How Much Concrete Do I Need for a Fence Post?
Work out the volume of the hole, subtract the part the post takes up, and divide by the bag yield. For a typical 10 in × 36 in hole with a 4×4 post that's about 1.38 cu ft per post.
Round posts are treated as circles; square posts subtract width × width × depth. Bag yields: 80 lb 0.60, 60 lb 0.45, 50 lb 0.375, 40 lb 0.30 cu ft (typical; check your bag).
How the calculation works
A post hole is a cylinder. Its volume is π × radius² × depth. A 10 in hole has a 5 in radius, so a 36 in deep hole holds 3.1416 × 25 × 36 = 2,827 cubic inches, or 1.64 cu ft (there are 1,728 cubic inches in a cubic foot).
The post fills part of that space. A 4×4 is actually 3.5 × 3.5 in, so 36 in of buried post takes up 441 cu in (0.26 cu ft). The concrete needed is the difference: 1.38 cu ft per post. That's 2.3 bags of 80 lb or 3.1 bags of 60 lb.
Bags per post chart
| Hole (dia × depth) | Post | Cu ft per post | 80 lb | 60 lb | 40 lb |
|---|---|---|---|---|---|
| 10" × 24" | 4×4 | 0.92 | 2 | 3 | 4 |
| 10" × 36" | 4×4 | 1.38 | 3 | 4 | 5 |
| 12" × 36" | 4×4 | 2.10 | 4 | 5 | 8 |
| 12" × 42" | 6×6 | 2.01 | 4 | 5 | 7 |
| 12" × 48" | 4×4 | 2.80 | 5 | 7 | 10 |
The chart rounds up per hole, which is how you'll actually use bags if you mix one hole at a time. For many posts, the calculator above also gives the total for the whole fence, which is usually a few bags fewer than rounding every hole up.
Sizing the hole
A widely used rule of thumb is a hole about three times the post's width and a depth of about a third to a half of the post's height above ground. Frost depth matters in cold climates, because footings that stop above the frost line can heave. Your local building department or fence supplier can tell you the frost depth and any rules for your area. Call your utility locator service before digging.
Fast-setting vs regular mix
Fast-setting post mixes are often poured dry into the hole and then soaked with water, which saves mixing. Standard concrete is mixed first. Either way, check the yield on the bag, since some products yield less per pound, and follow the maker's water and curing instructions before hanging gates or rails.
Related
- Concrete calculator
- Bags of concrete for a 10×10 slab
- Bags of concrete per cubic yard
- Feet-inch calculator for post spacing
FAQ
How many bags of concrete per fence post?
For a 10 in wide, 36 in deep hole with a 4×4 post, about 1.38 cu ft: 3 bags of 80 lb or 4 bags of 60 lb if you round up per hole (2.3 and 3.1 exact).
How deep should a fence post hole be?
A common rule is a third to a half of the post's above-ground height, and below the local frost line in cold climates. Check local rules.
How wide should a post hole be?
About three times the width of the post is a common guideline, e.g. 10–12 in for a 4×4.