Aquarium Volume Calculator
Tank gallons, liters, and water weight from inside length, width, and height.
What it does
Tank names lie a little: a nominal 50 gallon breeder can hold 50.5 gallons to the rim yet far less once substrate and the fill gap take their share. Inside dimensions tell the truth, and the tool above converts them to gallons, liters, and water weight in one pass.
The imperial default measures 36 by 18 by 18 inches. Multiplying gives 11,664 cubic inches, and dividing by 231 gives 50.5 gal, which is 191.1 L. Water at 8.34 lb per gallon puts the liquid alone at 421.1 lb before glass, sand, and rocks join.
Two constants carry the page. NIST defines the US gallon as exactly 231 cubic inches, and USGS water data pins a gallon near 8.34 lb with a liter near 1 kg. Metric mode skips the gallon entirely: centimeters cubed divided by 1000 is liters straight away.
Weight decides furniture as well as curiosity. A filled 50 gallon tank with stand and decor can press past 500 lb on four small feet, so floors and bargain shelves deserve a check first. Salt mixes run a touch heavier than fresh water, which the limits note covers.
How to use
- Pick imperial (in) or metric (cm).
- Measure inside length and width, glass to glass.
- Measure water height to the fill line, not the rim.
- Read gallons or liters first for stocking and dosing.
- Read water weight before trusting a stand or shelf.
Formula
- V
- Full interior water volume.
- 231
- Cubic inches in one US gallon (NIST).
- 8.34 / 1
- Pounds per gallon and kg per liter of fresh water (USGS).
Box tanks only; bowfronts and cylinders need their own geometry. Real water volume falls short of the result once substrate, rocks, and the air gap move in.
Worked example
Example: 36 x 18 x 18 in tank
Imperial defaults: 36, 18, 18 inches.
- Multiply: 36 x 18 x 18 = 11,664 cubic inches.
- Gallons: 11,664 / 231 = 50.5 gal.
- Liters: 11,664 x 16.387 / 1000 = 191.1 L.
- Weight: 50.5 x 8.34 = 421.1 lb of water.
- Tool shows 50.5 gal, 191.1 L, 421.1 lb.
Reference table
| Inside dims | Gallons | Liters |
|---|---|---|
| 24 x 12 x 12 in | 15 gal | 56.6 L |
| 24 x 12 x 16 in | 19.9 gal | 75.5 L |
| 36 x 18 x 18 in | 50.5 gal | 191.1 L |
| 48 x 13 x 21 in | 56.7 gal | 214.7 L |
Limits
- Rectangular glass only; curved panes change the math.
- Full to the measured line; air gaps and substrate excluded.
- Fresh water weights used; marine mix adds about 2.5%.
- Glass, stand, and decor weight sit outside the figure.
Common mistakes
- Measuring outside glass instead of inside.
- Filling math to the rim instead of the water line.
- Using imperial gallons where US gallons belong.
- Forgetting substrate displaces several gallons.
- Trusting the nominal size for medication dosing.
Try it yourself
Quick checks. Answers sit tucked under each question.
24 x 12 x 16 in?
4,608 in3 / 231 = 19.9 gal, 75.5 L, 166.4 lb of water.
60 x 30 x 30 cm?
54,000 cm3 = 54 L, 14.3 gal, 54.0 kg of water.
20 x 10 x 12 in?
2,400 in3 / 231 = 10.4 gal nominal 10 tank.
Questions
How many gallons is 36 x 18 x 18?
50.5 gal to the measured line, sold as a 50.
Why 231?
NIST defines the US gallon as exactly 231 cubic inches.
How heavy is a filled 50 gallon tank?
Water alone is 421.1 lb; glass and decor push past 500.
Liters from cm?
Multiply the three sides and divide by 1000.
Does gravel change it?
Yes. Deep substrate can displace 10% or more of the volume.
Saltwater heavier?
Slightly, near 8.55 lb per gallon at full marine mix.