Circle Size Calculator
Choose the measurement you know, the radius, diameter, circumference or area, and enter it. The calculator works out the other three, along with the sizes of the squares that fit inside and around the circle.
- Start from any value
- Every unit
- Inside and outside squares
Area is entered in the unit squared, such as cm².
Every circle is set by a single number. Know the radius and you know everything: the diameter is twice it, the circumference is 2π times it and the area is π times its square. Work backwards from any of them.
Circle size chart
From the diameter. Search for a size.
| Diameter | Radius | Circumference | Area |
|---|---|---|---|
| 1 | 0.5 | 3.142 | 0.785 |
| 2 | 1 | 6.283 | 3.142 |
| 5 | 2.5 | 15.71 | 19.63 |
| 10 | 5 | 31.42 | 78.54 |
| 20 | 10 | 62.83 | 314.16 |
| 50 | 25 | 157.08 | 1963.5 |
| 100 | 50 | 314.16 | 7853.98 |
Circumference = π × diameter. Area = π × diameter² ÷ 4.
Radius and circle
In centimetres. Slide the radius.
The dashed square is the square that fits round the circle. The circle fills about 78.5% of it.
How to work out a circle
From the radius. Step through it.
Find the radius
From what you know.
To find the radius from the area, divide the area by π and take the square root.
Radius, diameter or circumference
Which to measure. Compare them.
Centre to edge.
Edge to edge through centre.
Distance round.
For a large round object like a tree or tank, measuring round it with a tape is easier and more accurate.
Before using the result
Circle checks. Tick each item.
Tick each item as you go.
When marking out a circle, a nail, string and pencil makes a large compass.
What you get
Every circle size. Each tile explains one result.
Across.
Half.
Round.
Inside.
Inside and around.
The largest square inside a circle has sides of the radius times √2.
Frequently Asked Questions
No question matches that search.
How do I find the area of a circle?
Multiply π by the radius squared.
How do I find the diameter from the circumference?
Divide the circumference by π.
How do I find the radius from the area?
Divide the area by π, then take the square root.
What is π?
About 3.14159, the ratio of any circle's circumference to its diameter.
What is the difference between radius and diameter?
The radius is from the centre to the edge, and the diameter is twice that, right across.
How do I measure the diameter of a large circle?
Measure round it with a tape and divide by π.
Why does area grow faster than circumference?
Area depends on the radius squared, so doubling the radius makes it four times larger.
One number, every size
Enter whatever you know about the circle and get the rest. It works for pipes, tables, pizzas and planets alike.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.