What formula is used to calculate circle area?

The formula is A = π × r², where r is the radius. If you enter a diameter d, the tool first converts it to radius (r = d / 2) before applying the formula.

How do I calculate area from diameter instead of radius?

Select "Diameter" from the input toggle, then type the diameter value. The tool divides it by 2 internally and computes area as A = π × (d/2)².

What is the formula for circumference?

Circumference = 2 × π × r, or equivalently π × d. The result is the distance around the outside edge of the circle.

How precise are the results?

Results are calculated using JavaScript's built-in Math.PI (15+ significant digits) and displayed to 6 significant figures. You can see the full-precision value by hovering the result.

Can I switch units after entering a value?

Yes. The unit selector recalculates and relabels all outputs immediately — no need to re-enter the number.

How do I find the radius from the area?

Divide the area by π and take the square root: r = √(A ÷ π). An area of 50 m² gives r = √15.915 = 3.989 m.

Is a 16 inch pizza twice as big as a 12 inch pizza?

Not quite: it is about 1.78 times as big. Area grows with the square of the diameter, so a 16 inch pizza has 201.06 in² against 113.10 in² for a 12 inch one.

Area of a Circle Calculator

Enter a radius or diameter to instantly calculate circle area, circumference, and related measurements.

Area = π × r². A circle with a 5 cm radius has an area of 78.5398 cm² and a circumference of 31.4159 cm.

Circle Input

Input type

Radius

Unit

Enter a value on the left to see results.

Copied!

Quick answer

Area = π × r². A circle with a 5 cm radius has an area of 78.5398 cm² and a circumference of 31.4159 cm. If you know the diameter, halve it first: a 12 inch pizza has a 6 inch radius and 113.097 in² of area.

Examples

Radius	Diameter	Area (π r²)	Circumference
1	2	3.14159	6.28319
2	4	12.5664	12.5664
3	6	28.2743	18.8496
5	10	78.5398	31.4159
6	12	113.097	37.6991
10	20	314.159	62.8319

Enter a radius or diameter.

Summary

Enter a radius or diameter to instantly calculate circle area, circumference, and related measurements.

How it works

Choose whether you are entering a radius or diameter.
Type the numeric value into the input field.
Select your preferred unit (mm, cm, m, in, ft).
Results update instantly — area, circumference, and the other dimension.
Use the Copy button to grab any result value.

Use cases

Find the area of a circular garden bed or lawn section.
Calculate pipe cross-section area for flow rate problems.
Determine how much material is needed for a circular table top.
Solve geometry homework involving circles.
Compute the footprint of a circular pool or fountain.
Check architectural plans requiring circular floor areas.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu