Key takeaways
- Gallons = L × W × H ÷ 231 when all dimensions are in inches.
- A standard 24 × 12 × 16 in tank holds about 20 gallons (~75.5 L).
- Water weighs ≈ 8.34 lb per gallon — a 20-gallon tank is ~166 lb of water.
- Stock to the real volume (~10% less) after gravel, rocks, and the rim gap.
How to calculate aquarium volume
A rectangular fish tank is just a box, so its volume is length times width times height. Measure the inside dimensions in inches, multiply them together to get cubic inches, then divide by 231 — because one US gallon is exactly 231 cubic inches. To convert to liters, multiply the gallons by 3.785. This gives the brim-full volume; the water you actually keep is lower once substrate, hardscape, and the empty space below the rim are accounted for.
Knowing the true volume matters because stocking levels, dosing, and filter ratings are all based on actual water held — not the size printed on the box.
Worked example: a 24 × 12 × 16 inch tank
24 × 12 × 16 = 4,608 cubic inches. Divide by 231 and you get ≈ 20.0 gallons, which is about 75.5 liters and roughly 166 pounds of water. After ~10% for gravel and decor, plan on about 18 gallons of usable water for stocking.
Common aquarium sizes
| Tank | Typical dimensions (in) | Gallons | Water weight |
|---|---|---|---|
| 10-gallon | 20 × 10 × 12 | 10.4 | ~87 lb |
| 20-gallon | 24 × 12 × 16 | 20.0 | ~166 lb |
| 29-gallon | 30 × 12 × 18 | 28.1 | ~234 lb |
| 55-gallon | 48 × 13 × 21 | 56.8 | ~473 lb |
What to do with the number
Use the gallon figure to size your filter and heater and to set safe stocking limits. Check the water weight against your stand or furniture before filling. Once it's running, keep your fish hydrated and healthy alongside your other pets — see the dog water intake calculator for the household, or browse all CritterCalc tools.
Frequently asked questions
How do I calculate aquarium volume?
Multiply L × W × H in inches for cubic inches, then divide by 231 for US gallons. A 24 × 12 × 16 in tank ≈ 20 gallons.
How many gallons is my tank?
Measure inside length, width, and height in inches, multiply, and divide by 231. A 30 × 12 × 18 in tank ≈ 29 gallons.
Why subtract for substrate and decor?
Gravel, rocks, plants, and the rim gap displace water, so real stocked volume is usually ~10% less than the full figure.
How much does a full tank weigh?
Water is ~8.34 lb per gallon, so a 20-gallon tank is ~166 lb of water alone — confirm your stand is rated for it.
How do I convert liters to gallons?
Divide liters by 3.785 for US gallons, or multiply gallons by 3.785 for liters. 75 L ≈ 19.8 gallons.
What about a bow-front tank?
L × W × H assumes a box. A bow-front holds a bit more — add ~3–5%, or use the maker's stated volume.
One US gallon is defined as exactly 231 cubic inches and water density is taken at ~8.34 lb/gallon — see the US gallon definition from the National Institute of Standards and Technology.
Last reviewed June 2026