Quick answer
The perimeter (circumference) of a circle is C = 2πr. A circle of radius 1 has a perimeter of 6.283185 (2π).
Enter the radius. The calculator returns the perimeter.
Related tools: Circle Calculator: Radius, diameter, circumference and area of a circle from any one of them. Perimeter Calculator: Perimeter of common shapes. Area of a Circle Calculator: Area of a circle from its radius, diameter or circumference.
How to use the Circle Perimeter Calculator
- Enter radius (above 0).
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Radius | A number | > 0 |
Formula and method
Formula
C = 2πr
IEEE-754 double precision; shown to at most 6 decimal places.
Direct formula.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Radius = 1.
| Result | Value |
|---|---|
| Circle Perimeter | 6.283185 |
Example 2
Inputs: Radius = 5.
| Result | Value |
|---|---|
| Circle Perimeter | 31.415927 |
The first line is the headline result (circle Perimeter). IEEE-754 double precision; shown to at most 6 decimal places.
Scaling
Perimeter is proportional to the radius: doubling the radius doubles the perimeter, unlike area, which quadruples. A radius of 10 gives 62.83.
Quick estimate
C ≈ 6.28 × r, or 3.14 × diameter. Measure the radius from the centre to the edge. Use the same unit for the answer, which is a length, not an area.
Fencing a circular area
With radius 1 the circumference is 2π ≈ 6.283185. From a diameter use π × d, so a diameter of 10 gives 31.4159.
For a circular garden of radius 4 m the edging length is 2π × 4 ≈ 25.13 m; buy a little extra for cuts and overlap.
How the result changes with radius
Holding every other input at the first worked example, this table changes only radius. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Radius | Circle Perimeter |
|---|---|
| 0.5 | 3.141593 |
| 1 | 6.283185 |
| 2 | 12.566371 |
| 4 | 25.132741 |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Radius: entering "0" shows "Radius must be greater than 0." beside that field and no result.
Choosing between the Circle Perimeter Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Circle Calculator](/math/circle-calculator/) | i know the and value | Area |
| [Perimeter Calculator](/math/perimeter/) | shape and a | Perimeter |
| [Area of a Circle Calculator](/math/area-of-a-circle/) | i know the and value | Area |
| [Circle Diameter Calculator](/math/circle-diameter/) | i know the and value | Diameter |
| [Circle Equation Calculator](/math/circle-equation/) | centre x, centre y and radius | Standard form |
Background: geometry measurements
Geometry results carry units that depend on the quantity: a perimeter or length is in linear units such as cm, an area or surface area is in square units (cm²) and a volume is in cubic units (cm³). Use one length unit for every input, because the calculator multiplies the numbers as given.
Flat (2D) shapes have an area and a perimeter; solids (3D) have a surface area and a volume. Two familiar checks: a circle of radius r has area πr² and circumference 2πr, and a sphere has volume (4/3)πr³ and surface area 4πr².
Assumptions and limitations
- Positive value
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for radius to confirm this.
Frequently asked questions
What is the perimeter of a circle with diameter 10?
31.416, which is 10π.
Is perimeter the same as circumference?
For a circle, yes: its perimeter is called the circumference, 2πr.
What if I know the area?
Find r = √(A/π) first.
What is the circumference of a circle with diameter 10?
π × 10 ≈ 31.4159.
Is circumference the same as perimeter?
Yes. Circumference is the perimeter of a circle.
What does the Circle Perimeter Calculator calculate?
Perimeter (circumference) of a circle from its radius. It returns Circle Perimeter.
What formula does the Circle Perimeter Calculator use?
C = 2πr. Direct formula.
What do I need to enter in the circle Perimeter Calculator?
You enter Radius.
How is the result rounded or limited?
IEEE-754 double precision; shown to at most 6 decimal places. Positive value.
What is an example of a circle Perimeter result?
With radius 1, the circle Perimeter is 6.283185.
Related calculators
- Circle Calculator — Radius, diameter, circumference and area of a circle from any one of them.
- Perimeter Calculator — Perimeter of common shapes.
- Area of a Circle Calculator — Area of a circle from its radius, diameter or circumference.
- Circle Diameter Calculator — Diameter of a circle from radius, circumference or area.
- Circle Equation Calculator — Equation of a circle from its centre and radius.
- Triangle Perimeter Calculator — Perimeter of a triangle from three sides.
- Regular Polygon Calculator — Area, perimeter and angles of a regular polygon.
- Golden Ratio Calculator — Split a length into golden-ratio parts.
- Midpoint Calculator — Midpoint of a line segment.
Sources
- OpenStax, Prealgebra 2e (Rice University): perimeter, area, volume, circumference and the Pythagorean theorem
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.