GEOMETRY / CALCULATOR
Circle Calculator.
Calculate area, circumference and diameter from radius.
LET’S WORK IT OUT
Your numbers, your answer.
THE WHY BEHIND THE ANSWER
How it works.
The radius goes from the centre to the edge. Area uses squared units; circumference and diameter use the original length unit.
THE FORMULAArea = πr²; circumference = 2πr; diameter = 2r
Worked example
A radius of 5 gives area 25π and circumference 10π.
Good to know
Use the same unit consistently. Decimal results are rounded for display.
ONE MORE THING
Can I use the diameter instead?
Yes. Divide the diameter by two and enter the resulting radius.