This post hole concrete calculator works out the exact number of concrete bags for your post holes โ from the real hole volume, minus the space the post takes up. No sign-up. The working is shown under every number.
Quick example: 8 posts in 12 in wide, 36 in deep holes with 4 ร 4 posts need 31 ร 80 lb bags of concrete โ $186.00 at the example prices.
| Item | Buy | Cost |
|---|---|---|
| Estimated total | $0.00 |
Bag yields are the standard figures: an 80 lb bag makes about 0.60 cu ft of concrete, a 60 lb bag 0.45 cu ft and a 50 lb bag 0.375 cu ft. Fast-setting mixes use the same yields.
Estimate only โ not a guarantee. Mistakes are possible, so always check the quantities with your supplier or a builder before you buy or build.
For a 4 ร 4 post in a 12 in wide, 36 in deep hole, the hole holds 2.36 cu ft, the post itself takes up 0.26 cu ft, so the concrete needed is 2.10 cu ft โ about 3.5 ร 80 lb bags' worth of mixed concrete. With this calculator's 10% spare allowance, buy 4 bags per post. Published Quikrete-method estimates for this exact hole give about 3.6 bags before rounding up, the same figure.
A 6 ร 6 post (5.5 in actual) displaces more of the hole: in a 12 in ร 36 in hole it leaves 1.73 cu ft to fill, so with the spare allowance it still works out to 4 ร 80 lb bags per post โ but there is much less room for error, and many builders step up to a 16 in hole for 6 ร 6 posts.
The usual rule is to bury about one third of the post's total length, and the bottom of the hole should sit below your local frost line so frost cannot lift the post. For a 6 ft fence that normally means a 36 in hole. This calculator plans the concrete only โ check your frost depth and local code before you dig.
An 80 lb bag makes about 0.60 cu ft of concrete, a 60 lb bag about 0.45 cu ft and a 50 lb bag about 0.375 cu ft. So one cubic yard (27 cu ft) takes 45 ร 80 lb bags, 60 ร 60 lb bags or 72 ร 50 lb bags. Pick your bag size in the calculator and it does the conversion for your holes.
| Posts | Concrete per post | Total concrete | 80 lb bags | Total |
|---|---|---|---|---|
| 4 posts | 2.10 cu ft | 9.24 cu ft | 16 | $96.00 |
| 8 posts | 2.10 cu ft | 18.49 cu ft | 31 | $186.00 |
| 10 posts | 2.10 cu ft | 23.11 cu ft | 39 | $234.00 |
| 12 posts | 2.10 cu ft | 27.73 cu ft | 47 | $282.00 |
Total concrete includes the 10% spare allowance. Concrete per post is the hole volume (a 12 in ร 36 in cylinder = 2.36 cu ft) minus the part of the post below ground (0.26 cu ft).
These figures are planning estimates only โ not a guarantee and not professional advice. Mistakes are possible, including through an error in this calculator. Always check the quantities, sizes, spacings and depths with your supplier, a licensed builder, and your local building code before you buy materials or start work. The owner of this site accepts no responsibility or liability for any loss, cost, damage, injury, wasted material or failed inspection that results from using these estimates. Prices are examples to start from, not live prices or quotes.