Circle Calculator (Area, Circumference, Diameter)
Compute circle area, circumference, and diameter from any one of radius, diameter, area, or circumference.
What this calculates
All circle properties — radius, diameter, circumference, area — derive from any single one. This calculator solves them all from whichever value you have, using π = 3.14159265…
Formula & how it works
d = 2r. C = 2πr = πd. A = πr² = πd²/4 = C²/(4π). Going backwards: r = √(A/π) from area; r = C/(2π) from circumference.
Worked example
Radius 5: diameter 10, circumference 31.42, area 78.54. Area 50: radius 3.99, circumference 25.07.
Frequently asked questions
Why π?
π is the ratio of any circle's circumference to its diameter — same for every circle in flat geometry.
Ellipse instead?
Different formulas. Ellipse area = π × a × b (semi-axes). Perimeter has no closed form — Ramanujan's approximation is the standard.
Units?
Whatever you put in — area comes out as those squared. Inches in, square inches out.