Does this work for a circle's segment or sector?

No — this covers only the full circle's radius/diameter/circumference/area relationships, not partial arcs or sectors.

Why are my numbers slightly different from a textbook answer?

π is used to full JavaScript floating-point precision here, so results are more precise than a rounded-π hand calculation, not less.

Math & Everyday

Solve for radius, diameter, circumference, or area from any one of them.

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Solves for radius, diameter, circumference, or area from any one of them, using the standard circle formulas.

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How to use it

Pick which measurement you already know (radius, diameter, circumference, or area).
Enter it.
Every other measurement is computed instantly.

Formula

radius = diameter ÷ 2 · circumference = 2πr · area = πr²

Worked example

A circle with radius 7 has a circumference of 43.98 and an area of 153.94.

Interpreting your result

A textbook geometry identity, not an estimate — results are exact to floating-point precision.

Frequently asked questions

Enter it and select "Area" — the calculator solves radius = √(area ÷ π) first, then derives everything else from that.