Circle Calculator (Area, Circumference, Radius & Diameter)
Use this free circle calculator to find every measurement of a circle from just one. Enter the radius, diameter, circumference or area and you’ll get the other three — shown both as exact multiples of π and as decimals — with the circle drawn and the formulas worked out.
How to use the circle calculator
- Choose which measurement you already know.
- Enter its value.
- Read the radius, diameter, circumference and area, with steps and a scaled diagram.
The circle formulas
From the radius r: the diameter is d = 2r, the circumference is C = 2πr, and the area is A = πr². The calculator rearranges these formulas so it can start from whichever quantity you enter.
Frequently asked questions
What is the formula for the area of a circle?
A = πr², where r is the radius. For example r = 6 gives A = 36π ≈ 113.097.
How do you find the radius from the area?
Rearrange A = πr² to r = √(A/π). The calculator does this automatically when you enter the area.
Does it show answers in terms of π?
Yes — it gives both the exact value (like 12π) and the decimal approximation.
Read the full lesson: learn the method step by step.
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