octagon post formula

I am designing some crown post into a frame.
Is there a formula for cutting a square timber into an octagon with even measurements on all 8 faces?
-Collin

Think of the octagon as being composed of eight equally sized wedges rotated around an axis. The angle at the axis is 45. Draw the square, locate the axis, draw a line from axis perpendicular to side of square. The line from center bisects angle of wedge and face of wedge. so,

length of facet = 2 x length of perp x tan 22.5

much appreciated,
-Collin

I recently learned the octagon scale from a stanley square...
The octagon scale on my square is on the side opposite the brace table and has the numbers 5,10,15,20 etc.

here is the method to use the scale...

first measure in inches the square stock size you want to make into an octagon
while you are doing this, place a center mark on each side of the square stock(e.g. an 8 inch side has a mark at 4")
now take a set of dividers and set them to the octagon square scale. for example, if the side of the square stock you have is 8 inches, then set the dividers to measure from 0 to 8 on the octagon scale.
now use the dividers to place this measurement onto the square stock by swinging them from the center marks you previously placed.
now each side of the square stock should have three marks: the original center mark and two marks from the dividers.
now join the marks from the dividers across the corners and you should have an octagon.