Post Hole Concrete Calculator – How Many Bags?
Find out exactly how many 60 lb or 80 lb bags of concrete you need for fence, deck, gate and mailbox post holes — by hole size and number of posts.
How to calculate concrete for a post hole
To estimate how many bags of concrete a post hole needs, first find the volume of the hole you will fill. Measure the hole in inches and convert to feet:
Hole volume (cu ft) = π × (hole diameter ÷ 2 ÷ 12)² × (hole depth ÷ 12)
For a 10 in wide, 24 in deep hole the radius is 5 in (about 0.42 ft), so the hole area is roughly 0.55 sq ft. Times 2 ft of depth gives ≈ 1.09 cu ft of concrete. An 80 lb bag of concrete mix pours about 0.60 cu ft, so 1.09 ÷ 0.60 ≈ 1.8 — you round up to 2 bags of 80 lb concrete per post. Dig the same 10 in hole 30 in deep instead and it holds ≈ 1.36 cu ft, or about 2.3 bag volumes — 3 bags of 80 lb.
How many bags of concrete per post hole?
You can only buy whole bags, so the calculator always rounds up. This table shows the 80 lb bag counts for the most common post hole sizes:
| Hole size | Hole volume | 80 lb bags per hole |
|---|---|---|
| 8 in × 24 in | 0.70 cu ft | 2 bags |
| 8 in × 30 in | 0.87 cu ft | 2 bags |
| 8 in × 36 in | 1.05 cu ft | 2 bags |
| 10 in × 24 in | 1.09 cu ft | 2 bags |
| 10 in × 30 in | 1.36 cu ft | 3 bags |
| 10 in × 36 in | 1.64 cu ft | 3 bags |
| 12 in × 24 in | 1.57 cu ft | 3 bags |
| 12 in × 30 in | 1.96 cu ft | 4 bags |
| 12 in × 36 in | 2.36 cu ft | 4 bags |
These are estimates for a full cylindrical hole. Your actual yield depends on the mix and how you pour it, so check the coverage chart printed on the bag before you buy.
How deep should a post hole be?
A common rule of thumb is to bury at least one-third of the post's above-ground height, and in cold climates the bottom of the concrete should sit below your local frost line — commonly 24–36 in across much of the US, and deeper in far-northern regions. A 30–36 in hole suits most 6–8 ft fence, gate and mailbox posts. Adding 3–4 in of gravel at the bottom helps water drain away from the post.
60 lb or 80 lb bags — which should I use?
Both sizes mix the same concrete; the only difference is wet volume. An 80 lb bag yields about 0.60 cu ft and a 60 lb bag about 0.45 cu ft. For a 10 in × 30 in hole (≈ 1.36 cu ft) that means 3 bags of 80 lb or 4 bags of 60 lb. Choose 80 lb bags if you want roughly a quarter fewer bags to carry, or 60 lb bags if you prefer a lighter, easier mix. Switch the bag weight in the calculator above and it recalculates automatically.
Related guides
Post hole concrete calculator FAQ
How many bags of concrete do I need for one 4×4 post?
For a typical 10 in wide, 30 in deep hole around a 4×4 post you need about 3 bags of 80 lb mix (≈ 1.36 cu ft). A larger 12 in × 36 in hole takes about 4 bags of 80 lb. The calculator treats the whole hole as concrete — your pour may be slightly less because the post takes up space, so rounding up to whole bags is usually the safe call.
How wide should a post hole be?
A handy rule of thumb is about three times the post width: roughly 12 in for a 4×4 post and 14–18 in for a 6×6 post. Wider holes anchor the post better but use noticeably more concrete — most fence posts do fine in an 8–12 in wide hole.
Should I buy 60 lb or 80 lb bags?
Both make the same concrete mix — a 60 lb bag pours about 0.45 cu ft and an 80 lb bag about 0.60 cu ft. Go with 80 lb for roughly a quarter fewer bags, or 60 lb for easier lifting and mixing. Set the bag weight in the calculator and the bag count updates automatically; always follow the coverage chart on the packaging.
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