Math
Circle Calculator
Use the circle calculator to move between radius, diameter, circumference, and area without reworking the formulas by hand.
Area
—
- Radius
- —
- Diameter
- —
- Circumference
- —
Student quick launch
Grade planning, algebra checks, and formulas students reach for most.
Study path
Use this calculator with
Follow these when you want the formula behind the answer, a short lesson, or nearby tools in the same topic.
What the circle calculator solves
Use the circle calculator to move between radius, diameter, circumference, and area without reworking the formulas by hand.
Formula
How to use it
- Choose the circle measurement you already know.
- Enter its value.
- Read the equivalent radius, diameter, circumference, and area.
How to read the answer
All circle measurements are tied to the radius, so the calculator first converts the known value back to radius.
Common mistakes and edge cases
- Do not confuse radius with diameter.
- Circumference is distance around the circle, not area.
- Area units are square units.
Worked examples
Known radius
Area
113.0973
Known circumference
Area
78.5402
Frequently asked questions
What value of pi does this use?+
It uses JavaScript's Math.PI for full precision, then rounds the displayed answer.
Can I enter diameter instead of radius?+
Yes. Choose Diameter as the known measurement and enter the diameter value.
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Last updated: May 8, 2026