Fence Post Size: Use Actual Dimensions and a Project Specification

Use actual width for a square post and actual outside diameter for a round post. A nominal timber label does not automatically identify the supplied finished section, especially across markets. Ask the supplier for actual dimensions, material grade, treatment and any routed or hollow section information.

For concrete displacement, a solid square post occupies width² × embedded concrete depth. A round post occupies π × radius² × depth. Hollow or open-ended profiles may behave differently depending on whether concrete fills them; choose no subtraction when the assumed displacement is not justified.

A gate introduces hinge and latch loads and may need a different post and foundation. A corner may have infill on two sides. Tall solid panels and open chain link expose posts to different forces. The quantity schedule should identify these types so the correct specified products can be quoted.

There is no automatic rule here that a particular nominal post is adequate for every 6 ft fence. Local rules, manufacturer system design, soil and loading govern the project. A larger post without a suitable foundation does not complete the design.

A 3.5 in square post has a diagonal of about 4.95 in. A round hole must be wider than that diagonal even before allowing a concrete surround. The concrete calculator rejects a hole that cannot contain the post footprint. Its cylinder-minus-post formula estimates volume only; it does not select the required annular clearance, embedment or bearing capacity. Record those values from the approved project specification.