Circle Area
Calculator
Results
- Area
- 78.539816
- Circumference
- 31.415926
- Diameter
- 10
Results
| Area | 78.539816 |
| Circumference | 31.415926 |
| Diameter | 10 |
formula-map diagram
- Area
- 78.539816
- Circumference
- 31.415926
- Diameter
- 10
Formula map
Formula
A = π × r²= 78.539816339745
Note
Simplified model: these are exact results of the stated idealised formulas, assuming perfect shapes and exact inputs. Real measurements carry error, and the ellipse perimeter is a Ramanujan approximation. Verify independently before relying on these figures.
More in Mathematics
See all →Frequently asked questions
What inputs does this calculator need?+
Just the radius (or diameter, if the tool accepts it) of the circle. Everything else — area and circumference — is derived from that single measurement.
What is the formula behind the area calculation?+
Area equals pi times the radius squared (A = πr²). Circumference is 2 times pi times the radius. Both use the same radius value, so entering it once gives you both results.
Do I enter the radius or the diameter?+
Check which field is labeled — mixing them up is the most common mistake. The diameter is twice the radius, so if you only know the diameter, divide it by 2 before entering it as a radius, or use a diameter field if one is provided.
Why does doubling the radius more than double the area?+
Because area scales with the square of the radius, not linearly. Doubling the radius quadruples the area (2² = 4), which surprises people expecting a simple doubling.
What if I only know the circumference?+
Divide the circumference by 2π to get the radius, then use that radius to find the area. Some calculators let you enter circumference directly and back-solve for the rest.