The depth of a flagpole foundation is determined by the height of the pole, or simply 10% of the flagpole. (For example: a 35ft flagpole will require a hole 3-1/2ft deep, a 40ft flagpole will require a hole 4ft deep)
The width of a flagpole foundation is determined by the butt diameter of the pole times 4-6 inches. (For example: a 4 inch butt diameter flagpole will need a 16-24inch tapered hole.