Pick your tank shape, type the inside measurements and see how much water it holds in liters, US gallons and UK gallons, how much is left after gravel and rocks, and what the water weighs.
Actual water after 10% for substrate and decor: 193.3 L (51.1 US gal).
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This aquarium volume calculator is for fishkeepers sizing a new tank, dosing medicine or water conditioner, or checking that a stand and floor can take the load. It works for rectangle, cylinder, bow front and hexagon tanks in inches or centimeters, and shows both the full volume and a more realistic figure once substrate and decor take up space.
How Do You Calculate Aquarium Volume in Gallons?
Multiply length by width by water height in inches, then divide by 231 to get US gallons. A 48 by 13 inch tank filled to 21 inches holds 13,104 cubic inches, which is 56.7 US gallons or 214.7 liters. In centimeters, divide length times width times height by 1,000 to get liters.
What Are the Formulas for Each Tank Shape?
Every tank shape uses the same idea: find the area of the base, then multiply by the water height.
Cylinder: π × (diameter ÷ 2)² × height
Hexagon: 2.598 × side² × height
Bow front: (length × end width + bow segment area) × height
US gallons = cubic inches ÷ 231 liters = cubic cm ÷ 1,000
A US gallon is defined as exactly 231 cubic inches, or 3.785 liters. A UK gallon, also called the imperial gallon, is larger at 4.546 liters, so the same tank shows fewer UK gallons. One liter is 1,000 cubic centimeters. For a bow front tank, this tool treats the curved front as a circular arc and works out the extra area from the length and how far the center bulges past the ends. A regular hexagon base with side s has an area of 2.598 × s², the number being 3√3 ÷ 2.
How Much Water Is Really in My Aquarium?
A furnished aquarium usually holds about 10 to 20% less water than its full volume, because gravel, sand, rocks, wood and the gap at the top all take up space. That is why this calculator asks for the water height, not the glass height, and lets you set a loss percentage. Use the lower, actual figure when you dose medicine, and use the full volume when you buy a filter or heater, since those are rated by tank size.
How Much Does a Full Fish Tank Weigh?
Freshwater weighs about 1 kg per liter, or 8.34 lb per US gallon, so water alone in a 55 to 60 gallon tank weighs close to 500 lb. Saltwater is denser, with a specific gravity near 1.025, so a reef tank weighs about 2.5% more. Add the glass, stand, rock and substrate on top, and a large tank can pass 600 lb. Place big tanks across floor joists and close to a load bearing wall. To convert any result yourself, the liters to gallons converter handles both gallon types, and the inches to cm converter helps if your tape and the tank label use different units.
Common Aquarium Sizes in Gallons and Liters
These standard rectangle tanks are listed by outside size, so real water volume is a little lower.
| Size (L × W × H in) | US gallons | Liters | UK gallons |
|---|---|---|---|
| 20 × 10 × 12 | 10.4 | 39.3 | 8.7 |
| 30 × 12 × 12 | 18.7 | 70.8 | 15.6 |
| 30 × 12 × 18 | 28.1 | 106.2 | 23.4 |
| 36 × 18 × 16 | 44.9 | 169.9 | 37.4 |
| 48 × 13 × 21 | 56.7 | 214.7 | 47.2 |
| 48 × 18 × 21 | 78.5 | 297.3 | 65.4 |
| 72 × 18 × 21 | 117.8 | 446.0 | 98.1 |
Worked Example
Sam has a tank measured inside at 48 inches long and 13 inches wide, filled to 21 inches. The tool shows 56.7 US gallons, which is 214.7 liters or 47.2 UK gallons. With 10% lost to gravel and rocks, the actual water is 193.3 L (51.1 US gal), so Sam doses medicine for about 51 gallons. The water alone weighs 215 kg, or 473 lb. A 24 inch wide cylinder filled to 30 inches holds 222.4 liters (58.8 US gallons), and a hexagon with 12 inch sides filled to 24 inches holds 147.1 liters (38.9 US gallons).
How Do You Use the Aquarium Volume Calculator?
Choose the tank shape
Tap Rectangle, Cylinder, Bow front or Hexagon. The boxes change to match.
Enter the inside sizes
Type the inside measurements and the water height, in inches or centimeters.
Read the volume
The big number is the full volume, with the other units on the line below.
Check water and weight
The boxes show liters, US and UK gallons, actual water and water weight.
Frequently Asked Questions
How many liters is a 55 gallon tank?
A 55 US gallon tank holds about 208 liters, because one US gallon is 3.785 liters. In UK gallons that is about 45.8. After gravel and decor, the real water is often closer to 45 to 50 US gallons.
Should I measure the inside or outside of the tank?
Measure the inside if you can. Glass is often 6 to 12 mm thick on each side, which removes several liters from a large tank. Measure the height from the top of the substrate to the waterline for the most accurate dosing volume.
How do I measure a bow front aquarium?
Measure the straight length along the back, the width at one end from back glass to front glass, and the depth at the center of the bow. Enter all three with the water height and the tool adds the curved area.
How much does a gallon of water weigh?
A US gallon of freshwater weighs about 8.34 lb, or 3.79 kg. A UK gallon weighs about 10 lb, or 4.55 kg. Saltwater at a specific gravity of 1.025 weighs about 2.5% more than freshwater.
Why does my tank hold less water than the label says?
Labels usually give nominal volume from the outside size, filled to the brim. Glass thickness, the gap under the rim, substrate and decor all lower the real volume, often by 10 to 20%.
How much substrate reduces tank volume?
A 2 inch bed of gravel in a 48 by 13 inch tank takes up 1,248 cubic inches, about 5.4 US gallons. Gravel has gaps that hold some water, so the real loss is a little less than that figure.
Sources and Further Reading
- NIST Handbook 44, Appendix C: General Tables of Units of Measurement
- UK legislation: Weights and Measures Act 1985, Schedule 1 (gallon definition)
- USGS Water Science School: Water density
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Last reviewed: October 2026