Quick answer
Dig a hole about 3 times the width of the post and about one-third of the post's total length deep, plus 6 inches for gravel. For a 4x4 post on a 6 ft fence, that is a hole about 10 to 12 inches wide and roughly 30 to 36 inches deep (deeper where frost goes deeper). Add 6 inches of gravel, set the post plumb, and fill with concrete to just below ground level. Fast-setting mix sets in about 20 to 40 minutes; wait at least 4 hours before hanging rails, and about a day or more with regular concrete mix.
Hole size rules
Width
A common rule from concrete makers is a hole about 3 times the post's width. A 4x4 post actually measures 3 1/2 inches, so the hole should be about 10 to 12 inches across. A 6x6 (5 1/2 inches actual) needs about 16 to 18 inches. In loose or sandy soil, go wider at the bottom.
Depth
Bury about one-third of the post's length, then add 6 inches for the gravel layer.
| Fence height | Typical post length | Post in ground | Hole depth with gravel |
|---|---|---|---|
| 4 ft | 6 ft | about 2 ft | about 30 in |
| 6 ft | 8 ft | about 2 1/2 to 3 ft | about 36 to 42 in |
| 8 ft | 10 to 12 ft | about 3 to 4 ft | about 42 to 54 in |
In cold climates, the bottom of the post should sit below the local frost line so freezing soil does not heave it upward. Frost depths range from a few inches in the Deep South to 4 ft or more in the northern states; your local building department can tell you yours. Gate posts carry more load, so many builders dig them wider and deeper than line posts.
Dry-pour vs mixed concrete
| Fast-setting, poured dry | Regular concrete, mixed | |
|---|---|---|
| Typical bag | 50 lb, about 0.375 cu ft | 80 lb, about 0.60 cu ft |
| Method | Pour dry mix into hole, add about 1 gallon of water per bag | Mix in a wheelbarrow or tub to a thick oatmeal consistency, then shovel in |
| Sets | About 20 to 40 minutes | Several hours |
| Before loading | About 4 hours for heavy loads | About 24 to 48 hours; longer in cool weather |
| Best for | Fence line posts, mailbox posts, quick jobs | Gate posts, large holes, deck footings, strength-critical posts |
Pouring dry works because fast-setting mix is designed to pull in water. For holes deeper than about 2 1/2 ft, add the mix in stages: pour part, add water, let it soak in, then repeat. Regular concrete mix should not be dumped dry and watered in the hole; it needs to be fully mixed to reach its rated strength.
How many bags per post
Concrete volume = hole volume - post volume - gravel layer. For a 10 inch round hole with 24 inches of concrete around a 4x4:
- Hole: 3.14 x 5 x 5 x 24 = about 1,885 cubic inches
- Post: 3.5 x 3.5 x 24 = 294 cubic inches
- Concrete: about 1,590 cubic inches, or 0.92 cu ft
That is about 2 1/2 bags of 50 lb fast-setting mix (buy 3) or about 1 1/2 bags of 80 lb regular mix (buy 2). The fence post concrete calculator runs this for any hole size and number of posts.
Step by step
- Call 811 to have utilities marked before digging.
- Mark post locations with a string line between corner posts. Line posts are usually 6 to 8 ft apart; match your panel or rail length.
- Dig the holes with a post-hole digger or auger. Keep the sides straight and the bottom slightly wider than the top if possible.
- Add gravel. Put about 6 inches of gravel or crushed stone in the bottom and tamp it. This drains water away from the end of the post.
- Set the post on the gravel. Check plumb on two adjacent faces with a level and brace it with two stakes and scrap boards.
- Add concrete to about 3 to 4 inches below ground level (fast-setting) or near grade (regular mix).
- Shape the top. Slope the concrete surface away from the post so water runs off. You can cover the top later with soil or gravel.
- Recheck plumb in the first few minutes, before the concrete sets.
- Wait before attaching rails or panels: at least 4 hours for fast-setting, about 1 to 2 days for regular mix.
Tips for posts that last
- Use wood rated for ground contact. Ordinary treated lumber rated only for above-ground use rots faster in the hole.
- Do not pour a concrete pad under the post. A closed bottom traps water against the end grain.
- Set corner and end posts first, then pull a string between them to align line posts.
- In hot weather, fast-setting mix sets faster, so have everything ready before adding water.
Pouring a small slab for a gate landing or trash can pad too? See the concrete calculator.