Aquarium Tank Volume Calculator
Our free fish tank gallons calculator finds your aquarium's volume from its dimensions โ in gallons and liters. It works for rectangular and cylindrical tanks, adjusts for how full you actually fill the tank, and estimates the water weight so you can plan a safe stand.
Enter your numbers
Your results
| Full tank volume (gallons) | โ |
|---|---|
| Full tank volume (liters) | โ |
| Fill level | โ |
| Water weight (estimate) | โ |
Water weight is an estimate: fresh water weighs about 8.34 lb per gallon, before the tank, substrate, and equipment are added.
How to use this calculator
Pick your tank shape (rectangular is the default) and your units, inches or centimeters. For a rectangular tank enter the length, width, and height; for a cylindrical tank enter the diameter and height. Measure the inside of the tank for the most accurate result.
Set the fill level to match how you actually fill your tank. Almost nobody fills to the very rim โ water usually sits an inch or two below the top edge, and gravel, rocks, and decorations all displace water. A setting of 85โ95% gives a realistic picture of what your fish actually swim in.
Results update instantly as you type. You'll see the water volume in gallons and liters, plus an estimated water weight so you can check your stand and floor can safely support the tank.
How it works
For a rectangular tank, the calculator multiplies length ร width ร height in inches to get cubic inches, then divides by 231 โ because one US gallon is exactly 231 cubic inches. For a cylindrical tank it uses the cylinder volume formula ฯ ร rยฒ ร height (with radius = diameter รท 2), then divides by 231 the same way. Liters are calculated by multiplying gallons by 3.78541.
The fill level percentage is applied to the full tank volume to get the actual water volume, and water weight is estimated at 8.34 pounds per US gallon of fresh water (converted to kilograms by multiplying by 0.4536).
Our assumptions: we use US gallons, assume a perfectly rectangular or cylindrical shape, and ignore glass thickness and rounded corners. Bow-front, hexagonal, and other irregular tanks are only approximated โ treat those results as estimates. Centimetre inputs are converted to inches (1 in = 2.54 cm) before the same math runs.
Frequently asked questions
How do I calculate my fish tank's volume in gallons?
Measure the inside length, width, and height of your tank in inches, multiply the three numbers together, then divide by 231. For example, a 24ร 12ร 16-inch tank holds (24 ร 12 ร 16) รท 231 โ 19.95 gallons, so it is sold as a 20-gallon tank.
How do I convert fish tank gallons to liters?
Multiply US gallons by 3.78541 to get liters. A 20-gallon aquarium is about 75.7 liters, and a 10-gallon tank is about 37.9 liters.
Why should I set the fill level below 100%?
Most tanks are never filled to the very rim: water usually sits an inch or two below the top edge, and substrate, rocks, and decorations displace water too. A fill level of 85โ95% gives a more realistic picture of the water your fish actually swim in.
How much does a full fish tank weigh?
Fresh water weighs about 8.34 pounds per gallon, so the water alone in a 20-gallon tank weighs roughly 167 pounds. Add the empty tank, glass, substrate, and equipment, and a typical 20-gallon setup totals 200โ225 pounds โ make sure your stand and floor can handle it.
Does this calculator work for cylindrical tanks?
Yes. Enter the diameter and height for a cylindrical (column or bowl-style) tank, and we use the cylinder volume formula ฯ ร rยฒ ร height, divided by 231 for gallons. Note that very irregular tanks, like bow-front or hexagonal designs, can only be approximated this way.