Fence Post Concrete Calculator: How Many Bags per Post?
Enter your posts, hole size and depth to get the bags of regular or fast-setting concrete mix for the whole fence, with the gravel under each post and the post itself taken out.
A 4x4 fence post for a 6 ft fence goes in a hole about 10 in wide and 30 in deep (24 in of post in the ground plus 6 in of gravel). It takes about 1.7 bags of 80-lb concrete mix or 2.7 bags of 50-lb fast-setting mix per post, with 10% waste; ten posts need 17 bags of 80-lb mix or 28 bags of fast-setting.
Fence Post Concrete
Changing the post or fence height fills in a suggested hole: about 3 × the post width, and 1/3 of the height buried plus 6 in of gravel. Edit them for your soil or frost depth.
How Deep and Wide to Dig
Two rules of thumb set the hole. Depth: bury 1/3 to 1/2 of the post's above-ground height, then add 6 in for a gravel base that lets water drain away from the end grain. In cold climates the bottom must also be below the frost line, often 36–48 in in northern states, or frost will heave the post. Width: about 3 times the post's width, so a 4x4 (3.5 in actual) gets a 10 in hole and a 6x6 (5.5 in) a 16 in hole.
| Fence and post | Hole | 80-lb bags / post | 50-lb fast-set / post | Regular mix cost / post |
|---|---|---|---|---|
| 4 ft fence, 4x4 | 24 in deep × 10 in wide | 1.3 | 2.0 | $8.23 |
| 5 ft fence, 4x4 | 24 in deep × 10 in wide | 1.3 | 2.0 | $8.23 |
| 6 ft fence, 4x4 | 30 in deep × 10 in wide | 1.7 | 2.7 | $10.97 |
| 6 ft fence, 6x6 | 30 in deep × 16 in wide | 4.3 | 7.0 | $28.27 |
| 8 ft fence, 4x4 | 36 in deep × 10 in wide | 2.1 | 3.4 | $13.71 |
| 8 ft fence, 6x6 | 36 in deep × 16 in wide | 5.4 | 8.7 | $35.34 |
Regular Mix or Fast-Setting?
Both make a solid post. The difference is time and price:
- Regular 80-lb mix costs about $10.83 per cubic foot ($6.50 a bag). Mix it in a wheelbarrow, fill the hole, then brace the post for a day or two before hanging boards.
- Fast-setting 50-lb mix costs about $22.67 per cubic foot ($8.50 a bag), roughly twice as much. Pour it into the hole dry, add water, and it sets in 20–40 minutes; you can hang rails and boards after about 4 hours.
For a long fence, regular mix saves real money; for a gate post you need to hang today, or a few posts on a weekend, fast-setting is worth it. Either way, crown the top of the concrete so rain runs away from the post.
Chain Link Posts
Chain link uses steel pipe. Line posts are the thinner pipe; corner, end and gate posts are thicker and take the wire tension, so they get the bigger hole.
| Post | Hole | 80-lb bags / post | 50-lb fast-set / post |
|---|---|---|---|
| Line post (2-3/8 in steel pipe (chain link line post)) | 8 in × 30 in | 1.2 | 1.9 |
| Corner / end / gate post (2-7/8 in steel pipe (chain link terminal post)) | 10 in × 30 in | 1.8 | 2.9 |
Setting a Post Step by Step
- Dig the hole to depth with a post-hole digger or auger, a little wider at the bottom than the top.
- Put in 6 in of gravel and tamp it, so water drains away from the post's end.
- Set the post, check it for plumb on two faces, and brace it with two stakes.
- Fill with concrete to just below ground level and slope the top away from the post.
- Let regular mix cure a day or two before you hang heavy gates or boards.
Building a bigger footing or a deck pier? Use the footing and pier calculator. Pouring a slab? Start with the bags calculator.
Frequently Asked Questions
For a 4x4 post on a 6 ft fence in a 10 in wide, 30 in deep hole: about 1.7 bags of 80-lb mix or 2.7 bags of 50-lb fast-setting mix. Taller fences and 6x6 posts need more; see the table.
About 17 bags of 80-lb mix or 28 bags of 50-lb fast-setting mix for ten 4x4 posts on a 6 ft fence, with 10% waste.
Bury a third to a half of the post's above-ground height and add 6 in for gravel: about 30 in for a 6 ft fence. In cold climates go below the frost line.
Fast-setting post mix is made to go in dry with water poured on top. Regular concrete mix should be mixed with water first; dry-packed regular mix hardens unevenly and ends up weaker.
They are the same concrete; only the bag size differs. One 80-lb bag makes 0.60 ft³ and a 60-lb bag 0.45 ft³, so you need about a third more 60-lb bags. Pick the size you can lift and mix comfortably.