Area of a Circle Calculator
The area of a circle is A = π r², and this area of a circle calculator works it out the moment you enter a radius. You can also start from the diameter or the circumference, and switch the portion to a full circle, a semicircle, a quarter, or a custom sector. You'll get the area in square inches, feet, centimeters, and meters, along with the circumference and diameter, and the formula shown with every result.
- From radius, diameter, or circumference
- Semicircle & quarter
- Sector by angle
- Many area units
- Formula shown
Last updated June 15, 2026 Method: A = πr² Reviewed by the Calcowa math team
Enter a positive value to see the area.
Show all area units
A = π × 5² = 78.54 in²
What is the area of a circle?
The area of a circle is the space inside it, found with A = π × r², where r is the radius. For a radius of 5 inches, the area is π × 25, or about 78.5 square inches.
Only the radius matters, because a circle is the same all the way around. Square the radius, multiply by pi, and that's the area. If you've got the diameter or the circumference instead, you work back to the radius first, which the calculator handles for you.
How do you calculate the area of a circle?
To find the area of a circle, square the radius and multiply by pi. Here's the full sequence:
- 1
Find the radiusMeasure the radius, or halve the diameter if that is what you have.
- 2
Square the radiusMultiply the radius by itself to get r squared.
- 3
Multiply by piMultiply the squared radius by pi (about 3.14159) for the area.
- 4
Convert the unitsConvert to square feet or square meters if that's what you need.
Area of a circle with the diameter
If you measured across the middle, you've got the diameter, not the radius. Halve it (r = d ÷ 2) and use A = πr², or go straight from the diameter with A = π × d² ÷ 4, which gives the same answer. Set the known value to Diameter in the calculator and it halves the number for you before finding the area.
Area of a circle from the circumference
When all you know is the distance around, the circumference C, find the radius first with r = C ÷ (2π), then use A = πr². Rolled into one step, that's A = C² ÷ (4π). Choose Circumference in the calculator and it does the conversion, which is handy for round objects you can wrap a tape around but can't measure across.
Is the surface area of a circle the same as its area?
Yes. A circle is flat and two-dimensional, so it has a single area, A = πr², and that's all there is to measure. The phrase surface area of a circle usually means the same thing. Surface area really belongs to 3D shapes like a sphere or a cylinder, where there's an outer skin wrapping a solid, so a circle just has its one face.
Area of a semicircle, quarter, and sector
Part of a circle is just a fraction of the full area. A semicircle, or half circle, has an area of ½ × πr². A quarter circle is a fourth, ¼ × πr². A sector is a pie slice set by its angle, so its area is (angle ÷ 360) × πr², which a quarter and a semicircle are special cases of (90 and 180 degrees). Use the Portion setting to pick a full circle, a half, a quarter, or a custom sector, and the result updates to that slice.
Area and circumference of a circle
Area and circumference both start from the radius but measure different things. The area, πr², is the space inside in square units. The circumference, 2πr, is the distance around in plain length units. This calculator shows the circumference beside the area, and the circumference of a circle calculator focuses on the distance around in full.
An area of a circle example
Say you've got a round table with a diameter of 120 cm. Halve it for a radius of 60 cm, square that to get 3,600, then multiply by pi.
A = π × 60² = 3,600π ≈ 11,310 cm²
that's about 1.13 square meters
Set the known value to Diameter, type 120 in centimeters, and you'll get the matching square meters and feet at once.
Units and accuracy
Calcowa shows the area of a circle in square millimeters, centimeters, meters, inches, feet, yards, and acres all at once, plus the exact answer in terms of pi. The results use the full value of pi, not a rounded 3.14, so they're accurate for landscaping, engineering, and school work alike.
| Unit | Best for | Good to know |
|---|---|---|
| Square inches (in²) | Small circles, discs, lids | Default when you enter inches |
| Square feet (ft²) | Tables, pools, garden beds | 1 ft² = 144 in² |
| Square centimeters (cm²) | Lab and school work | 1 in² = 6.4516 cm² |
| Square meters (m²) | Large circles and ponds | 1 m² = 10.7639 ft² |
| Acres | Round fields and plots | 1 acre = 43,560 ft² |
Frequently asked questions
Why is the area of a circle πr² and not 2πr?
Because 2πr is the circumference, the distance around the edge, while πr² is the area, the space inside. They're easy to mix up since both use pi and the radius, but one is a length and the other is a length squared.
The area of a circle is A = π × r², where r is the radius. For a radius of 5 inches, that's π × 25 = about 78.5 square inches. If you have the diameter instead, halve it first to get the radius.
Halve the diameter to get the radius, then use A = πr². You can also go straight from the diameter with A = π × d² ÷ 4, which gives the same answer. Set the calculator to Diameter and it does the halving for you.
First get the radius with r = C ÷ (2π), then use A = πr². You can combine them into A = C² ÷ (4π). Choose Circumference in the calculator and it works the radius out for you before finding the area.
Yes. A circle is a flat, two-dimensional shape, so it has just one area, A = πr². People sometimes say surface area of a circle, but there's only the one face to measure, unlike a 3D sphere or cylinder.
A semicircle is half a circle, so its area is ½ × πr². A quarter circle is a fourth, ¼ × πr². Use the Portion setting in the calculator to switch between a full circle, a half, a quarter, or a custom sector.
A sector is a pie slice, so its area is (angle ÷ 360) × πr², using the angle at the center in degrees. Pick Sector in the calculator, enter the angle, and it scales the full area down to that slice.
Rearrange the formula to r = √(A ÷ π). Divide the area by pi, then take the square root, and you've got the radius. From there the diameter is just twice the radius.
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