Geometry
Circle area & circumference
- where
- area (m²)
- diameter (m)
- circumference (m)
Explain this equation
This gives the area and circumference of a circle from its diameter. Use it for round bases, circular planters, manholes, curved paving or the length of a round wall.
d is the diameter, measured straight across through the centre. A is the area inside the circle and C is the distance around it. π, about 3.1416, is the fixed ratio of any circle's circumference to its diameter.
Circumference is π times the diameter by definition. The area is πr², and since the radius is half the diameter, πr² becomes πd²/4. That is where the 4 comes from.
A 2 m diameter circle has a circumference of about 6.28 m and an area of about 3.14 m². Doubling the diameter doubles the circumference but quadruples the area.
Watch out: on a circular wall the inside face, centreline and outside face each have a different diameter. Use the one that matches what you are measuring or setting out.
Calculate a circular feature from its diameter.
Enter your values to see the result.
Good to know
Use the correct setting-out line: inside, outside or centreline.
Sources
- Geometry / arithmetic
Estimates are not design approval or British Standards compliance checks. Structural arch, retaining-wall, wind, foundation, lintel and masonry-capacity design are not performed. Use the project engineer’s specification.