How big should the hole be?
QUIKRETE's post-setting instructions give two rules of thumb:
- Width: Make the hole about three times as wide as the post.
- Depth: Dig the hole 1/3 to 1/2 as deep as the post stands above ground, plus 6 in for a gravel base.
- Base: Put about 6 in of gravel in the bottom of the hole, and fill concrete to 3 to 4 in below ground level.
Enter your post's height above ground and the calculator checks the hole depth for you. Use the post's actual size, not its name: a 4×4 measures 3½ in and a 6×6 measures 5½ in.
How the concrete per hole is calculated
The concrete fills the hole above the gravel, minus the space the post takes up. So the calculation is (hole area − post area) × (hole depth − gravel) × number of posts. For one 4×4 in a 12 in hole, 30 in deep, with 6 in of gravel:
Fence posts
- Hole diameter: 12 in ÷ 12 = 1 ft
- Hole depth: 30 in ÷ 12 = 2.5 ft
- Gravel base: 6 in ÷ 12 = 0.5 ft
- Post width or diameter: 3.5 in ÷ 12 = 0.2917 ft
- Number of posts: 1
- Post shape: square
- Hole area: π × (1 ft ÷ 2)² = 0.7854 ft²
- Square post area: 0.2917 ft × 0.2917 ft = 0.0851 ft²
- Concrete area around the post: 0.7854 ft² − 0.0851 ft² = 0.7003 ft²
- Concrete depth: 2.5 ft − 0.5 ft gravel = 2 ft
- One hole: 0.7003 ft² × 2 ft = 1.4007 ft³
- All holes: 1.4007 ft³ × 1 = 1.4007 ft³
Total and order
- In cubic yards: 1.4007 ft³ ÷ 27 = 0.0519 yd³
- With 10% extra: 0.0519 yd³ × 1.1 = 0.0571 yd³
- Order, rounded up to the next 0.25 yd³: 0.25 yd³
That's 1.4 ft³: 5 50-lb bags or 3 80-lb bags per post with 10% extra. A 12-post fence needs 50 50-lb bags or 31 80-lb bags.
Bags per post hole
Per hole, with 6 in of gravel at the bottom and 10% extra. Bag yields from the QUIKRETE data sheet (0.375 ft³ per 50-lb bag, 0.60 ft³ per 80-lb bag).
| Post | Hole | Depth | ft³ concrete | 50-lb bags | 80-lb bags |
|---|---|---|---|---|---|
| 4×4 (3½ in) | 10 in | 24 in | 0.69 | 3 | 2 |
| 4×4 (3½ in) | 12 in | 30 in | 1.4 | 5 | 3 |
| 4×4 (3½ in) | 12 in | 36 in | 1.75 | 6 | 4 |
| 6×6 (5½ in) | 16 in | 30 in | 2.37 | 7 | 5 |
| 6×6 (5½ in) | 18 in | 36 in | 3.89 | 12 | 8 |
| 6×6 (5½ in) | 18 in | 42 in | 4.67 | 14 | 9 |
Fence post tips
- Round posts: switch the post shape to round for metal posts, and enter the outside diameter.
- Leave the top of the hole open. QUIKRETE suggests stopping the concrete 3 to 4 in below ground level, so you can backfill with soil.
- Holes dug by hand are rarely exact. Measure a couple of finished holes before you buy bags for the whole fence.
- Different post sizes? Add a part for the corner and gate posts if their holes are bigger than the line posts.
Questions people ask
How many bags of concrete per fence post?
For a 4×4 post (3½ in actual) in a 12 in hole 30 in deep with 6 in of gravel at the bottom, you need 1.4 ft³ of concrete. With 10% extra that's 5 50-lb bags or 3 80-lb bags per post.
How deep should a fence post hole be?
QUIKRETE recommends a hole 1/3 to 1/2 as deep as the post stands above ground, plus 6 in for a gravel base. A post 6 ft above ground needs a hole at least 2 ft 6 in deep.
How wide should a fence post hole be?
About three times the width of the post, according to QUIKRETE. A 4×4 (3½ in actual) needs a hole about 10½ in wide, so 10 to 12 in is common.
Should I subtract the post from the concrete?
Yes. The post takes up space in the hole. In the example above, the post saves 0.17 ft³ per hole, which adds up over a long fence line.
How many bags for 10 fence posts?
With the same 12 in × 30 in holes and 4×4 posts, 10 posts need 42 50-lb bags or 26 80-lb bags, including 10% extra.
Related calculators
Sources
- QUIKRETE, Setting Posts in Concrete, retrieved October 2, 2026
- Archtoolbox, Lumber Dimensions: Nominal vs Actual (based on the American Softwood Lumber Standard, PS 20), retrieved October 2, 2026
- QUIKRETE Concrete Mix No. 1101 product data sheet, retrieved October 2, 2026
- SAKRETE High-Strength Concrete Mix technical data sheet, retrieved October 2, 2026
- NRMCA, Concrete in Practice 36: Structural Lightweight Concrete, retrieved October 2, 2026
Last updated October 2, 2026. Read how we calculate, including formulas, waste defaults and rounding.