Helm size formula

If a head was round , the circumference C would equal 2*pi*r.  So the radius of the head would equal C/(2*pi). 

The circumference of a padded helm with .75" of padding CH should equal 2*pi* (r+.75).  Replacing the r with the formula above gives CH=2*pi* (C/(2*pi)+.75).   Simplify to CH=C+(2*pi*.75) Again for our 23" head this gives a helm circumference of 27.71" or an additional 4.71 inches of circumference to allow for padding.  Round it up to 5.

To put it another way, multiply the amount of padding to be added by 2pi or 6.28 to get the amount to add to the helm circumference.