Diameter of a Circle Calculator

Created by Dominika Śmiałek, MD, PhD candidate
Reviewed by Arturo Barrantes
Last updated: Jun 05, 2023

If you're wondering how to calculate the diameter of a circle, this diameter of a circle calculator is a perfect choice.

You will need either the radius, the circumference, or the area of a given circle for the calculations. Take a look at the picture above the calculator to visualize each element.

How do I calculate the diameter of a circle?

There are three methods for diameter estimation. Choose the useful one based on the data you have. This calculator will automatically estimate all of the other values.

  1. The first one bases on the radius of the circle. The diameter equals the length of two radiuses.


  1. Area of a circle:


  1. Circumference of a circle:



  • r stands for radius of the circle,
  • c for circumference,
  • A for area, and
  • d stand for diameter of the circle.


What is the diameter of a 9 inch circle?

If the circle radius is 9 inches, then the diameter is 18 inches. The easiest formula for the diameter is d=2rd=2r, where d is diameter and r radius.

How to measure the diameter of a circle?

The diameter is the radius multiplied by two. How to calculate radius? Measure the distance from the center to any point on the circle.

How do you find the radius and diameter of a circle?

To calculate the radius of a circle, divide its circumference by 2π; for the circle's diameter, divide the circumference by π.
Let's say that the circumference of a given circle is 20 inches. Therefore, its radius is 3.18 in and diameter 6.37 in.

Dominika Śmiałek, MD, PhD candidate
Image of a circle with radius, diameter, circumference and area marked.
Radius (r)
Circumference (c)
Area (A)
Diameter (d)
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