What is the formula for the area of a circle?

Area = π × r², where r is the radius. If you know the diameter d, use Area = π × (d/2)².

What is the formula for circumference?

Circumference = 2 × π × r, or equivalently π × d where d is the diameter.

What value of π does this calculator use?

This tool uses JavaScript's Math.PI, which is accurate to 15+ significant digits (approximately 3.14159265358979).

Can I enter a diameter instead of a radius?

Yes. Toggle the "Input type" selector to Diameter and the tool converts it to radius before computing.

Why does the diagram look small for large values?

The SVG diagram scales the circle to fit inside the canvas regardless of the actual value, so it is always visible. The numeric results reflect the true size.

How many decimal places are shown?

Results are displayed to 4 decimal places for precision. Use the copy button to grab the full value.

Circle Area Calculator

Name: Circle Area Calculator
Availability: InStock
Author: Nham Vu

Enter a radius or diameter to instantly get the area and circumference of a circle, with formulas and a scaled SVG diagram.

Enter Circle Dimensions

Input type

Radius

Unit

Enter a value above to see results.

Diagram

Radius (r) Diameter (d)

Quick Reference

Area A = π · r²

Circumference C = 2π · r

Diameter d = 2 · r

π (pi) ≈ 3.14159265…

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Summary

Enter a radius or diameter to instantly get the area and circumference of a circle, with formulas and a scaled SVG diagram.

How it works

Choose whether you are entering a radius or a diameter.
Type the value into the input field (any positive number).
Select the unit — millimeters, centimeters, meters, inches, or feet.
Results appear instantly: area, circumference, and the other dimension.
Expand the "Formulas" section to see the step-by-step calculation.
The SVG diagram updates in real time to reflect the entered value.

Use cases

Calculate the area of a circular garden bed or lawn.
Find the cross-sectional area of a pipe or rod.
Determine the material needed to cover a circular table top.
Compute the circumference of a wheel or tire.
Check geometry homework with step-by-step formula breakdowns.
Convert between radius and diameter instantly.
Estimate paint or fabric coverage for circular shapes.
Engineering calculations for circular flanges and plates.

Frequently Asked Questions

Last updated: 2026-06-09 · Reviewed by Nham Vu