Mixing or ordering it
6 × 80 lb bags is 0.12 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.
Post holes
Concrete for 6 × 8 in post holes, 24 in deep
6 fence posts in 8 in diameter holes, 24 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
0.12cubic yards· 0.09 m³· 3.17 ft³
| Bag size | Bags needed |
|---|---|
| 40 lb | 11 |
| 60 lb | 8 |
| 80 lb | 6 |
| 25 kg | 8 |
| 30 kg | 7 |
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.
Bagged or ready-mix?
Bags
0.12 yd³ is under a ready-mix supplier's usual one-yard minimum, so bags are the answer whatever the price per yard says. Buy a couple more than the count above and return what you do not open.
At this size
What changes on 6 × 8 in post holes, 24 in deep
The dimensions used
| Hole diameter | 8 in |
|---|---|
| Depth | 24 in |
| Holes | 6 |
| Post | 4×4 (3.5 in) |
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 - 3.17 ft³ - is divided by 27 to give 0.12 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 6 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, 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, 36 in deep0.7 yd³Concrete for 9 × 12 in post holes, 42 in deep0.82 yd³Concrete for 12 × 12 in post holes, 48 in deep1.25 yd³