Circle Calculator

Know one measurement of a circle? Enter it and get the other three.

Circle

Area78.5398163397
Circumference
31.4159265359
Diameter
10

Working

  1. r = 5
  2. d = 2r = 10
  3. C = 2πr = 31.4159265359
  4. A = πr² = 78.5398163397

Formulas

d = 2r

C = 2πr = πd

A = πr² = πd² ÷ 4

r = C ÷ 2π = √(A ÷ π)

π (pi) ≈ 3.14159265358979 is the ratio of any circle’s circumference to its diameter.

Worked example

Measuring a tree trunk

  1. A tape around the trunk reads 157 cm (circumference)
  2. d = C ÷ π = 157 ÷ 3.14159 ≈ 49.97 cm

The trunk is about 50 cm across.

Everyday uses

  • Pizza size: a 16-inch pizza has (16 ÷ 12)² ≈ 1.78 times the area of a 12-inch one.
  • Wheel travel: one turn of a 70 cm wheel moves about 2.2 m.
  • Round tables, ponds and patios: area for materials, circumference for edging.