Circle Calculator
Find a circle’s area, circumference and diameter from its radius.
Your result
MATHEnter your values and select calculate.
On this page How it works & practical examples
How to use the Circle Calculator
The radius is the distance from a circle’s center to its boundary. Enter half the diameter when you measured across the whole circle. The calculator distinguishes area, which fills the circle, from circumference, which measures its boundary. Doubling radius doubles circumference but multiplies area by four because area depends on the radius squared.
The calculation
Worked example
A radius of 5 gives an area of about 78.5398 square units, a circumference of 31.4159 units and a diameter of 10 units.
Assumptions and limits
The model assumes a perfect flat circle. An oval or irregular rounded shape cannot be represented by one radius. Use one consistent length unit and interpret area in its squared unit. Results use floating-point π and are rounded; the precision of your original measurement remains the practical limit.
Common questions
Is circumference the same as perimeter?
For a circle, yes: circumference is its boundary length.
Why does doubling radius quadruple area?
The formula contains r², so multiplying radius by two multiplies area by 2², which is four.