Circle Length Calculator
Calculate the circle length with this fantastic tool, and know its circumference, radius, or diameter.
Area78.539816from radius 5
Radius5
Diameter10
Circumference31.415927
Area78.539816
Area of the enclosing square100the circle fills 78.54% of it
Working
| Radius | 5 |
| C = 2πr | 2 × π × 5 = 31.415927 |
| A = πr² | π × 5² = 78.539816 |
The formulas
C = 2πr, A = πr², d = 2r
Everything follows from the radius
A circle has one degree of freedom, so any single measurement fixes all the others:
d = 2rC = 2πr = πdA = πr² = πd²/4 = C²/4π
Going backwards, r = C/2π and r = √(A/π). The calculator above accepts whichever you happen to have.
Area grows with the square
Double the radius and the circumference doubles, but the area quadruples. That is why a 16-inch pizza is more than twice the food of a an 11-inch one, and why pipe capacity rises so sharply with bore.