Formulas, without the guesswork.
Choose a relationship from the measurements you know. The variable names below belong to each calculator; a letter can have a different role in another method. π is evaluated using the browser’s full numerical constant, rather than replacing it with 3.14.
From a radius
Inputs: Radius r (length units).
From a diameter
Inputs: Diameter d (length units).
Find radius from area
Inputs: Area A (square units).
Sector of a circle
Inputs: Radius r (length units); Central angle θ (degrees).
Reading the notation
√ means a square root, ∛ means a cube root, and a superscript ² or ³ means a square or cube. Sine, cosine and tangent use angles converted from entered degrees. Where an inverse trigonometric function is used, its displayed angle is converted back to degrees. The ellipse perimeter is approximate; other formula labels describe the ideal mathematical model.
Do not enter the diameter in a field asking for radius. A sector boundary includes two radii as well as its arc.
Reference reading: Circle at Wolfram MathWorld.