Circle Calculator
Enter a radius or diameter to find the area, circumference, and the other dimension. Add a central angle for arc length and sector area, with the full working shown.
About this calculator
Choose whether you are entering the radius or the diameter, type the value, then press Calculate. The calculator uses A = π r² for area and C = 2π r for circumference. If you also enter a central angle θ in degrees, it computes the arc length r·θ·π/180 and the sector area ½·r²·θ·π/180.
How circle area, circumference, and sectors work
A circle is defined by its radius r — the distance from the centre to the edge. The diameter d is twice the radius (d = 2r). The area enclosed by the circle is A = π r², and the distance around it (the circumference) is C = 2π r = π d. A sector is a 'pie slice' bounded by two radii and the arc between them. For a central angle θ measured in degrees, the arc length is L = r · θ · π/180 and the sector area is A = ½ · r² · θ · π/180 — both are just the matching fraction (θ/360) of the full circle, rewritten with the radian conversion π/180.