The formula for a cylinder
A cylinder is a circle extended upwards. Its volume is the area of the circular base multiplied by the height.
volume = π × r² × h
r (radius) = diameter ÷ 2
Because the radius is squared, the diameter has a big effect on volume. A tank twice as wide holds four times as much water.
Example 1 – a rain barrel: diameter 60 cm, height 90 cm. Radius 30 cm. Volume = 3.1416 × 30² × 90 = 254,469 cm³ = 254.5 litres (67.2 US gallons).
Example 2 – an above-ground pool: diameter 12 ft, water depth 3.5 ft. Volume = 3.1416 × 6² × 3.5 = 395.8 ft³ = 11.2 m³, roughly 2,960 US gallons.
Example 3 – a pipe: a 100 m drainage pipe with an internal diameter of 11 cm holds π × 0.055² × 100 = 0.95 m³ of water.
Diameter, radius and circumference
Measuring the diameter of a large tank can be awkward. Measure the circumference with a tape around the outside instead and divide by π: diameter = circumference ÷ 3.1416. For pipes and tanks, use the internal diameter if you want the capacity rather than the outside size; wall thickness matters for small diameters.
Converting the result
| 1 unit | Litres | US gallons | UK gallons |
|---|---|---|---|
| 1 m³ | 1,000 | 264.17 | 219.97 |
| 1 ft³ | 28.32 | 7.48 | 6.23 |
| 1 US gallon | 3.785 | 1 | 0.833 |
| 1 UK gallon | 4.546 | 1.201 | 1 |
The calculator also gives the base area and the area of the curved side (lateral area), useful when you need to paint or insulate a tank.
Practical tips
- Partial filling. For the water you actually have, use the water depth as the height, not the tank height.
- Lying tanks. For a horizontal cylinder that is completely full the formula is the same, with the tank length as the height. Partly filled horizontal tanks need a more complex formula.
- Treatment doses. Pool chemicals and pond treatments are dosed per 1,000 litres or per 10,000 gallons. Calculate the volume first to avoid over- or under-dosing.
- Square containers? Use the cubic meter calculator. Filling a round column with concrete? See the concrete calculator.
Everyday cylinders
Many everyday objects are cylinders, and knowing their volume is surprisingly useful. Water tanks, rain barrels and hot water cylinders are sold by liters or gallons; measure the inside diameter and height to check the real capacity. Round ponds and above-ground pools need a volume to dose water treatment and to choose a pump; a pool 3.6 m across with 0.9 m of water holds about 9,160 liters. Concrete for round footings and sonotubes is calculated the same way, so you can order the right number of bags. Pipes are cylinders too: the volume of water in a pipe tells you how long it takes before hot water reaches the tap.
Tips for measuring
- Measure the inside diameter for containers and the outside diameter for solid objects.
- For tapered buckets or tanks, measure the diameter halfway up to get a reasonable estimate.
- A tank is rarely filled to the brim, so use the actual fill height for a realistic volume.
Frequently asked questions
How do I calculate the volume of a cylinder in litres?
Measure diameter and height in centimetres, calculate π × (diameter ÷ 2)² × height and divide by 1,000. A cylinder 40 cm wide and 50 cm high holds 62.8 litres.
How many gallons does my round pool hold?
Multiply π × radius² × water depth in feet to get cubic feet, then multiply by 7.48 for US gallons. A 15 ft pool with 4 ft of water holds about 5,290 US gallons.
What is the difference between radius and diameter?
The diameter is the full width across the circle through the centre. The radius is half of that, from the centre to the edge.
Does the wall thickness matter?
For capacity, yes. Use the inside diameter. For the space a tank takes up, use the outside diameter.
Can I calculate a cone or sphere with this?
No. A cone holds one third of a cylinder with the same base and height, and a sphere is 4/3 × π × r³. This calculator is for straight cylinders only.
Last reviewed: 2026-10-06. Results are estimates for information only.