The hole rule: 3× the post, ⅓ in the ground
A fence post hole should be about three times the width of the post (a 4×4 post wants a 12-inch hole) and buried at least one-third of the fence's height, plus 6 inches of gravel at the bottom for drainage. For a 6-foot fence that means a 2-to-3-foot hole — right at the frost line in most of the US, which is the real requirement in freeze country.
Bags are the right format for this job
A single 12-inch × 3-foot hole takes about 1 cubic foot of concrete — under two 80-lb bags. That is exactly why fence work is a bagged-concrete job: you carry a few bags per hole instead of paying a truck minimum. Fast-setting (no-mix) bags are popular here: pour dry, add water, set the post level in minutes.
Crown the top
Finish each pour with the concrete sloped slightly away from the post so rain runs off rather than pooling around the wood. It is a two-second trowel move that adds years to the post.
Sources and standards
Where these figures come from
Every formula on this page is implemented from the published reference material below. Where a value depends on a regional code, the calculator lets you override it rather than assuming one jurisdiction.
- ASTM International — C94 for ready-mixed concrete, C150 for cement and C143 for slump testing.
- American Concrete Institute — ACI 318 code requirements and ACI 302 slab construction guidance.
- Portland Cement Association — mix design, yield and curing data published by the US cement industry.
Figures, formulas and reference tables last verified: 19 September 2026.