Rectangular Tank Volume Calculator
Work out the volume and capacity of a rectangular (box) tank — in US gallons, imperial gallons, litres, cubic feet and cubic metres. Enter the length, width and height for the total capacity, or turn on partial fill to find how much liquid is in a part-full tank.
Tank volume calculator
How to calculate rectangular tank volume
A rectangular tank — a box or cuboid — is the simplest shape to work out. The volume of a rectangular tank is just its three inside dimensions multiplied together:
That is the rectangular tank volume formula, often written V = l × w × h. Every horizontal slice of the tank is the same rectangle, so the capacity is the base area (length × width) multiplied by the height. Keep all three measurements in the same unit and the result comes out in that unit cubed. This page works as both a rectangular tank capacity calculator and a rectangular water tank capacity calculator — the geometry is identical for water, fuel or any other liquid.
Partial fill is linear for a box tank
Because the cross-section never changes, partial fill is proportional to depth. The volume of a partially filled rectangular tank at a liquid height hf is:
So a box tank filled to half its height holds exactly half its capacity, and to a quarter of its height a quarter of its capacity. This is the same straightforward behaviour as a vertical cylinder, and unlike a horizontal cylinder where the liquid forms a circular segment. For the derivations and sources behind every formula on this site, see the methodology page.
Worked example
Take a rectangular tank measuring 6 ft long, 4 ft wide and 4 ft high.
- Total volume: V = 6 × 4 × 4 = 96 ft³. At 7.48052 US gallons per cubic foot, that is about 718 US gallons (≈ 598 imperial gallons, ≈ 2,718 litres).
- Half full (liquid height = 2 ft): V = 6 × 4 × 2 = 48 ft³ ≈ 359 US gallons — exactly half the total, because fill is linear with depth.
- Quarter height (liquid height = 1 ft): V = 6 × 4 × 1 = 24 ft³ ≈ 180 US gallons, a quarter of the full capacity.
Enter these numbers in the calculator above to confirm the figures and read off imperial gallons, litres, cubic feet and cubic metres at the same time.
Units and gallons
This calculator reports rectangular tank volume in gallons and rectangular tank volume in litres at once, alongside imperial gallons, cubic feet and cubic metres, plus optional petroleum barrels. One US liquid gallon is 231 cubic inches ≈ 3.785 litres, while one imperial gallon ≈ 4.546 litres, so both are shown to avoid ambiguity. Enter inside dimensions for the closest estimate of liquid capacity.
Other tank shapes
For round tanks, use the vertical cylindrical or horizontal cylindrical tank volume calculators. For capsule, oval, elliptical, cone-bottom and cone-top tanks, the main tank volume calculator handles every supported tank shape with full and partial-fill results. Planning how much water to store rather than sizing a tank? See the water storage calculator.
Frequently asked questions
How do you calculate the volume of a rectangular tank?
Multiply the three inside dimensions: V = length × width × height. The result is the full capacity of the box. Keep all three measurements in the same unit, then convert to gallons or litres — or let the calculator above show every unit at once from the length, width and height you enter.
What is the formula for the volume of a rectangular tank?
The rectangular tank volume formula is V = length × width × height (often written V = l × w × h). Because every cross-section is the same rectangle, the capacity is simply the base area (length × width) multiplied by the height.
How do I find the volume of a partially filled rectangular tank?
For a box tank, partial fill is linear: V = length × width × fill height, where the fill height is the depth of liquid measured from the bottom. Fill it to half the height and it holds exactly half its capacity. Turn on “Calculate partial fill”, enter the liquid level, and the calculator reports the filled and empty volumes.
How do I get rectangular tank volume in litres or gallons?
Work out the volume in cubic units first (for example cubic feet or cubic metres), then convert. One cubic foot ≈ 7.481 US gallons ≈ 28.317 litres; one cubic metre = 1,000 litres ≈ 264.2 US gallons. The calculator does this automatically and shows US gallons, imperial gallons, litres, cubic feet and cubic metres together.
Does this work as a rectangular water tank capacity calculator?
Yes. Water, fuel or any liquid — the geometry is the same. Enter the inside length, width and height of the tank and read the capacity. For drinking-water storage planning (how much to keep per person), see the water storage calculator linked below.
Should I use inside or outside dimensions?
Use inside dimensions. The calculator computes the volume of an ideal box from the numbers you enter and does not account for wall thickness, internal bracing, fittings or radiused corners. Inside dimensions give the closest estimate of actual liquid capacity.