Circle Calculator

Calculate area, circumference, diameter, and radius of a circle from any one value.

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How it works

Enter any one circle measurement — radius, diameter, circumference, or area — and the tool solves for all four using the standard circle relationships (circumference = 2πr, area = πr²). Results display in real time. Supports decimal input and shows answers to several significant figures. All calculation runs instantly in your browser.


              C = 2πr   A = πr²   d = 2r

Common use cases

  • Finding the area of a circular garden, pool, or room feature from its diameter.
  • Calculating how much fencing (circumference) you need to enclose a circular plot.
  • Solving geometry homework problems about circles.

Frequently asked questions

What value of π is used?

JavaScript's built-in Math.PI (≈ 3.141592653589793), which is accurate to 15 significant figures — far more than any practical measurement.

Can I calculate a semicircle or sector?

This tool covers the full circle. For sectors and segments, you would also need the central angle, which is a more advanced geometry problem.