Post Hole Concrete Calculator
How many bags to set that fence or deck post?
How this is calculated
- Hole volume = π × (diameter ÷ 12)² × (depth ÷ 12) — a cylinder, not a box
- Total = per-hole volume × number of holes
- Bags = ⌈total ÷ yield⌉ — 80 lb regular yields 0.60 cu ft, 50 lb fast-setting yields 0.375 cu ft
- This counts the concrete to fill the hole; the post itself displaces a little, which is your built-in safety margin
Common questions
How much concrete per fence post?
A standard fence post hole — 12 in across, 24 in deep — takes about 6.3 cu ft, or 11 eighty-pound bags per hole... which is why most fence setters either use a narrower hole (10 in ≈ 7-8 bags) or set posts in tamped gravel for non-critical fences. Four posts at 12 × 24: roughly 42 bags.
Should fence posts be set in concrete or gravel?
Concrete for gates, corners, and end posts (where the load is), and it is the durable choice for 6 ft+ privacy fences. Tamped crushed stone works well for line posts in well-draining soil — and it lets water drain past the post instead of pooling against it. In clay soils or frost zones, concrete with a dome or gravel-collar above grade keeps the post dry.
How deep should a post hole be?
One-third to one-half the above-ground height, and below the local frost line — the number that matters most. A 6 ft privacy fence post wants 30-36 in of depth; a 4 ft post 24 in. Digging to frost depth matters more than diameter: heave pushes shallow posts up year after year.
How big should the hole be compared to the post?
Three times the post's width is the standard — a 4×4 (actually 3.5 in) wants a 10-12 in hole. Wider holes waste concrete fast: at 12 in wide vs 8 in, the same depth costs over double the bags. Match the auger to the post, not to the shovel you happen to own.
Fast-setting or regular concrete for posts?
Fast-setting for posts — it cures in 20-40 minutes, so you can set the post plumb, brace it briefly, and move on to the next hole the same day. Regular concrete needs bracing for a day or two. Either way, slope the top of the concrete into a dome above soil level so water runs off, not down the post.