Circle area, circumference and diameter from any one measurement — radius, diameter, area or circumference.
Enter the length in the unit below. Leave the other measurements blank — they are worked out for you.
Area of the circle
How to use this tool
Choose which measurement you already know: radius, diameter, circumference or area.
Enter that one number — you do not need the others.
The calculator fills in the radius, diameter, circumference and area at once.
Read the formula breakdown to see exactly how each value was worked out.
Formula & worked example
A circle with radius 5 cm has diameter 10 cm, circumference 2π × 5 ≈ 31.4159 cm, and area π × 5² ≈ 78.5398 cm². Starting from the diameter 10 cm instead gives exactly the same circle.
Limitations — read this before you rely on it
Results are for a perfect mathematical circle. Real measured circles (a plate, a wheel, a tree trunk) are never perfectly round, so measured values will differ slightly. π is irrational, so area and circumference are shown rounded to 4 decimal places — for exact work, keep the answer in terms of π.
Frequently asked questions
Pick which measurement you know — radius, diameter, circumference or area — enter it, and the calculator works out the other three, with the formula steps shown.
Area = πr², where r is the radius. For example, a radius of 5 gives π × 25 ≈ 78.54 square units.
Circumference = 2πr, or equivalently πd, where d is the diameter. The circumference is the perimeter (distance around) of the circle.
The diameter is twice the radius: d = 2r. If you know the circumference, divide it by π; if you know the area, take the square root of (area ÷ π) and double it.
Yes. The diameter is the widest distance across the circle, through its centre; the radius is half of that, from the centre to the edge.