Aquarium Volume Calculator
Use this tool to find your aquarium's actual water volume or convert known quantities between units. Select your calculation mode, enter tank measurements or an existing amount, and account for displacement from hardscape and gravel.Use this tool to find your aquarium's actual water volume or convert known quantities between units. Select your calculation mode, enter tank measurements or an existing amount, and account for displacement from hardscape and gravel.[1]
45.5L
Water after sand and decor · 12 US gal
- Glass box, filled to the brim
- 60.7 L
- Sand or gravel
- − 8.6 L
- Rocks and wood
- − 2.4 L
Also allows for glass thickness and 2.5 cm of air below the rim.
How is the result calculated?
The default "From tank size" mode asks for inside length, width and height in centimetres or inches, alongside sand or gravel depth and a rocks-and-wood level. For basic rectangular brim capacity, multiply internal length by width by height in centimetres, then divide by 1000 to get litres.The default "From tank size" mode asks for inside length, width and height in centimetres or inches, alongside sand or gravel depth and a rocks-and-wood level. For basic rectangular brim capacity, multiply internal length by width by height in centimetres, then divide by 1000 to get litres.[1]
Displacement-based volume is an estimate, not an accurate measurement. To calculate real water, subtract the unfilled space above the waterline and the volume occupied by substrate and decoration. Measuring the water you add during filling provides a helpful check on the estimate.Displacement-based volume is an estimate, not an accurate measurement. To calculate real water, subtract the unfilled space above the waterline and the volume occupied by substrate and decoration. Measuring the water you add during filling provides a helpful check on the estimate.[1]
For example, entering 60 cm × 30 cm × 36 cm in "From tank size" gives a glass-adjusted brim capacity of 61 L (61 L). Subtracting a 2.5 cm air gap below the rim (4.3 L), 5 cm of substrate (8.6 L) and Light decoration (2.4 L) leaves 46 L (46 L) of real water.For example, entering 24 in × 12 in × 14 in in "From tank size" gives a glass-adjusted brim capacity of 16.0 gal (16.0 gal). Subtracting a 1 in air gap below the rim (1.1 gal), 2 in of substrate (2.3 gal) and Light decoration (0.6 gal) leaves 12.0 gal (12.0 gal) of real water.
Decor options include None, Light, Some and Lots, displacing fractions of 0, 0.05, 0.1 and 0.2 of the water remaining after the air gap and substrate allowances. The breakdown shows brim capacity, substrate, and rocks and wood, while noting the air-gap allowance separately.Decor options include None, Light, Some and Lots, displacing fractions of 0, 0.05, 0.1 and 0.2 of the water remaining after the air gap and substrate allowances. The breakdown shows brim capacity, substrate, and rocks and wood, while noting the air-gap allowance separately.
The "Convert units" mode converts an existing amount between litres, US gal and UK gal. Changing the unit converts the amount while keeping the same water volume, subject to rounding to one decimal place. Entering 75 L displays 19.8 on the US scale and 16.5 on the UK scale.The "Convert units" mode converts an existing amount between litres, US gal and UK gal. Changing the unit converts the amount while keeping the same water volume, subject to rounding to one decimal place. Entering 19.8 gal displays 19.8 on the US scale and 16.5 on the UK scale.[3]
Why is usable water less than tank capacity?
Sand, gravel and rocks occupy space in the tank that would otherwise hold water. In addition, the air gap under the lid sits above the water surface, further reducing the final filled volume compared to the brim capacity.Sand, gravel and rocks occupy space in the tank that would otherwise hold water. In addition, the air gap under the lid sits above the water surface, further reducing the final filled volume compared to the brim capacity.[1]
For routine water changes, base your calculations on the furnished aquarium's real water amount rather than the sales label. Keep a written note of the volume added when setting up the tank or after rearranging hardscape.For routine water changes, base your calculations on the furnished aquarium's real water amount rather than the sales label. Keep a written note of the volume added when setting up the tank or after rearranging hardscape.[1]
Does aquarium shape matter for fish?
Tank proportions change the environment significantly. A long aquarium offers a different swimming track and floor footprint than a tall aquarium with identical capacity. Bottom-dwelling species rely on floor area, while active species need horizontal swimming distance.Tank proportions change the environment significantly. A long aquarium offers a different swimming track and floor footprint than a tall aquarium with identical capacity. Bottom-dwelling species rely on floor area, while active species need horizontal swimming distance.[1][2]
Vertical height cannot replace a species' requirement for horizontal length. Review adult sizes and social group needs in the Vertical height cannot replace a species' requirement for horizontal length. Review adult sizes and social group needs in the plannerplanner before purchasing fish, ensuring the floor area fits your livestock. before purchasing fish, ensuring the floor area fits your livestock.[1][2]
Questions
How do I calculate aquarium volume in litres?
Multiply internal length by width by water depth in centimetres, then divide by 1000. This calculation gives the basic water volume before accounting for displacement from hardscape and substrate.Multiply internal length by width by water depth in centimetres, then divide by 1000. This calculation gives the basic water volume before accounting for displacement from hardscape and substrate.[1]
Why are US and Imperial volume conversions different?
They are entirely different measurement units, despite sharing the gallon name. The US liquid gallon converts using a factor of 3.785, whereas the Imperial gallon converts using 4.546.They are entirely different measurement units, despite sharing the gallon name. The US liquid gallon converts using a factor of 3.785, whereas the Imperial gallon converts using 4.546.[3]
Should I use tank capacity or actual water for a water change?
Use actual water volume. Substrate, rocks and the unfilled surface air gap reduce the liquid volume, so routine water change percentages should always follow the real water amount.Use actual water volume. Substrate, rocks and the unfilled surface air gap reduce the liquid volume, so routine water change percentages should always follow the real water amount.[1]
Can a tall aquarium hold the same fish as a long one?
No, identical capacity does not mean two differently proportioned tanks suit the same livestock. Active swimmers require horizontal length to cruise, and bottom dwellers require adequate floor footprint.No, identical capacity does not mean two differently proportioned tanks suit the same livestock. Active swimmers require horizontal length to cruise, and bottom dwellers require adequate floor footprint.[1][2]
How do I check the result before using it?
Measure the water you pour into the aquarium when filling it. Recording the actual buckets or containers added provides a practical check against your calculated estimate.Measure the water you pour into the aquarium when filling it. Recording the actual buckets or containers added provides a practical check against your calculated estimate.[1]
Sources
- Choosing an aquarium — RSPCA. Accessed 2026-10-05.
- Understanding pet fish behaviour — RSPCA. Accessed 2026-10-05.
- NIST Guide to the SI, Appendix B.9: Factors for units listed by kind of quantity or field of science — NIST. Accessed 2026-10-05.
Written from the sources above. Not yet reviewed by an aquarist.