Circumference Calculator
Find the circumference of a circle from its radius, diameter or area, with the exact answer in terms of π and the other circle measurements.
The circumference is the distance around a circle — the length of its edge. Enter the radius, the diameter or the area and the calculator gives the circumference, the exact answer as a multiple of π and the other measurements of the same circle, in the unit you choose. Use it for trim, fencing, wheel travel or any round object you need to wrap.
How to use this tool
- Choose what you know: radius, diameter or area.
- Enter the measurement and pick its unit.
- Read the circumference, then the diameter, radius and area of the same circle.
Formula
C = 2πr = πd
r is the radius (centre to edge), d = 2r is the diameter and π ≈ 3.14159265. From an area A, the radius is r = √(A ÷ π), so C = 2√(πA). The area itself is A = πr².
Example
A round table 120 cm in diameter needs decorative trim around its edge. The circumference is π × 120 = 376.991118 cm, so buy at least 3.77 m of trim (plus a little for the overlap). The exact answer is 120π cm, the radius is 60 cm and the tabletop area is 11,309.733553 cm².
Frequently asked questions
What is the difference between circumference and perimeter?
Circumference is the perimeter of a circle. Perimeter is the general word for the distance around any flat shape.
How do I find the diameter from the circumference?
Divide by π: d = C ÷ π. A tree trunk that measures 157 cm around is about 49.97 cm, or roughly 50 cm, across.
Is 3.14 accurate enough for π?
For rough work, yes: it is about 0.05% too small. For the 120 cm table it gives 376.8 cm instead of 376.99 cm. For machining or long distances, use the full value as this calculator does.
Why is the area in square units?
Area measures a surface, so a circle measured in centimetres has its area in square centimetres (cm²), while the circumference stays in plain centimetres.
Last updated: 2026-10-30