About

Setting fence posts requires a precise amount of concrete to ensure stability without wasting material. A common mistake is calculating the volume of the entire hole without subtracting the volume occupied by the post itself (the displacement). This tool calculates the "Annular Volume"—the empty space between the post and the earth.

Properly set posts withstand wind loads and frost heaves. This calculator helps landscapers and homeowners order the exact number of premix bags needed for a run of 4x4 or 6x6 posts, accounting for the cylindrical hole and the square or round post inside it.

Formulas

The required concrete volume V_c is the volume of the hole minus the volume of the post buried.

V_c = V_hole − V_post

Where:

V_hole = π × (d2)² × depth

Reference Data

Post Size	Min Hole Diameter	Rec. Concrete Depth	Bags (Approx per Post)
4x4 Wood	8 - 10 inches	24 - 36 inches	1.5 - 2 (50lb)
6x6 Wood	10 - 12 inches	30 - 48 inches	3 - 4 (50lb)
2-3/8" Metal	6 - 8 inches	18 - 24 inches	1 (50lb)
Gate Post	+2 to 4 inches extra	36 - 48 inches	3 - 5 (50lb)
Corner Post	+2 to 4 inches extra	36 - 48 inches	3 - 5 (50lb)
Line Post	Post Width + 4"	1/3 of Post Length	1 - 2 (50lb)
Freeze Line	N/A	Below Frost Line	N/A
Density	N/A	N/A	150 lb/ft³

Frequently Asked Questions

A general rule of thumb is the post width plus 3 times the post width? No, simpler: Post width times 3? Actually, the standard industry rule is the diameter of the hole should be 3 times the width of the post for gate posts, or 2 times for line posts. Alternatively, simply add 4-6 inches to the post width.

Yes. It is highly recommended to place 3-6 inches of gravel at the bottom of the hole before setting the post. This promotes drainage and prevents the wood from rotting due to trapped moisture.

No, this tool specifically calculates the concrete mix required to fill the annulus around the post.

You can enter the total number of posts in the 'Quantity' field to get the total bags required for the entire fence line.