Annulus

 

An annulus is a flat shape like a ring.

Its edges are two circles that have the same center.

 

Area

Because it is a circle with a circular hole, you can calculate the area by subtracting the area of the "hole" from the big circle's area:

Area = πR2πr2
= π( R2 − r2 )

 

Example: a steel pipe has an outside diameter (OD) of 100mm and an inside diameter (ID) of 80mm, what is the area of the cross section?

Convert diameter to radius for both outside and inside circles:

  • R = 100 mm / 2 = 50 mm
  • r = 80 mm / 2 = 40 mm

Now calculate area:

Area = π( R2 - r2 )
Area = 3.14159... × ( 502 − 402 )
Area = 3.14159... × (2500 − 1600)
Area = 3.14159... × 900
Area = 2827 mm2 (to nearest mm2)