The dimensions used
| Hole diameter | 12 in |
|---|---|
| Depth | 36 in |
| Holes | 9 |
| Post | 4×4 (3.5 in) |
Post holes
Concrete for 9 × 12 in post holes, 36 in deep
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; here is the total volume and bag count.
Concrete needed
0.7cubic yards· 0.535 m³· 18.91 ft³
| Bag size | Bags needed |
|---|---|
| 40 lb | 64 |
| 60 lb | 43 |
| 80 lb | 32 |
| 25 kg | 46 |
| 30 kg | 39 |
Planning estimate, with no waste allowance included. Add 5–10% for spillage and over-excavation, and check the yield printed on your bag. Structural work must follow your local building code and an engineer's specification.
How it is worked out
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.
Formwork
What is being measured
Nearby
More post holes examples
Concrete for 6 × 8 in post holes, 24 in deep0.12 yd³Concrete for 6 × 8 in post holes, 36 in deep0.18 yd³Concrete for 6 × 10 in post holes, 24 in deep0.2 yd³Concrete for 8 × 10 in post holes, 30 in deep0.34 yd³Concrete for 6 × 10 in post holes, 36 in deep0.31 yd³Concrete for 9 × 12 in post holes, 42 in deep0.82 yd³Concrete for 12 × 12 in post holes, 48 in deep1.25 yd³