Circle Calculator

A circle is fully defined by a single quantity: its radius. From it follow the area (pi times radius squared), the perimeter (two times pi times radius) and the diameter (two times radius). This calculator takes either the radius or the diameter as input - if you enter the diameter it is halved internally. So a single number gives you all three figures at once.

The calculation runs entirely locally in your browser, in pure JavaScript - nothing is uploaded and nothing is stored. The calculator is unit-free: if you enter the radius in centimetres, the area is in square centimetres, and the perimeter and diameter are in centimetres. So you can use metres, millimetres or any unit - just read them consistently. Change the value or the input type and everything updates instantly.

An honest note: this is a pure calculation helper for an ideal circle. The exact value of pi from the browser is used; results are shown rounded to two decimals. Real objects are rarely perfectly circular - the calculator gives the mathematically exact value for your inputs, not a measurement. Negative or empty inputs yield zero.

An often surprising consequence of the square relationship: double the radius and the perimeter doubles, but the area quadruples. That is exactly why a pizza 40 centimetres across has not twice but four times as much topping as one of 20 - a solid argument at the next price comparison. The perimeter, in turn, is the length you need for an edging, a band or a border.

Specifications

In 3 steps

1. Enter the value (radius or diameter).
2. Choose whether the value is a radius or a diameter.
3. Read off the area, perimeter and diameter.

Limitations: A pure calculation helper for an ideal circle: area = pi times radius squared, perimeter = two times pi times radius, diameter = two times radius. Unit-free (area in square units). Results rounded to two decimals. Not a measurement of real objects.

FAQ

Related tools

Rectangle Calculator · Triangle Calculator · Length converter