How to Calculate the Radius of a Circle (With Examples and Calculator)

Need to find the radius of a circle? Whether you’re measuring tires, pizza pans, tables, or roundabouts—knowing how to calculate the radius can help in tons of real-life situations.

In this article, we’ll explain what the radius is, give you the formulas to find it, walk you through examples, and provide a handy calculator and infographic to make it all click.

Want to skip the math? Use the calculator below to enter one of the following: Diameter, Circumference, or Area and we’ll calculate the radius for you

🙋‍♀️ What Is the Radius?

The radius is the distance from the center of the circle to any point on its edge. It’s half the diameter.

You’ll often see it labeled as r in math formulas.

🧾 How to Calculate Radius

There are a few ways to calculate radius depending on what information you have:

1. If You Know the Diameter

This is the easiest one:

Radius = Diameter ÷ 2

Example: If the diameter is 20 cm

Radius = 20 ÷ 2 = 10 cm

2. If You Know the Circumference

Use this formula:

Radius = Circumference ÷ (2 × π)

Example: If the circumference is 31.42 cm

Radius = 31.42 ÷ (2 × 3.1416) ≈ 5 cm

3. If You Know the Area

This one’s a little trickier but still doable:

Radius = √(Area ÷ π)

Example: If the area is 78.5 cm²

Radius = √(78.5 ÷ 3.1416) ≈ √25 = 5 cm

🔍 When Do You Need Radius?

• Sizing round tables or rugs
• Designing wheels or mechanical parts
• Solving math or geometry problems
• Estimating space in circular gardens, plates, etc.

✨ Final Thoughts

Finding the radius is easier than you think once you know which formula to use! Just plug in what you know (diameter, circumference, or area), and you’ve got your answer.