Geometry & Shapes
Circle Calculator
Circle — it uses area = πr² and circumference = 2πr from the radius you enter. This page runs that same calculation on your numbers, in your browser: no signup, no uploads, nothing stored.
Circle Calculator helps you find a circle's area and circumference. Enter your radius and it returns the area, circumference, diameter and radius instantly, right in your browser with nothing uploaded.
About this calculator
It uses area = πr² and circumference = 2πr from the radius you enter. It matters whenever a shape has to be measured, cut, covered or compared and the geometry has to be right first. Here is the calculation on real numbers: A circle of radius 5: area = π × 5² = 78.54 and circumference = 2π × 5 = 31.42 . For your own figures, use the calculator above — same method, your inputs, computed on-device.
How to use
- Enter the radius of the circle.
- Press Calculate to find the circle's measurements.
- Read the area, circumference, and diameter.
- Press Reset to try another radius.
Why use Circle Calculator
Fast
Calculations run in your browser. No round trip to a server.
Private
Your inputs never leave your device. Nothing is uploaded.
Free
No signup, no paywall, no ads-in-results, no watermarks.
Accurate
Uses standard formulas where applicable; check critical results before relying on them.
Mobile-ready
Optimised for phones, tablets, and desktops alike.
Transparent
Honest disclaimers wherever browser limits or regional rules apply.
Common uses
Use it for school and college geometry
Use it when comparing two shapes
Use it when planning a garden bed
Use it for sign and panel sizing
Use it for fencing and edging lengths
Use it when drafting plans and sketches
Technical notes
Circle Calculator uses area = πr² and circumference = 2πr from the radius you enter.
Results assume exact, idealised shapes; real-world measurement error is not included.
All arithmetic runs in your browser using double-precision floating point, so extreme inputs may show tiny rounding differences.
FAQ
What is the difference between circumference and diameter?
The diameter is the distance across the circle through its centre; the circumference is the distance all the way around, equal to pi times the diameter.
How is the area of a circle calculated?
The area is pi times the radius squared, so a radius of 5 gives an area of about 78.54.
Can I enter the diameter instead of the radius?
Enter half the diameter as the radius; the diameter is then shown back to you in the result.
Does the Circle Calculator send my data to a server?
No. The Circle Calculator runs entirely in your browser, so nothing you enter is uploaded, stored, or shared.
Worked example
A circle of radius 5: area = π × 5² = 78.54 and circumference = 2π × 5 = 31.42.