Post holes
9 deck posts in 12 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 18.91 cubic feet, or 0.7 cubic yards: 32 bags of 80 lb premix or 43 bags of 60 lb, before any waste.
Concrete needed
| Bag size | Bags needed |
|---|---|
| 40 lb | 64 |
| 60 lb | 43 |
| 80 lb | 32 |
| 25 kg | 46 |
| 30 kg | 39 |
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 | 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 (this page) | 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 2.1 ft³ around the post.
Bought for the whole job that is 32 bags of 80 lb; if every hole gets whole bags of its own it is 4 per hole, 36 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
32 × 80 lb bags is 0.7 yd³, which is about 16 batches in a 6 ft³ site mixer at two bags a batch. That is a comfortable half-day for two people - one mixing, one placing - and at 0.7 yd³ you are under the one-yard minimum most ready-mix suppliers will deliver, so bagging it is the route regardless of price.
| Hole diameter | 12 in |
|---|---|
| Depth | 36 in |
| Holes | 9 |
| 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 - 18.91 ft³ - is divided by 27 to give 0.7 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 32 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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