What is circumference?
It is the length of the boundary of a circle—the circle’s perimeter.
Circumference Calculator
Find the distance around a circle from its radius or diameter.
Enter radius or diameter
Choose input mode and provide a positive length.
About this calculator
How to use Circumference Calculator, plus the formulas and logic behind the results.
Tool description
Circumference is the perimeter of a circle. It scales linearly with radius or diameter.
How to use
- Select radius or diameter mode.
- Enter a positive length.
- Select Calculate to view circumference.
- Review the related diameter or radius as well.
Formulas & reference
C = 2πrC = πdd = 2r
Quick checks
- r = 1 → C = 2π ≈ 6.2832
- d = 10 → C = 10π ≈ 31.4159
- C grows with diameter
- Full circle only (not arcs)
Calculation logic
Uses π = M_PI. Radius mode doubles the radius then multiplies by π; diameter mode multiplies diameter by π.
Radius and diameter must be positive.
Circumference Calculator FAQ
Common questions about this calculator and how to use the results.
Do I use radius or diameter?
Either works. C = 2πr and C = πd are equivalent because d = 2r.
What value of π is used?
This calculator uses PHP’s M_PI constant.
Can radius be zero?
Zero gives circumference zero, but negative values are rejected.
Is circumference the same as perimeter?
For a circle, yes—circumference is the specialized name for its perimeter.
How is this different from arc length?
Arc length is only part of the circle; circumference is the full loop.