Circle Calculator — Calculate Area, Circumference & Diameter

What do you know?

RadiusEnter a valid positive number.

Results

Radius

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Diameter

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Area

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Circumference

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How to Use This Tool

Select which value you already know — radius, diameter, area, or circumference.
Enter that value.
Click Calculate to see all four properties of the circle.

Circle Formulas

Area = π × r²

Circumference = 2 × π × r

Diameter = 2 × r

Working Backward From Any Value

This calculator can start from any of the four properties and derive the others by rearranging the formulas — for example, if you know the area, the radius is found using r = √(Area ÷ π).

Common Applications

Geometry homework and exams
Calculating material needed for circular objects (tabletops, pipes, pools)
Engineering and construction calculations involving circular components

Frequently Asked Questions

Can I start with the area instead of the radius?

Yes — select "Area" as what you know, enter your area value, and the calculator will derive the radius, diameter, and circumference for you automatically.

What value of π does this calculator use?

It uses JavaScript's built-in high-precision value of π (accurate to about 15 decimal places), giving highly precise results.

This calculator is provided for general informational and estimation purposes only. Results should not be treated as professional financial, tax, legal, medical, or engineering advice. Always verify critical calculations with a qualified professional or official source before making decisions.