⭐ 0
🔍
Text Image PDF & Docs Audio Music Electronics Time Utilities Validators Esoteric Games

Area of a Circle Calculator

Enter any one measure of a circle — radius, diameter, circumference or area — and get the other three instantly, with every formula shown with your numbers substituted in.

Processing... 0%
Reading the known value
Solving for the radius
Computing the other measures

Result

This page opens the circle calculator with the radius preselected and the area as the highlighted result. Enter the radius, and the tool returns the area with the full working: A = πr² = π × r² with your number in place.

The area grows with the square of the radius — double the radius and the area quadruples. A radius of 5 gives A = π × 5² = 25π ≈ 78.539816; a radius of 10 gives 100π ≈ 314.159265. Because the input is a radius, the exact form in terms of π is printed alongside the decimal, which is exactly the format most geometry courses expect.

Starting from a diameter, circumference or even a known area works too — pick the value you have and the calculator inverts the formula for you. All the math happens in your browser and nothing you type is sent anywhere.

🛡️
Executed in Browser

Your files and inputs never leave this device. Processing runs completely offline. 100% Secure.

Accepted Formats & Rules

  • Solves from any of the four known values: radius, diameter, circumference or area.
  • Shows each formula with substitution, e.g. C = 2πr = 2 × π × 5 ≈ 31.415927.
  • Exact answers in terms of π (like A = 25π) whenever the radius or diameter is given.
  • Copy button grabs all four measures in one click.

Key Benefits

✓ One input is enough: give any single measure and the radius, diameter, circumference and area all appear at once.
✓ Every result comes with the formula written out with your numbers substituted — ideal for showing your work in homework.
✓ Runs entirely in your browser: no sign-up, no upload, instant results.

Use Cases

  • • Find how much edging or fencing a round flower bed needs from its measured diameter.
  • • Get the exact answer in terms of π, like A = 25π, for geometry homework — with every formula substituted step by step.
  • • Convert a tape measurement around a trunk, pipe or column into its diameter and cross-section area.