Circumference Calculator
Last updated: November 4, 2026
Calculate a circle’s circumference and area from an explicitly selected radius or diameter.
On this page
Quick summary: Calculate a circle’s circumference and area from an explicitly selected radius or diameter.
Read the complete guide
Updated reference sections in this guide are in English.
Using this calculator
Choose whether your measurement is a radius or a diameter before entering its length. Radius runs from the center to the edge; diameter crosses the circle through its center and is twice the radius. The form changes its label with the selection and reports both dimensions beside the circumference.
Calculation method
Circumference C = 2πr = πd
Area A = πr²
The calculation uses the browser’s value of π rather than rounding it to 3.14 first. This retains more precision until display. Circumference is a length; area is a different measurement and uses the square of the entered length unit.
Worked examples
Radius of 5 units
Diameter is 10. Circumference is 10π, approximately 31.415926535898 units; area is 25π, approximately 78.539816339745 square units.
The same circle from its diameter
Switch to diameter and enter 10. The result matches the radius-5 example. Entering 5 as a diameter instead would describe a circle half as wide.
Check this scenario in the calculator:
- Known measurement
- Radius
- Radius
- 5
Understanding the result
Doubling the radius doubles circumference but multiplies area by four. This is why a linear size change does not produce the same percentage change in covered surface. The result can help check ideal circular geometry, provided the measurement actually represents a circle.
Boundaries and assumptions
Use a nonnegative radius or diameter no larger than 10⁹. Zero is accepted as a degenerate zero-size case. The form does not infer units, estimate measurement uncertainty or compensate for thickness, stretching or an irregular shape. If you are measuring material around an object, choose the relevant inside, outside or centerline dimension yourself.
Questions and reference
Should I enter a unit symbol?
Enter only the number. Read the circumference in the same unit and area in its square.
Can I enter the circumference and solve backward?
This form takes radius or diameter as the known input; divide circumference by 2π or π separately to recover them.