The radius of a circle can be calculated from any one of three measurements: diameter, circumference, or area. Each uses a simple formula, and you only need basic arithmetic (plus the value of pi, approximately 3.14159) to solve it. Below you’ll find the formulas, worked examples, and a few other common meanings of “get radius” that may have brought you here.
Radius From Diameter
The diameter of a circle is always exactly twice the radius. That gives you the simplest formula of the three:
- r = d ÷ 2
If a circle’s diameter is 10 inches, the radius is 5 inches. If the diameter is 3.7 centimeters, the radius is 1.85 centimeters. No pi required.
Radius From Circumference
Circumference is the distance around the outside of the circle. It relates to the radius through the formula C = 2πr. To isolate the radius, divide both sides by 2π:
- r = C ÷ (2π)
Say a circular track has a circumference of 400 meters. Divide 400 by 2π (roughly 6.2832), and you get a radius of about 63.66 meters. On a calculator, just divide the circumference by 6.2832 and you’re done.
Radius From Area
The area of a circle is A = πr². To reverse this and solve for the radius, divide the area by π, then take the square root of the result:
- r = √(A ÷ π)
For example, if a circular garden covers 50 square feet, divide 50 by π (about 3.14159) to get 15.915, then take the square root: roughly 3.99 feet. So the garden’s radius is essentially 4 feet.
Quick Reference Table
- Known value: Diameter → r = d ÷ 2
- Known value: Circumference → r = C ÷ (2π)
- Known value: Area → r = √(A ÷ π)
Drawing a Radius on a Map
If you searched “how to get radius” because you want to draw a radius circle around a point on a map, Google Maps does not have a built-in radius tool. You have two practical options.
The first is a free third-party web tool like MapDevelopers.com’s circle tool. You enter an address or coordinates, set a distance (in miles or kilometers), and the site draws the circle on a Google Map. You can expand it to full screen, adjust the radius, and move the center point.
The second option is Google Earth Pro, which is free desktop software. It lets you draw circles and other shapes directly on the globe, then export them as KML files. You can import that KML file into Google My Maps to view the circle inside the regular Google Maps interface, under the “Your Places” section.
For a quick workaround without any extra tools, right-click any point in Google Maps and select “Measure distance.” This lets you draw a straight line to see how far something is from your starting point. It won’t draw a circle, but it tells you whether a location falls within a given distance.
Locating the Radius Bone
The radius is one of two bones in your forearm. To identify it, hold your arm out in front of you with your palm facing up. The radius is the bone on the thumb side (the outer, or lateral, side). The other bone, the ulna, runs along the pinky side.
The radius is slightly shorter but thicker than the ulna. When you rotate your wrist so your palm faces down, the radius crosses over the ulna. When your palm faces up, the two bones sit roughly parallel to each other. If a doctor or physical therapist asks you to locate your radius, just feel along the thumb side of your forearm from wrist to elbow.