Octagon Dimensions

I want to draw an octagon for the outside walls of a model. I want to be able to set the distance between opposite walls at a specific value; however, when I add a measurement it is the ‘radius’ of the distance between opposite angles. It’s been 65+ years since I studied geometry. Can anyone tell me how to enter a measurement that will give me the span I want? Trial and error has mostly been error on my part.

TIA

while drawing your octagon, if you press… i’ll say ctrl on PC, if switches between the inside and outside circle.

basically the normal way asks you the half distance between two points as you say. the other way asks you the half distance between the walls.

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This is a mathematical calculator that will give you all the dimensions you will need for this shape.