About the Circle Calculator
Circle Calculator finds every basic measurement of a circle from any one of them. Choose whether you know the radius, the diameter, the circumference or the area, type the value, and the tool gives you all four, with the unit you pick. If you also enter a central angle between 0 and 360 degrees, it adds the arc length, the area of the sector, the length of the chord and the area of the circular segment. Results use the full value of pi and are shown to six decimal places when needed. You can start from the area to find the radius, which many calculators cannot do, or from the circumference, such as the length of a belt or fence, to find the diameter. Everything runs in your browser.
Updated September 2026
How to use the Circle Calculator
- 1Choose what you know: radius, diameter, circumference or area.
- 2Type the value and choose a unit.
- 3Read the area and circumference in the result box, and the radius and diameter below.
- 4Enter an angle if you want the arc length, sector area, chord and segment area.
- 5Change the number to try another circle.
The four basic formulas
- Diameter d = 2r
- Circumference C = 2πr = πd
- Area A = πr²
- From the area: r = √(A ÷ π)
- From the circumference: r = C ÷ (2π)
Everyday uses
Circle measurements come up when buying a round table cover or a pizza, cutting a circular hole, laying a circular patio, working out how much fencing goes around a round pond, or finding how far a wheel moves in one turn. A pizza with a 30 cm diameter has an area of about 707 cm², and a 40 cm pizza has 1,257 cm², which is 78 percent more food for a 33 percent larger diameter.
For measuring length you can start from the circumference: wrap a string around a pipe, measure the string and enter it as the circumference to find the diameter of the pipe.
Arcs, sectors and segments
With an angle you can describe part of a circle. The arc length is the stretch of the edge, the sector area is the pie slice from the centre, the chord is the straight line joining the two ends of the arc, and the segment is the area between the chord and the arc. A clock face of radius 10 cm, for a 30-degree slice, has an arc of 5.24 cm and a sector area of 26.18 cm².
A full 360 degrees gives the whole circle: the arc is the circumference and the sector is the full area, which is a handy check.
Tips and related tools
Use the same unit for every value. The area is shown in square units, so a radius in centimetres gives an area in square centimetres. Area Calculator covers other shapes, Volume Calculator handles cylinders and spheres, and Angle Converter switches between degrees and radians.
For the diagonal or chord of right-angled shapes, use Pythagorean Theorem Calculator. If you need a length in another unit, Unit Converter converts millimetres, inches, feet and more.
Features
- Start from radius, diameter, circumference or area
- All four measurements at once
- Arc length, sector area, chord and segment for any angle
- Units from millimetres to miles
- Uses the full value of pi
- Decimals supported
- Runs in your browser
Frequently asked questions
How do I find the area of a circle?
Multiply pi by the radius squared: A = πr². For a circle with a radius of 5 cm, the area is 3.14159 × 25 = 78.54 cm². If you know the diameter, halve it first to get the radius. This tool does that automatically when you choose Diameter.
How do I find the circumference?
Multiply pi by the diameter, or two times pi times the radius: C = 2πr. A wheel with a diameter of 60 cm travels about 188.5 cm in one turn, since 3.14159 × 60 = 188.5.
How do I find the radius from the area?
Divide the area by pi and take the square root: r = √(A ÷ π). An area of 100 square metres gives a radius of √(100 ÷ 3.14159) = 5.642 metres. Choose Area in the tool and type the value to see it.
What is the difference between radius and diameter?
The radius runs from the centre to the edge, and the diameter runs across the circle through the centre, so the diameter is exactly twice the radius. The circumference is the distance around the edge, and it is about 3.14 times the diameter.
What are arc length and sector area?
An arc is a part of the circle's edge, and a sector is a slice of the circle like a piece of pie. Both depend on the central angle. Arc length is the radius times the angle in radians, and the sector area is half of the radius squared times the angle in radians. Enter an angle in the tool to get both.
Which value of pi does the tool use?
It uses the full precision value of pi built into your browser, about 3.141592653589793, not the rounded 3.14. This gives more exact results, and small differences from answers worked out with 3.14 are normal.