Choose a solid, enter its dimensions, and get the volume in cubic units plus the capacity in litres, US gallons and imperial gallons. Aquariums, water tanks, concrete pours and skips are what people usually want this for, so the capacity conversions are done alongside.
Key terms
- Volume — the space inside a solid, measured in cubed units.
- Capacity — the same quantity expressed in litres or gallons, which is how liquids are sold.
- Radius — half the distance across a circular cross-section.
How it works
Five solids
box = w × l × h · cylinder = πr²h · sphere = 4⁄3 πr³ · cone = πr²h ÷ 3 · pyramid = w × l × h ÷ 3
A cone and a pyramid each hold exactly one third of the cylinder or box that would enclose them — same base, same height.
Volume grows with the cube of the sides
A tank 20% bigger in every direction holds 73% more, not 20%. The arithmetic is 1.2 × 1.2 × 1.2 = 1.728. This is why stepping up one aquarium size costs so much more than expected in water, in weight and in the stand that has to hold it — and why a skip that looks “a bit bigger” is often nearly double.
It runs the other way too. Shrink every dimension by 10% and you lose 27% of the capacity, not 10%.
A third, not a half
The intuitive guess for a cone is half the cylinder around it. It is a third. Same for a pyramid against its box. A conical pile of sand two metres across and one metre high holds a third of what the drum around it would — which is the difference between one delivery and three.
A gallon is two different volumes
The US gallon is 3.785411784 litres. The imperial gallon is 4.54609 litres. Both figures are exact by definition, and they differ by about 20% — so a tank described as “50 gallons” is either 189 litres or 227 litres depending on which side of the Atlantic the label was written.
Rather than picking one silently, this calculator shows both. If a spec sheet says gallons and does not say whose, that ambiguity is real and worth resolving before you buy.
How to use this calculator
- Pick the solid and the unit you measured in.
- For round shapes, say whether you measured the radius or straight across.
- Read the volume in cubic units, and the litres or gallons if you are filling it.
- For tanks, remember that usable capacity is less than total volume — nothing is filled to the brim.
Frequently asked questions
How many litres are in a cubic metre?
Exactly 1,000. The litre is defined as a cubic decimetre, and there are 1,000 of those in a cubic metre, so the conversion is exact rather than rounded.
How do I work out the volume of my aquarium in litres?
Measure the inside width, length and water height in centimetres and choose the box shape. The result in litres is the water volume; the actual amount you pour in will be a little less once substrate and decor take up space.
What is the volume of a cylinder?
The area of the circular end times the height: πr²h. A cylinder 3 m in radius and 4 m tall holds about 113 m³, or 113,000 litres.
Why is a cone a third of a cylinder?
It falls out of integrating the circular cross-section as it shrinks to a point. The useful part is the result: for any cone, a third of the cylinder with the same base and height. The same one-third relationship holds for a pyramid and its box.
How much concrete do I need?
Work out the volume in cubic metres and add about 10% for spillage and uneven ground. Suppliers sell by the cubic metre, so that is the figure to order in.
