Circle Calculator
Give it a radius and it hands back the circle's area, circumference, and diameter.
Area
78.5398
Area = π × r². A radius of 5 gives about 78.54 square units.
Circumference
31.4159
Diameter
10
How it works
Everything about a circle traces back to one number: the radius, the distance from the center to the edge. Once you know it, the rest is a couple of multiplications away.
Area is π × r², so a circle with radius 5 has an area of about 78.54. Circumference — the distance around the outside — is 2 × π × r, roughly 31.42 for that same circle. The diameter is just twice the radius.
We keep the full value of π through the calculation and only round the answers to four decimals at the very end, so nothing drifts along the way.
Frequently asked questions
I have the diameter, not the radius. What do I do?
Halve it. The radius is always half the diameter, so a circle 10 across has a radius of 5. Type that 5 into the radius box and you're set.
What units do the answers come in?
Whatever unit you enter. A radius in inches gives area in square inches and circumference in inches. The tool never assumes a unit, so the choice is entirely yours.
Why is π used instead of 3.14?
3.14 is a rounded stand-in that introduces small errors. This calculator uses the full-precision value built into JavaScript, so circle results stay accurate down to the last displayed decimal.