Calculating the
Radius, Diameter, and
Circumference of a Circle

circle

The radius of a circle is the distance from the center of the circle to any point at the outer edge of a circle.
The distance must be measured along a straight line, similar to what you would get if you used a ruler, connected 2 dots together, and then measured the distance between both dots.
This is what you are being asked to do when you are measuring two of the main characteristics of a circle: the radius, and a distance called the Diameter. It's as easy as that.
The real-world applications related to these two measurements are infinite and fascinating!

Examples:
Below are a few examples of real-world uses applied to circles; which viewed in 3D, are called Spheres; in order to deleniate between the two views of a Circle (2D) and a Sphere (3D).

Earth

Planets
The radius of the Earth is approximately 4,000 miles.
The diameter of Earth is approximately 8,000 miles.
The Circumference of the Earth is approximately 25,132 miles.

Bugatti-4
Bugatti Divo's Wheels have:

A Radius = the distance from the center of the tire to the outer edge.
A Diameter = the distance across the tire measured from one side to the opposite side, by measuring through the center of the tire where the hubcap would be.
A Circumference = the measurement around the tire. This is best accomplished using a tape measure because it can be wrapped around the outside portion of the tire referred to as the tread (the part of the tire that touches the road) to make the measurement.

1929
1929 Bugatti

wheels
Bugatti Wheels

ferriswheel
Ferris Wheel in Singapore: The Singapore Flyer
The Singapore Flyer is the second largest Ferris Wheel in the world.

Radius = 270.5 feet
Diameter = 541 feet
Circumference = 1700 feet

highroller
The world's largest Ferris Wheel, The High Roller, is in Las Vegas, Nevada.
It has a height of 550 feet.

Radius = 275 feet
Diameter = 550 feet
Circumference = 1728 feet

coke coke2
Coke Can: the bases of the cylindrical can (the top and bottom: which are 2D circles) have a radius, diameter, and circumference.
The second image is a Coca-Cola can in China, which is pretty cool to see.

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Directions:
To calculate the Radius, Diameter, or Circumference of your circle
enter one value into its corresponding box below.
If you know two of the three values, pick either one, but you do not need to use both.
The Submit button will give you all three measurements, regardless of which measurement(s) that you have!

Radius =

Diameter =

Circumference =