Fence Post Concrete Calculator
Work out the hole's volume, subtract the post, and divide by the bag's yield. An 8 ft 4×4 in a 10½ inch hole, 32 inches deep, takes 4 bags of 50 lb fast-setting concrete; a 10-post fence takes 32.
Updated October 19, 2026 · How we calculate
Worked example
One 8 ft 4×4 post, hole 10½ inches wide and 32 inches deep, filled to 4 inches below the ground:
- Hole area: 3.1416 × 5.25² = 86.6 sq in
- Minus the post (3½ × 3½ = 12.25 sq in) = 74.3 sq in
- × 28 in of concrete ÷ 1,728 = 1.205 cu ft
- ÷ 0.375 cu ft per 50 lb bag = 3.21 bags, so 4
Source: Quikrete Fast-Setting Concrete Mix 1004 data sheet (2025); American Softwood Lumber Standard, PS 20-20 (NIST)
How to choose and how deep
Size the hole
Make it about 3 times the post's width, and a third of the post's full length deep.
Source: Quikrete Fast-Setting Concrete Mix 1004 data sheet (2025); Sakrete Fast Setting Concrete Mix data sheet
Use the post's real size
Lumber is sold by its name size, but it is planed smaller: a 4×4 is 3½ inches square and a 6×6 is 5½. The calculator subtracts the real size.
Source: American Softwood Lumber Standard, PS 20-20 (NIST)
Two ways to fill it
Quikrete's fast-setting method pours dry mix into the hole and stops 3 to 4 inches below the ground for soil. Sakrete puts 4 to 6 inches of gravel in the bottom first. Pick either in step 3.
Source: Quikrete Fast-Setting Concrete Mix 1004 data sheet (2025); Sakrete Fast Setting Concrete Mix data sheet
How we round
Like the manufacturers' charts, we round up to the next bag unless the last bag would cover 1.5% or less of the job. The exact figure is shown too.
Quick reference
Sakrete's chart vs ours
4×4 post, 10 in hole, 50 lb Fast Setting bags (0.38 cu ft), filled to the top.
| Hole depth | Sakrete's chart | Our exact figure |
|---|---|---|
| 12 in | 1.25 bags | 1.21 bags |
| 18 in | 2 bags | 1.82 bags |
| 24 in | 2.5 bags | 2.42 bags |
| 30 in | 3 bags | 3.03 bags |
Source: Sakrete Fast Setting Concrete Mix data sheet
Quikrete's chart vs ours
4×4 post, 12 in hole, 50 lb Fast-Setting bags (0.375 cu ft), filled to 4 in below the ground.
| Hole depth | Quikrete's chart | Our exact figure |
|---|---|---|
| 18 in | 3 bags | 2.18 bags |
| 24 in | 4 bags | 3.11 bags |
| 30 in | 4 bags | 4.05 bags |
| 36 in | 5 bags | 4.98 bags |
| 42 in | 6 bags | 5.91 bags |
| 48 in | 7 bags | 6.85 bags |
Source: Quikrete Product Guide (Fast-Setting Concrete bag chart)
Questions people ask
How many bags of concrete do I need per fence post?
For an 8 ft 4×4 in a 10½ inch hole, 32 inches deep and filled to 4 inches below the ground: 3.21 bags of 50 lb fast-setting mix, so 4 bags.
Source: Quikrete Fast-Setting Concrete Mix 1004 data sheet (2025); American Softwood Lumber Standard, PS 20-20 (NIST)
How big should a fence post hole be?
About 3 times the post's width. For a 4×4, which is 3½ inches actual, that is about 10½ inches.
Source: Quikrete Fast-Setting Concrete Mix 1004 data sheet (2025); Sakrete Fast Setting Concrete Mix data sheet
How deep should a fence post hole be?
A third of the post's overall length: 32 inches for an 8 ft post. Follow your local code for posts that carry a load.
Source: Quikrete Fast-Setting Concrete Mix 1004 data sheet (2025)
Do I need gravel under a fence post?
Sakrete sets posts on 4 to 6 inches of gravel. Quikrete's fast-setting method skips gravel and fills to 3 to 4 inches below the ground.
Source: Sakrete Fast Setting Concrete Mix data sheet; Quikrete Fast-Setting Concrete Mix 1004 data sheet (2025)