If you're talking about a circle, then the perimeter is more often called the "circumference".
If you know the circumference, then Diameter = (circumference) divided by (pi).
(Pi) is approximately 3.14159 26535 89792 . But if you use (22/7), you're off by less than 1/10th of 1 percent.
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The perimeter is equal to Pi times the diameter.
2*pi*radius or pi*diameter
The formula for the circumference of a circle is pi * diameter or (2 radius.)
There is no formula for an octagon. There are formulas for figuring out different aspects of an octagon, such as its perimeter, diameter, etc.
perimeter / pi (3.14) = diameter