A circle’s area is one of the most commonly needed geometry calculations, from flooring estimates to garden bed planning.
How it works
Squaring the radius and multiplying by pi gives the area. If you only have the diameter, this calculator halves it to the radius automatically.
What this does not include
This does not include a partial circle (a sector or segment), which would need the angle of the slice as an additional input beyond simple radius or diameter.
How to use this calculator
- Choose whether you’re measuring by radius or diameter, then enter that value.
A worked example
Radius 5 → area = π × 5² = 78.5398. Diameter 14 (radius 7) → area = π × 7² = 153.938.
What the variables mean
| Variable | Meaning |
|---|---|
| Radius | Distance from center to edge — squared in the formula |
| Diameter | Full width through the center; halved to get the radius first |
Edge cases worth knowing
Doubling the radius quadruples the area, not doubles it — a frequent source of underestimated material orders, since area scales with the radius squared, not the radius itself.
A radius of zero gives zero area, the degenerate case of a circle with no size.
Frequently asked questions
Why does area scale so fast with radius?
Because area depends on the radius *squared* — doubling the radius quadruples the area, not doubles it, a common source of underestimated material orders.
What’s the difference between this and the circumference calculator?
Circumference measures the distance around a circle’s edge (a length); area measures the space inside it (a surface) — related but answering different questions.
Is pi exact in this calculation?
The calculator uses a high-precision value of pi (JavaScript’s built-in constant), accurate to about 15 decimal digits — far more precise than any real-world measurement input would need.
