Post holes
6 fence posts in 8 in diameter holes, 36 in deep, set around a nominal 4×4 post (3.5 in actual). The concrete fills the ring around each post. That is 4.75 cubic feet, or 0.18 cubic yards: 8 bags of 80 lb premix or 11 bags of 60 lb, before any waste.
Concrete needed
| Bag size | Bags needed |
|---|---|
| 40 lb | 16 |
| 60 lb | 11 |
| 80 lb | 8 |
| 25 kg | 12 |
| 30 kg | 10 |
Same job, other sizes
| Size | ft³ | yd³ | yd³ to order (+10%) | 80 lb bags (+10%) | 60 lb bags (+10%) |
|---|---|---|---|---|---|
| 6 × 8 in, 24 in deep | 3.17 | 0.12 | 0.13 | 6 | 8 |
| 6 × 8 in, 36 in deep (this page) | 4.75 | 0.18 | 0.19 | 9 | 12 |
| 6 × 10 in, 24 in deep | 5.52 | 0.2 | 0.23 | 11 | 14 |
| 6 × 10 in, 36 in deep | 8.29 | 0.31 | 0.34 | 16 | 21 |
| 8 × 10 in, 30 in deep | 9.21 | 0.34 | 0.38 | 17 | 23 |
| 9 × 12 in, 36 in deep | 18.91 | 0.7 | 0.77 | 35 | 47 |
| 9 × 12 in, 42 in deep | 22.06 | 0.82 | 0.9 | 41 | 54 |
| 12 × 12 in, 48 in deep | 33.62 | 1.25 | 1.37 | 62 | 83 |
Each hole takes 0.79 ft³ around the post.
Bought for the whole job that is 8 bags of 80 lb; if every hole gets whole bags of its own it is 2 per hole, 12 in all.
Change the dimensions in the post holes calculator. Why every count rounds up: how many bags of concrete you need.
At this size
8 × 80 lb bags is 0.18 yd³ - a wheelbarrow-and-shovel job. Mix two bags at a time, keep the water tight to the bag's own figure, and place it in one go so there is no cold joint. No mixer, no delivery, no minimum load.
| Hole diameter | 8 in |
|---|---|
| Depth | 36 in |
| Holes | 6 |
| Post | 4×4 (3.5 in) |
Each post hole is a cylinder - π·(diameter/2)² × depth - and the concrete only fills the space around a set post, so the post's cross-section is subtracted. Enter a post size of 0 to fill the whole hole as a mass footing.
That volume - 4.75 ft³ - is divided by 27 to give 0.18 cubic yards, and by each bag's yield to give the bag count. An 80 lb bag of premix yields about 0.60 cubic feet, so this job takes 8 of them; bag counts always round up, because you cannot buy part of a bag.
Ordering ready-mix instead? Suppliers sell by the cubic yard (or cubic metre) and usually have a minimum load and a short-load surcharge - worth a call before you choose between bags and a truck.
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