How to calculate aquarium volume
Measure the inside of the tank, and use the height of the water, not the glass. Then multiply and convert cubic inches to gallons.
Cylinder: gallons = 3.1416 × (diameter ÷ 2)² × height ÷ 231
Bow front: gallons ≈ length × height × (side width + ⅔ × (center width − side width)) ÷ 231
Liters = gallons × 3.785 · Water weight = gallons × 8.34 lb · Full setup ≈ gallons × 11.5 lb
Worked example
A tank 30 in long, 12 in wide, with water 12 in deep: 30 × 12 × 12 = 4,320 cubic inches ÷ 231 = 18.7 gallons (71 liters). The water weighs about 156 lb, and the full setup with glass and gravel about 215 lb. It needs a heater of about 100 W and a filter rated for 75 to 112 gallons per hour.
Standard aquarium sizes
| Tank | Size (L × W × H, in) | Full weight with gravel (approx.) |
|---|---|---|
| 10 gallon | 20 × 10 × 12 | 110 lb |
| 20 gallon long | 30 × 12 × 12 | 225 lb |
| 29 gallon | 30 × 12 × 18 | 330 lb |
| 40 gallon breeder | 36 × 18 × 16 | 460 lb |
| 55 gallon | 48 × 13 × 21 | 625 lb |
| 75 gallon | 48 × 18 × 21 | 850 lb |
| 125 gallon | 72 × 18 × 21 | 1,400 lb |
Heater, filter and stocking rules of thumb
- Heater: about 5 watts per gallon for small tanks and 3 watts per gallon for large ones. In a cold room, or for tanks over 50 gallons, two smaller heaters are safer than one big one.
- Filter: pick one that moves 4 to 6 times the tank volume per hour.
- Gravel: about 1 lb per gallon gives a 1 inch bed; use 2 lb per gallon for 2 inches or for planted tanks.
- Stocking: the old "1 inch of fish per gallon" rule only works roughly for small, slim fish. Goldfish, cichlids and other large or messy fish need much more. Check each species' adult size and minimum tank.
Building a pond instead? Try the pond volume calculator. Keeping turtles or axolotls? See the reptile tank size calculator.
Frequently asked questions
How do I calculate how many gallons my fish tank holds?
Multiply the inside length, width and water height in inches, then divide by 231. A 30 × 12 × 12 in tank holds about 18.7 gallons.
How much does a full aquarium weigh?
Water weighs about 8.34 lb per gallon. With glass, gravel and decor, a full setup weighs about 11 to 12 lb per gallon, so a 55 gallon tank comes to around 625 lb.
What size heater do I need for my aquarium?
About 5 watts per gallon for tanks up to about 30 gallons and 3 watts per gallon for larger tanks. A 20 gallon tank needs about 100 W.
How many fish can I put in my tank?
The "1 inch of fish per gallon" rule is only a rough guide for small fish. Consider each fish's adult size, how messy it is and your filter, and add fish slowly.
How do I convert gallons to liters?
Multiply US gallons by 3.785. A 20 gallon tank is about 76 liters.
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Last updated October 2026.