Aquarium Volume Calculator
A free aquarium and fish tank volume calculator. Get the water volume of any tank in US gallons and litres. Twelve shapes cover standard rectangles, hexagons, flat-back hexagons, bowfronts, corner pentagons, and sumps — with live fill depth, displacement, weight, and a printable spec sheet.
Rectangle — top view
- L = Length
- W = Width
- H = Height
Cube — top view
- L = Length
Hexagon — top view
- L = Length (across corners)
- W = Width (across flats)
- H = Height
Octagon — top view
- W = Width (across flats)
- H = Height
Flat-Back Hexagon — top view
- L = Back length
- W = Depth
- X = Front pane
- Y = End pane
- H = Height
Cylinder — top view
- R = Radius
- H = Height
Half Cylinder — top view
- R = Radius
- H = Height
Corner Cylinder — top view
- R = Radius
- H = Height
Elliptical (Oval) — top view
- R₁ = Radius 1
- R₂ = Radius 2
- H = Height
Corner Pentagon — top view
- L = Long side
- S = Short side
- H = Height
L-Shaped — top view
- L₁ = Length 1
- W₁ = Width 1
- L₂ = Length 2
- W₂ = Width 2
- H = Height
Corner Prism — top view
- L₁ = Length 1
- L₂ = Length 2
- H = Height
Fill & displacement (optional)
Substrate volume is estimated from the top-view area × depth; rock is taken as a percentage of the water volume. Glass thickness is not included — these are estimates.
Filled volume
Rectangle
— liters
Showing fill depth — max capacity below.
- Max capacity
- — gal / — L
- Water weight
- — lb / — kg
- Per inch of depth
- — gal / — L per cm
- Surface area
- — in² / — cm²
Est. displacement − — gal
Net volume — gal / — L
Estimates only; glass thickness and interior bracing are not included.
History
Saved results appear here. Up to 5 are kept for this session.
How to measure an aquarium or fish tank
From tape measure to an accurate gallon count for any shape.
Step 1
Select the tank shape
Pick from twelve shapes, including rectangle, hexagon, flat-back hexagon, bowfront, corner pentagon, and L-footprint sumps.
Step 2
Measure the inside dimensions
Record the interior length, width, and height, plus a radius for round or oval tanks. Inside measurements account for glass thickness.
Step 3
Enter the dimensions
Type each value and switch between inches and centimeters. The result updates as you type.
Step 4
Add fill depth and displacement
Optionally enter the actual water depth, substrate depth, and an estimated rock percentage to get a net volume.
Step 5
Read and save the result
The filled volume shows in US gallons and litres with weight and surface area. Use Copy link to share the exact inputs or Print for a spec sheet.
Fish tank volume in gallons and litres
The calculator reports every result in both units at once. Working in US gallons, measure the inside of the tank in inches: a rectangular tank’s volume is length × width × height ÷ 231. Working in litres(sometimes spelled liters), measure in centimeters and divide the product by 1,000. To convert between them, 1 US gallon equals 3.785 litres; the imperial gallon still used for some UK tanks is larger at 4.546 litres. Because most tanks are not filled to the brim and substrate and rock take up space, the net filled volume is what matters for dosing, water changes, and equipment sizing.
Inside dimensions & displacement
Advertised gallon ratings are rounded and usually measured from the outside, so they overstate the water a tank actually holds. Measuring the inside and subtracting the substrate and rock gives a net volume that matches what you dose, mix, and filter. Enter a substrate depth and a rock percentage and the calculator reports the net water volume directly. These remain estimates — glass thickness and internal bracing are not included.
Frequently asked questions
- How do I calculate fish tank volume in gallons?
- Measure the inside length, width, and height in inches, multiply them together, and divide by 231 for US gallons. For example, a 36 × 18 × 18 inch rectangular tank holds about 50.6 gallons before substrate and rock displacement. This calculator does the arithmetic for twelve tank shapes.
- How do I calculate aquarium volume in litres?
- Measure the inside dimensions in centimeters and multiply length × width × height, then divide by 1,000 for litres. If you already know the volume in US gallons, multiply by 3.785 to convert to litres (an imperial gallon used in the UK is larger: 4.546 litres).
- Does the calculator show gallons and litres at the same time?
- Yes. Every result is reported in US gallons and litres side by side, along with water weight in both pounds and kilograms, so you can switch between unit systems without recalculating.
- Why is my real volume lower than the advertised gallons?
- Listed gallons are rounded and usually computed from outside dimensions. Measuring inside and subtracting substrate and rock gives a lower, more accurate number for dosing, salt mixing, and filtration sizing.
- Should I measure inside or outside dimensions?
- Inside. Glass thickness, trim, and internal bracing reduce capacity, so interior measurements give the true water volume.
- Can I calculate sump, bowfront, and hexagon tank volume?
- Yes. Treat a rectangular sump as a rectangle, a bowfront or oval tank as Elliptical, and a flat-backed six-pane tank as Flat-Back Hexagon, then enter the inside dimensions for each.
- How are substrate and rock handled?
- Enter the substrate depth and a rock estimate as a percentage of the water volume. The calculator subtracts that displacement and reports the net water volume you actually fill and filter.