How do you calculate the area of a circle from its circumference?

First calculate the radius by dividing circumference by 2π (r = C / 2π). Then compute area using A = πr², which simplifies to A = C² / (4π).

⚪ 2D Circle & 3D Cylinder Geometry

Circle & Cylinder Calculator

Calculate radius, diameter, circumference, area, arc length, sector area, and cylinder volume.

Radius (r):

Diameter (d):

Circumference (C):

Area (A):

Cylinder Height (h, optional):

Sector Angle (θ in degrees, optional):

Presets:

Unit Circle (r=1) r=5 r=10 r=25

Radius

Diameter (2r)

Circumference (2πr)

31.4159

Circle Area (πr²)

78.5398

Cylinder Volume (πr²h)

785.398

Surface Area (2πrh + 2πr²)

471.239

Arc Length (s)

5.236

Sector Area (A_sec)

13.090

Diameter ($d$): $d = 2r$
Circumference ($C$): $C = 2\pi r = \pi d$
Circle Area ($A$): $A = \pi r^2 = \frac{\pi d^2}{4}$
Arc Length ($s$): $s = \frac{\theta}{360^\circ} \times 2\pi r$
Sector Area ($A_{\text{sector}}$): $A_{\text{sector}} = \frac{\theta}{360^\circ} \times \pi r^2$
Cylinder Volume ($V$): $V = \pi r^2 h$
Cylinder Surface Area ($A_{\text{surface}}$): $A_{\text{surface}} = 2\pi rh + 2\pi r^2$