Post holes
12 deck posts in 12 in diameter holes, 48 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 | 113 |
| 60 lb | 75 |
| 80 lb | 57 |
| 25 kg | 82 |
| 30 kg | 68 |
At this size
57 × 80 lb bags is 1.25 yd³ and roughly 29 mixer batches. This is the crossover point: bagging it is a full day with a mixer and at least two people, and the concrete has to keep arriving faster than the placed edge stiffens. Price 1.25 yd³ of ready-mix, short-load surcharge included, against 57 bags at your own bag price before you commit to a day of mixing.
| Hole diameter | 12 in |
|---|---|
| Depth | 48 in |
| Holes | 12 |
| 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 - 33.62 ft³ - is divided by 27 to give 1.25 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 57 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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