Circle Calculator: Radius, Diameter, Circumference and Area
Tell the calculator which value you know and enter it. It works out the radius first, then everything else, and shows each step.
- Nothing you type is stored
- Free
- No signup
Area
153.938cm²
- Radius
- 7 cm
- Diameter
- 14 cm
- Circumference
- 43.9823 cm
Show working
- d = 2r = 14 cm
- C = 2πr = 2 × π × 7 = 43.9823 cm
- A = πr² = π × 7² = 153.938 cm²
How to use
- Choose what you know: radius, diameter, circumference or area.
- Enter the value and pick a unit.
- Read all four values and the working.
Good to know
The four values are linked through the radius r: d = 2r, C = 2πr and A = πr². Going backwards, r = d/2, r = C/2π and r = √(A/π). These are from NCERT Class 7 and Class 10 "Areas Related to Circles".
Choose π = 22/7 when a question says so: for example C = 44 cm gives r = 7 cm and A = 154 cm² exactly. With exact π the numbers are a little different.
Everyday uses: the length of a wheel's rim, how far a wheel travels in one turn (its circumference), the area of a round table top or a circular plot.
Questions
How do I find the radius from the circumference?
r = C ÷ 2π. For C = 44 cm and π = 22/7, r = 44 × 7 ÷ 44 = 7 cm.
How do I find the radius from the area?
r = √(A ÷ π). For A = 154 cm² and π = 22/7, r = √49 = 7 cm.
How far does a wheel go in one turn?
One circumference, πd. A wheel of 70 cm diameter covers 220 cm per turn with π = 22/7.
Is the diameter always twice the radius?
Yes, the diameter passes through the centre, so d = 2r.
Is anything I enter stored?
No. The calculation runs in your browser and nothing is sent to a server.