How Many Bags of Concrete Per Post Hole
Setting a fence, deck, or mailbox post starts with one question: how much concrete goes in each hole? Here is the simple math and a quick lookup table.
Each post hole holds a cylinder of concrete minus the space the post itself takes up. The volume of a cylinder is pi times the radius squared times the depth. Once you know the volume per hole, you divide by the yield of your chosen bag to get bag count, then multiply by the number of posts.
The rule of thumb for depth and diameter
A common guideline is to dig the hole about three times the post width in diameter and bury one-third of the post length. For a 4 by 4 post, that means a hole roughly 10 to 12 inches across. For frost-prone regions, the hole should extend below the frost line, which can mean 36 to 48 inches deep.
Volume per hole
Take a 10-inch diameter hole (radius 5 inches) that is 24 inches deep. Convert to feet: radius 0.417 ft, depth 2 ft. Volume = 3.1416 × 0.417² × 2 = about 1.09 cubic feet. Subtract the post: a 4 by 4 is about 0.11 square feet in cross-section, so over 2 feet it displaces about 0.22 cubic feet. Net concrete needed is roughly 0.87 cubic feet per hole.
Bag reference table
Concrete bag yields are approximate: a 50 lb bag is about 0.375 cubic feet, a 60 lb bag about 0.45 cubic feet, and an 80 lb bag about 0.6 cubic feet.
| Hole size | Concrete needed | 80 lb bags |
|---|---|---|
| 8 in dia × 24 in deep | ~0.6 cu ft | 1 |
| 10 in dia × 24 in deep | ~0.9 cu ft | 2 |
| 12 in dia × 30 in deep | ~1.6 cu ft | 3 |
| 12 in dia × 42 in deep | ~2.3 cu ft | 4 |
Fast-setting versus standard mix
Fast-setting concrete can be poured dry into the hole and soaked with water, which is popular for fence posts. Standard mix is stronger for heavy structural posts but needs mixing. Either way the volume calculation is the same, so plan your bag count first.
Tips for a solid set
- Add a few inches of gravel at the bottom of each hole for drainage.
- Brace posts plumb before the concrete sets.
- Crown the top of the concrete so water sheds away from the post.
- Buy one extra bag per several holes to cover slightly larger digs.
Scaling to a whole fence
Multiply the bags per hole by the number of posts. A 100-foot fence with posts every 8 feet needs about 13 posts; at 2 bags each that is 26 bags. Corner and gate posts often use bigger holes, so add a few bags for those.