Radius vs Diameter: What's the Difference?

The radius and diameter both measure across a circle, but they’re not the same thing — and mixing them up is one of the most common circle mistakes. The short version: the radius reaches from the centre to the edge, the diameter reaches all the way across, and the diameter is always twice the radius. Here’s the full picture.

What is the radius?

The radius is the distance from the centre of a circle to any point on its edge. Think of it as how far you’d walk from the middle straight out to the rim. It’s usually written as r.

What is the diameter?

The diameter is the distance all the way across the circle, passing through the centre — edge to edge. It’s the longest straight line you can draw inside a circle. It’s usually written as d.

The relationship: diameter = 2 × radius

This is the only rule you really need:

• diameter = 2 × radius (the diameter is double the radius)
• radius = diameter ÷ 2 (the radius is half the diameter)

So they always travel together. If one changes, the other changes with it.

Examples:

• A radius of 5 → diameter of 5 × 2 = 10
• A diameter of 10 → radius of 10 ÷ 2 = 5
• A radius of 7 → diameter of 14
• A diameter of 3 → radius of 1.5

A quick way to remember it

The diameter is the distance all the way across — both start with “d”, and it’s the bigger one. The radius is just the reach from the middle, so it’s half.

Quick reference

Radius	Diameter
1	2
2	4
5	10
7	14
10	20

How they connect to circumference and area

Both pop up in the other circle formulas:

• Circumference uses the radius: C = 2πr (or πd with the diameter).
• Area uses the radius: A = πr².

So once you know either the radius or the diameter, you can work out everything else about the circle.

Do it instantly

Our free circle calculator converts between radius, diameter, circumference, and area the moment you type in any one of them — with unit conversion built in.

Frequently asked questions

What is the difference between radius and diameter?

The radius goes from the centre of the circle to the edge. The diameter goes all the way across, through the centre. The diameter is always twice the radius.

How do you convert radius to diameter?

Multiply the radius by 2. A radius of 5 gives a diameter of 10.

How do you convert diameter to radius?

Divide the diameter by 2. A diameter of 10 gives a radius of 5.

Is the radius half the diameter?

Yes. The radius is always exactly half the diameter, and the diameter is always exactly double the radius.

Which is bigger, the radius or the diameter?

The diameter is bigger — it is twice the length of the radius.