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How to measure calculator

Which measurements you actually need, where to take them, and the difference between inside and outside dimensions.

Before any formula, you need the right measurements. This guide, paired with the how to measure calculator tool, shows exactly what to measure on a cylinder, where to take each reading, and why inside dimensions matter for anything that holds liquid.

Using the calculator

  1. Measure across the circular end — the diameter or radius.
  2. If you measured the diameter, halve it to get the radius r.
  3. Square the radius and multiply by π to get the base area.
  4. Multiply the base area by the height h: V = π r² h.
  5. Convert the result into your preferred unit above.

Worked example. Take a cylinder with a diameter of 20 cm (so a radius of 10 cm) and a height of 40 cm. The base area is π × 10² = 314.16 cm², and multiplying by the height gives a volume of about 12.57 litres (12,566 cm³ · 3.32 US gallons).

For anything that holds liquid, measure the inside of the cylinder — the wall takes up space that the contents cannot.

Every how to measure page

Each page below targets a specific question and opens with the calculator ready to go:

Questions people ask

What do I need to measure on a cylinder?
Two things: the width (diameter or radius) of the circular end, and the height or length. For capacity, measure the inside; for a solid object, the outside is fine.
How do I measure the radius if I can only reach the top?
Measure the diameter straight across the opening and halve it, or measure the circumference around the outside and divide by 2π. The tool accepts any of the three.

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