How many bags of concrete per fence post?
For a typical 6 ft fence with 4×4 posts set 2 ft deep in an 11″ hole, plan on about three 50 lb bags of fast-setting concrete or two 80 lb bags of regular mix per post. Bigger posts and deeper holes add up fast:
| Post | Concrete depth | Hole width | Cu ft per post | 50 lb bags | 80 lb bags |
|---|---|---|---|---|---|
| 4×4 | 2 ft | 11″ | 1.03 | 2.8 | 1.7 |
| 4×4 | 2.5 ft | 11″ | 1.29 | 3.4 | 2.2 |
| 4×4 | 3 ft | 11″ | 1.55 | 4.1 | 2.6 |
| 4×4 | 4 ft | 11″ | 2.07 | 5.5 | 3.4 |
| 6×6 | 2 ft | 17″ | 2.55 | 6.8 | 4.2 |
| 6×6 | 2.5 ft | 17″ | 3.19 | 8.5 | 5.3 |
| 6×6 | 3 ft | 17″ | 3.82 | 10.2 | 6.4 |
| 6×6 | 4 ft | 17″ | 5.10 | 13.6 | 8.5 |
The calculator rounds up the total for all posts, not each hole, because leftover concrete from one hole goes into the next.
How the math works
Each hole is a cylinder about three times as wide as the post. Its volume is π × radius² × depth, minus the space the post takes up. Divide that by what one bag makes: about 0.30 cu ft for a 40 lb bag, 0.375 for a 50 lb bag of fast-setting mix, 0.45 for 60 lb and 0.60 for 80 lb.
Fast-setting or regular concrete?
Fast-setting mix goes into the hole dry and you add water on top, with no mixing. It firms up in well under an hour, so you can move on quickly and won't need to brace the post for long. Regular mix has to be mixed in a wheelbarrow first and takes longer to set, so posts need bracing until it hardens. The 80 lb bags make the most concrete per bag. Follow the water amounts on the bag either way.
How to set a fence post in concrete
- Dig the hole about 6″ deeper than the concrete depth and add 6″ of gravel for drainage.
- Stand the post on the gravel, check it's plumb on two sides, and brace it.
- Fill the hole with concrete, then slope the top away from the post so rain runs off instead of pooling against the wood.
- Let it cure before hanging rails or gates. Check the bag for the time.
Planning the rest of the fence?
The fence post depth calculator works out how deep each post needs to go. The fence calculator gives you every post, rail and picket for your fence length, with a full material cost.