Per unit
Each hole takes 1.15 ft³ around the post.
That is 2 bags of 80 lb per hole, 16 in all.
Change the dimensions in the post holes calculator. Why every count rounds up: how many bags of concrete you need.
Post holes
Concrete for 8 × 10 in post holes, 30 in deep
8 fence posts in 10 in diameter holes, 30 in deep, set around a nominal 4×4 post (3.5 in actual). The concrete fills the ring around each post. That is 9.21 cubic feet, or 0.34 cubic yards: 16 bags of 80 lb premix or 21 bags of 60 lb, before any waste.
Concrete needed
0.34cubic yards· 0.261 m³· 9.21 ft³
| Bag size | Bags needed |
|---|---|
| 40 lb | 31 |
| 60 lb | 21 |
| 80 lb | 16 |
| 25 kg | 23 |
| 30 kg | 19 |
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
0.34 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.
Same job, other sizes
Post holes around a 4×4 post
| 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 (this page) | 9.21 | 0.34 | 0.38 | 17 | 23 |
| 9 × 12 in, 36 in deep | 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 |
At this size
What changes on 8 × 10 in post holes, 30 in deep
Mixing or ordering it
16 × 80 lb bags is 0.34 yd³, which is about 8 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 0.34 yd³ you are under the one-yard minimum most ready-mix suppliers will deliver, so bagging it is the route regardless of price.
The dimensions used
| Hole diameter | 10 in |
|---|---|
| Depth | 30 in |
| Holes | 8 |
| 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 - 9.21 ft³ - is divided by 27 to give 0.34 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 16 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
Similar sizes
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