Circle Calculator
Enter a radius, diameter, circumference or area and get the other three — with the circle drawn, the unit label you pick, and arc, chord and sector areas.
A slice of it
The sector is the pie slice between the two radii; the segment is what the straight chord cuts off on its own.
Radius: 1 Diameter: 2 Circumference: 6.283185 Area: 3.141593 Arc length: 1.570796 Chord: 1.414214 Sector area: 0.785398 Segment area: 0.285398
🔒 Calculated in your browser — nothing is uploaded. Uses Math.PI.
Any one measurement gives you the rest
Say which measurement you have — radius, diameter, circumference or area — type it in, and the other three appear immediately. Everything is derived from the radius: the diameter is twice it, the circumference is 2πr, and the area is πr², using the full precision of π rather than 3.14. The circle is drawn beside the numbers with the radius and diameter marked on it, so it is clear which measurement is the radius and which the diameter.
Units, so an area is not a square something
Pick a unit — mm, cm, m, km, in, ft, yd or mi — and every length is labelled with it and every area with its square. The tool does not convert between units; it labels, which is the part that was missing when an area came out as an ambiguous number with no dimension attached. Leave the unit unset and the results stay plain numbers, as they were.
A slice of the circle
Give a central angle and the page adds the four measurements of that piece: the arc along the edge, the chord straight across it, the area of the sector — the pie slice between the two radii — and the area of the segment, which is what the chord alone cuts off. The wedge is shaded on the diagram so it is clear which piece the numbers describe. The number of decimal places is yours to set, from none to ten.
Frequently asked questions
What formulas does it use?
Diameter = 2r, circumference = 2πr, and area = πr². Working backwards, r = diameter ÷ 2, r = circumference ÷ 2π, and r = √(area ÷ π). π is the full double-precision constant, not 3.14 or 22/7.
What is the difference between the sector and the segment?
The sector is the pie slice: the two radii and the arc between them. The segment is what the straight chord cuts off on its own — the sector with the triangle removed. Past a half turn the triangle falls on the other side of the chord, so the major segment is the larger of the two, which is correct rather than a bug.
Can it convert between units?
No, and it does not pretend to. Choosing a unit labels the answers so an area reads as cm² rather than an unnamed number; whatever unit goes in comes out. Use a unit converter first if your measurement is in something else.
Why does the answer have so many decimals?
Because π does. Set the decimals box to however many places you actually need — anywhere from none to ten — and everything, including the slice, is rounded to that.
Is anything uploaded?
No. Everything is calculated in your browser with JavaScript. Nothing you type is sent anywhere or stored.