A 4x4 post in a 10-inch hole with 24 inches of concrete takes about 0.92 cubic feet: 3 bags of 50 lb fast-setting mix, or 2 bags of 80 lb regular mix.
Step 1: Size the hole
- Width: about 3 times the post width. (QUIKRETE (opens in a new tab))
- Depth: about one third of the post's total length, plus 6 inches for gravel. (QUIKRETE (opens in a new tab))
- Gravel: put 6 inches of gravel or crushed stone in the bottom so water drains away from the post. (QUIKRETE (opens in a new tab))
- Frost: posts that support a structure must reach at least 6 inches below your local frost line. (QUIKRETE (opens in a new tab)) Your town's building department can tell you the depth.
Step 2: Work out the concrete
Concrete fills the hole around the post, so subtract the post from the hole:
- Hole volume = π x (hole radius in feet)² x concrete depth in feet
- Post volume = post width x post width x concrete depth (all in feet)
- Concrete = hole volume minus post volume
Example: 4x4 post, 10-inch hole, 24 inches of concrete
- Hole: 3.1416 x (0.417 ft)² x 2 ft = 1.09 cubic feet
- Post: 4x4 lumber is actually 3.5 x 3.5 inches, so 0.292 x 0.292 x 2 = 0.17 cubic feet
- Concrete: 1.09 minus 0.17 = 0.92 cubic feet
A 50 lb bag of fast-setting concrete yields about 0.375 cubic feet, so this post needs 2.5 bags. Buy 3. (QUIKRETE data sheet (opens in a new tab)) QUIKRETE's own rule of thumb is two bags for this hole size, which assumes part of the 2-foot depth is gravel.
Bags per post
Hole width is about 3 times the post width. Fast-setting mix at 0.375 cubic feet per 50 lb bag; regular mix at 0.60 cubic feet per 80 lb bag. Rounded up to whole bags.
| Post | Hole | Concrete depth | Concrete | 50 lb fast-setting | 80 lb regular |
|---|---|---|---|---|---|
| 4x4 | 10 in | 24 in | 0.92 ft³ | 3 | 2 |
| 4x4 | 10 in | 30 in | 1.15 ft³ | 4 | 2 |
| 4x4 | 10 in | 36 in | 1.38 ft³ | 4 | 3 |
| 6x6 | 16 in | 24 in | 2.37 ft³ | 7 | 4 |
| 6x6 | 16 in | 30 in | 2.97 ft³ | 8 | 5 |
| 6x6 | 16 in | 36 in | 3.56 ft³ | 10 | 6 |
For a whole fence, multiply by the number of posts and add one or two spare bags.
Fast-setting or regular mix?
- Fast-setting sets in 20 to 40 minutes and can be poured dry into the hole with water added, which makes it popular for fence posts. Wait about 4 hours before hanging heavy rails or gates. (QUIKRETE data sheet (opens in a new tab))
- Regular concrete mix is mixed first and gives you more working time, which helps when you are setting many posts at once.
For holes deeper than 2.5 feet, fill and water the concrete in layers no thicker than 2.5 feet. (QUIKRETE (opens in a new tab))