Aquarium Volume Calculator
An aquarium volume calculator multiplies length × width × height in inches and divides by 231, so a 48 × 18 × 21 in tank holds 78.55 gallons (297 L).
How many gallons does my aquarium hold?
An aquarium volume calculator turns three tape-measure readings into gallons. Multiply the length, width, and height in inches, then divide by 231, because a US gallon is 231 cubic inches. Multiply the gallons by 3.785 for liters. A standard 48 × 18 × 21 inch tank works out to 78.55 gallons, or 297 liters.
Enter the three dimensions in inches and the calculator shows gallons and liters together. Use the result to size the heater, filter, and stocking level, and to check that your stand and floor can carry the weight.
The formula in plain words
A rectangular tank is a box. Its volume is the floor area of the box times its height:
- Cubic inches = length × width × height
- Gallons = cubic inches ÷ 231
- Liters = gallons × 3.785
The width here means front-to-back depth. The height is measured from the bottom glass to the top rim.
Worked examples
These match the calculator’s output.
- Standard 75-gallon footprint, 48 × 18 × 21 in
- 48 × 18 × 21 = 18,144 cubic inches
- 18,144 ÷ 231 = 78.55 gallons
- 78.55 × 3.785 = 297.33 liters
The tank is sold as a 75 because the label rounds down and the measurement is taken outside the glass. Once you allow for the glass, a 2-inch sand bed, and filling an inch below the rim, expect closer to 65 to 70 gallons of actual water.
- 40-gallon breeder, 36 × 18 × 16 in
- 36 × 18 × 16 = 10,368 cubic inches
- 10,368 ÷ 231 = 44.88 gallons
- 44.88 × 3.785 = 169.90 liters
At 8.34 lb per gallon, the water alone in a full breeder weighs about 374 lb.
Common aquarium sizes
Nominal dimensions vary a little by brand. These are typical outside measurements, with the calculator’s gross volume.
| Sold as | L × W × H (in) | Calculated gallons | Calculated liters |
|---|---|---|---|
| 10 gallon | 20 × 10 × 12 | 10.39 | 39.3 |
| 20 long | 30 × 12 × 12 | 18.70 | 70.8 |
| 20 high | 24 × 12 × 16 | 19.95 | 75.5 |
| 40 breeder | 36 × 18 × 16 | 44.88 | 169.9 |
| 55 gallon | 48 × 13 × 21 | 56.73 | 214.7 |
| 75 gallon | 48 × 18 × 21 | 78.55 | 297.3 |
| 125 gallon | 72 × 18 × 21 | 117.82 | 446.0 |
| 180 gallon | 72 × 24 × 25 | 187.01 | 707.9 |
A 20 long and a 20 high hold about the same water. The long tank has more floor area, which helps bottom-dwelling fish and gas exchange at the surface.
Equipment sized from volume
| Item | Common rule of thumb | For a 55-gallon tank |
|---|---|---|
| Heater | 3–5 W per gallon | 165–275 W |
| Filter flow | 4–6 × volume per hour | 220–330 gph |
| Water weight | 8.34 lb per gallon | about 459 lb |
| Weekly water change | 10–25% of volume | 5.5–14 gallons |
These are hobby rules of thumb. Heavily stocked tanks, cold rooms, and marine systems often need more.
How to measure your tank
- Use inches. The calculator expects inches for all three numbers.
- Measure inside the glass if you can reach. Outside measurements run high because 6 to 12 mm glass is counted as water.
- Measure height to the water line, not the rim, if you want the volume you actually fill.
- For bowfronts, average the front-to-back depth at the ends and the center.
- For sumps, run the sump’s own dimensions separately and add the gallons. Only count the water level it runs at, not the full box.
Common mistakes
- Dosing medication by the label size. A “75” with 3 inches of sand and a large rockscape may hold 60 gallons. Dosing for 75 overdoses by a quarter.
- Mixing up width and height. Width is front to back. Swapping it with height on a tall tank changes nothing in the math, but it confuses the substrate figure later.
- Forgetting the stand load. Water is heavy. A 125-gallon setup can pass 1,200 lb, so check the floor before you fill.
- Using liters with a US-gallon dose. Many conditioners are dosed per 10 US gallons. Convert first.
Related calculators
To work out how much sand or gravel goes in the bottom, use the aquarium substrate calculator. Cylinders and odd-shaped tanks fit the tank volume calculator. Outdoor water features measured in feet belong in the pond volume calculator.
Frequently asked questions
How do I calculate aquarium gallons?
Multiply length, width, and height in inches, then divide by 231, the number of cubic inches in a US gallon. A 36 × 18 × 16 in tank is 10,368 cubic inches, and 10,368 ÷ 231 = 44.88 gallons. Multiply gallons by 3.785 for liters (169.9 L).
Is the calculated volume the same as the water volume?
No. It is the volume of the box you measured. If you used outside dimensions, the glass walls take some of it, you fill an inch or so below the rim, and substrate, rock, and decor displace more. Real water is often 10 to 20% less than the calculated figure, so a 78.55-gallon result may hold roughly 63 to 70 gallons of water.
How much does a filled aquarium weigh?
Fresh water weighs about 8.34 lb per gallon, so a tank holding 75 gallons of water carries about 625 lb of water alone. Add the glass, stand, and substrate (sand or gravel adds roughly 1 to 1.5 lb per gallon of tank at a typical 2-inch bed), and a 75-gallon setup commonly lands in the 800 to 900 lb range. Check that the floor and stand are rated for it.
What size heater and filter does my tank need?
Common hobby rules of thumb are 3 to 5 watts of heater per gallon and a filter rated to turn the full volume over 4 to 6 times an hour. For a 55-gallon tank that is a 165 to 275 W heater and a filter rated around 220 to 330 gallons per hour. Use your net water volume, not the glass volume.
Does this work for a bowfront or hexagon tank?
It is exact for a rectangular tank. For a bowfront, measure the depth at the deepest point and at the ends and use the average, which gets you close. For a hexagon or cylinder, use the tank volume calculator, which handles round and odd shapes.
Estimates only. Verify quantities with your supplier before purchasing.