|
pi.r ² has long been, and currently remains, the formula for calculating the area of a 2D circular disc.
pi.d, or 2.pi.r, is the correct formula for calculating the circumference of a 2D circle or the minimum circumference of a right circular cylinder in 3D.
For a right circular cylinder with wall thickness of say 0.25mm the outside circumference will be 2.pi.R and the inside circumference will be 2.pi.r where r = (R - 0.25).
Both paper and card don't particularly like either tension or compression and it's difficult to know where the axis of bending will occur within the thickness of the material.
So theoretical calculations will only find the ball park and a bit of fumble will likely be needed with the actual material being used.
|