Short answer
One standard fence post hole — 12 inches wide and 2 feet deep — needs about 1.6 cubic feet of concrete: 3 bags of 80 lb mix, or 4 bags of 60 lb mix. A post hole is a cylinder, so its volume is π × radius² × depth: for this hole, π × 0.5² × 2 = 1.57 cu ft. Always round up to whole bags.
Concrete by hole size
Bag counts use standard yields: an 80 lb bag gives about 0.6 cu ft, a 60 lb bag about 0.45 cu ft. Check your bag, since yields vary slightly by brand.
| Hole diameter × depth | Volume | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 8 in × 2 ft | 0.70 cu ft | 2 | 2 |
| 10 in × 2 ft | 1.09 cu ft | 2 | 3 |
| 12 in × 2 ft | 1.57 cu ft | 3 | 4 |
| 12 in × 3 ft | 2.36 cu ft | 4 | 6 |
How to size a post hole
Two rules of thumb cover most fences:
- Width: dig the hole about 3 times the width of the post. A 4×4 post (3.5 inches across) gets a 12-inch hole.
- Depth: dig about one-third of the post’s above-ground height. A 6 ft fence gets a 2 ft hole. In cold climates, dig below the frost line so freeze-thaw cycles do not push the post up.
Example: a 100 ft fence
Posts spaced 8 ft apart along a 100 ft run mean 14 posts (13 sections, counting both end posts). At the standard 12 in × 2 ft hole, that is 14 × 1.57 = 22.0 cu ft — 37 bags of 80 lb mix (22.0 ÷ 0.6 = 36.7, rounded up). As ready-mix, 22.0 cu ft is about 0.81 cubic yards — roughly $122 at $150 per yard, though most people buy bags for post holes.
Dry pour or mix first?
Both work. Standard mix should be mixed with water in a wheelbarrow or mixer before pouring. Fast-setting mix can be poured dry into the hole and then watered from the top, following the bag’s instructions — it sets in about 20–40 minutes. Either way, wait at least 24 hours before hanging panels or gates on the post.
Frequently asked questions
How deep should a fence post hole be?
About one-third of the post’s height above ground — 2 ft for a 6 ft fence. Go deeper in frost country, below the local frost line.
Can I use one bag of concrete per fence post?
Generally no, not for a standard hole. A 12-inch hole 2 ft deep needs 3 bags of 80 lb mix. One bag only covers small holes, around 8 inches wide and shallow.
Do I need to add a waste margin?
No separate margin is needed here — rounding up to whole bags already covers spillage and uneven holes. If your holes come out wider than planned, buy one or two extra bags for the run.
How long before I can hang the fence on the post?
Fast-setting mix is firm in about 20–40 minutes, but wait at least 24 hours before putting weight on the post. Standard mix also needs about 24 hours before loading.
Run your exact numbers
Hole sizes vary, so run your own diameter and depth through the free concrete calculator — it works out the cubic feet, yards, and bags for your exact hole.