How Many Bags of Concrete per Fence Post? 4x4 and 6x6 Chart
Work out the volume of the hole, take away the post, and divide by what one bag makes. A 4x4 post in a 10 inch hole 2 1/2 ft deep takes about 1.27 cu ft of concrete: 3 bags of 80 lb mix or 4 bags of 50 lb fast-setting. The fence calculator adds it up for a whole fence.
Bags per post by hole size
Includes 10% extra for uneven hole walls. An 80 lb bag of concrete makes about 0.6 cu ft; a 50 lb bag of fast-setting mix makes about 0.375 cu ft.
| Post | Hole width | Hole depth | Concrete (cu ft) | 80 lb bags | 50 lb fast-setting |
|---|---|---|---|---|---|
| 4x4 | 8 in | 2.0 ft | 0.58 | 1 | 2 |
| 4x4 | 10 in | 2.0 ft | 1.01 | 2 | 3 |
| 4x4 | 10 in | 2.5 ft | 1.27 | 3 | 4 |
| 4x4 | 12 in | 3.0 ft | 2.31 | 4 | 7 |
| 6x6 | 12 in | 3.0 ft | 1.90 | 4 | 6 |
| 6x6 | 14 in | 3.0 ft | 2.83 | 5 | 8 |
| 6x6 | 16 in | 4.0 ft | 5.22 | 9 | 14 |
How big to dig the hole
- Width: about three times the post width. That's 10 to 12 inches for a 4x4 and 14 to 18 inches for a 6x6.
- Depth: one third of the post's height above ground, at least 2 ft. Go below the frost line where the ground freezes, or frost can heave the posts.
- Gravel: many builders put 4 to 6 inches of gravel in the bottom for drainage. Measure the depth to fill with concrete above it.
Bags for a whole fence
4x4 posts every 8 ft, 10 inch holes 2 1/2 ft deep, no gates. Each gate adds one post.
| Fence length | Posts | 80 lb bags | 50 lb fast-setting |
|---|---|---|---|
| 50 ft | 8 | 17 | 28 |
| 100 ft | 14 | 30 | 48 |
| 150 ft | 20 | 43 | 68 |
| 200 ft | 26 | 55 | 88 |
Fast-setting or regular concrete?
Fast-setting mix is poured dry into the hole around the post and soaked with water, and it sets in 20 to 40 minutes, so you can hang rails the same day. It costs more per cubic foot. Regular mix has to be mixed first and needs a day or two before you load the post, but it's cheaper and stronger, so it's the better buy for gate posts and long fences.
Either way, slope the top of the concrete away from the post so rain runs off, which slows rot at ground level.
Frequently asked questions
How many bags of concrete do I need for a 4x4 fence post?
For a 10 inch hole 2 1/2 ft deep, about 1.27 cu ft: 3 bags of 80 lb concrete or 4 bags of 50 lb fast-setting mix. A 2 ft deep hole takes 2 bags of 80 lb.
How many bags of concrete for a 6x6 post?
A 6x6 post in a 12 inch hole 3 ft deep takes about 1.90 cu ft of concrete: 4 bags of 80 lb or 6 bags of 50 lb fast-setting.
How many bags of concrete for a mailbox post?
A 4x4 mailbox post in a 10 inch hole 2 ft deep needs about 1.01 cu ft, so 2 bags of 80 lb or 3 bags of 50 lb fast-setting concrete.
How deep should a fence post hole be?
About one third of the post's height above ground, and at least 2 ft. A 6 ft privacy fence needs holes 2 to 3 ft deep, and below the frost line in cold climates.
Do you mix concrete for fence posts?
Fast-setting mix (like Quikrete Fast-Setting) goes in dry and you pour water on top, so no mixing. Regular concrete mix should be mixed in a wheelbarrow first; it is stronger and cheaper per cubic foot.