Rectangular Tank Volume Calculator (Gallons and Litres)
Work out how many gallons or litres a rectangular tank or aquarium holds from its length, width and height, and how much water is in it at a given fill height.
Measured from the lowest inside point.
Enter the dimensions to see the volume.
About the rectangular tank volume calculator
The Rectangular Tank Volume Calculator gives the capacity of any box-shaped container from its length, width and height. It opens with the rectangular shape selected, which suits aquariums, sumps, cisterns, water butts, bunded drip trays and open troughs. The result appears in US gallons, imperial gallons, litres, cubic feet and cubic metres, and each side can be entered in feet, inches, yards, metres, centimetres or millimetres, so you can mix units freely.
Enter a fill height as well to see how full the tank is and how much liquid is in it. That answers questions like how many litres an aquarium holds at its normal water line, rather than at the brim.
How to calculate rectangular tank volume
Steps
- Make sure Rectangular is the selected shape.
- Enter the length, width and height, each with its own unit.
- Use inside measurements. Glass, steel or concrete walls take up space, so outside dimensions overstate the capacity. For a 10 mm glass aquarium, subtract 20 mm from the length and width, and subtract the base thickness from the height.
- Optionally enter the fill height, measured from the inside floor. It cannot be taller than the inside height.
- Read the total capacity and, if you gave a fill height, the percent full and the volume of liquid.
Formula
With a fill height , the liquid volume is , and the percent full is . A box has constant cross-section, so depth and volume rise in a straight line.
Worked example: a small cistern
A tank measures 4 ft long, 3 ft wide and 2 ft high inside. The volume is cubic feet. That is 179.5 US gallons, 149.5 imperial gallons or 679.6 litres. Because the US and imperial gallons differ, check which one your supplier or label uses before ordering a refill.
Worked example: mixed units
You have a sump 2 m long, 50 cm wide and 1000 mm deep. There is no need to convert by hand. Pick metres for the length, centimetres for the width and millimetres for the height, and the calculator returns 1 cubic metre, which is 1,000 litres. This is handy when drawings use one unit and a tape measure uses another.
Worked example: a partly filled tank
Take the 4 ft by 3 ft by 2 ft tank above and fill it to 1 ft. The liquid volume is cubic feet, which is half the total and 50 percent full. Unlike a round horizontal tank, a rectangular one fills evenly, so a quarter of the height is always a quarter of the volume.
Tips for aquariums and sumps
Aquariums are rarely filled to the brim, and substrate, rocks and equipment displace water. Enter the water line as the fill height to get the water volume, then expect the real figure to be somewhat lower once decorations are in. For a sump or cistern, the overflow or outlet usually sits below the top, so the working volume is also smaller than the total. If your tank has sloped sides or a curved front, treat the figure as an upper limit, or try another shape on the Tank Volume Calculator.
Converting and planning
If you need another volume unit, the Volume Converter converts the result, and the Length Converter helps when dimensions come in unfamiliar units. To plan the base, the Area Calculator gives the footprint, and the Concrete Calculator estimates the volume for a slab or a below-ground tank surround.
Accuracy and limits
The result is geometric. It assumes square corners and flat walls, and it ignores internal braces, fittings, overflow pipes and outlets that sit above the floor. Measure inside dimensions, and check the manufacturer's rating for the usable volume, particularly for drinking water, fuel or chemical storage where regulations may apply.
Privacy
Nothing is uploaded; every calculation runs in your browser and never leaves your device.
Related tools
For round tanks, open the Cylinder Tank Volume Calculator. For capsule, oval and cone-bottom shapes, the full Tank Volume Calculator has all six. The Gravel Calculator helps estimate bedding material under a tank.
Frequently asked questions
How do I calculate the volume of a rectangular tank?
Multiply length by width by height, all measured inside the tank. A tank 4 ft by 3 ft by 2 ft holds 24 cubic feet, which is 179.5 US gallons or 679.6 litres.
Can I enter the sides in different units?
Yes. Each of the three dimensions has its own unit selector, so you can use metres, centimetres and millimetres together. A tank 2 m by 50 cm by 1000 mm comes out as 1 cubic metre, or 1,000 litres.
How do I find the volume of water in an aquarium?
Use the inside length and width and enter the water depth as the fill height. The calculator gives the water volume and the percent full. Substrate and decorations displace some water, so the real volume will be a little lower.
What happens if the fill height is larger than the tank?
The calculator shows an inline error that names the maximum allowed, which is the inside height of the tank. Reduce the fill height or correct the height you entered.
Why is the actual capacity lower than the calculated one?
Wall thickness, braces, fittings and an overflow or outlet set below the top all cut into the usable volume. Measure inside dimensions and check the manufacturer's rating for the figure to rely on.
How many litres are in a US gallon and an imperial gallon?
A US gallon is 3.785411784 litres and an imperial gallon is 4.54609 litres. The calculator shows both next to litres, cubic feet and cubic metres, so you can pick the unit your supplier uses.
Is my data sent anywhere?
No. The box tank capacity sums run in your browser, so your length, width, height and fill level are never uploaded or stored on a server. They stay on this device and are not shared with anyone.