the number of cubic inches in a US gallon per NIST. A 24 x 12 x 16 in. tank holds 4,608 cubic inches, or about 19.9 gallons.
How much does a full aquarium weigh?
Water alone weighs about 8.33 pounds per gallon at 70°F, per the USGS, so a 20 gallon tank holds roughly 167 pounds of water before you add glass, gravel, rock and the stand.
Is the real water volume less than the calculated volume?
Yes. Measure the inside of the tank to the water line, not the outside edges, and remember gravel, rock and decor displace water. The calculator gives the volume of the space you measure.