Taken from Portland tree code

Roots over 4 inches in diameter should not be cut.

root encroachment zone is one foot for every inch of diameter.

See this handy pamphlet.

What is the diameter of the oak tree? Circumference is ~83“. Therefore, Radius is 13.2”, Diameter is 26.4“. New measurement: C 84.5”, D 27.05“, R is 13.53”.

Aha! It's measured from the CENTER of the trunk, which saves us a bit. yay!

So, we MUST be ~149“ away from the tree.