Post holes
9 deck posts in 12 in diameter holes, 42 in deep, set around a nominal 4×4 post (3.5 in actual). The concrete fills the ring around each post; here is the total volume and bag count.
Concrete needed
| Bag size | Bags needed |
|---|---|
| 40 lb | 74 |
| 60 lb | 50 |
| 80 lb | 37 |
| 25 kg | 54 |
| 30 kg | 45 |
At this size
37 × 80 lb bags is 0.82 yd³, which is about 19 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 0.82 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 | 42 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 - 22.06 ft³ - is divided by 27 to give 0.82 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 37 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.
Formwork
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