Circle Calculator
Everyday
Know one measurement of a circle and need the rest? Pick whether you're starting from the radius, diameter, circumference or area, type the value, and this calculator works out all four using π. The formulas (d = 2r, C = 2πr, A = πr²) are shown alongside each result so it's clear how the numbers relate. Handy for geometry homework, DIY and woodworking, sewing and crafts, pipe and tank sizing, and anywhere you need to go between a circle's size measurements. Everything is computed locally in your browser.
Radius
5
r
Diameter
10
d = 2r
Circumference
31.415927
C = 2πr
Area
78.539816
A = πr²
How to use
- Choose which measurement you already know: radius, diameter, circumference or area.
- Type its value.
- Read the other three measurements, each with its formula; copy any value.
Frequently asked questions
- Which value should I start from?
- Whichever you have. The calculator first finds the radius from your input (r = d/2, r = C/2π, or r = √(A/π)), then derives the rest from it.
- What value of π is used?
- The full-precision π built into your device (about 3.14159265358979), so results are as accurate as floating-point math allows, trimmed to a readable number of digits.
- What units does it use?
- Whatever unit you enter — the tool is unit-agnostic. If you put the radius in centimetres, the area comes out in square centimetres and the circumference in centimetres.
- Why must the value be non-negative?
- A circle can't have a negative radius, diameter, circumference or area, so negative input has no geometric meaning.
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