How to calculate concrete for fence posts
The concrete for one post is the volume of the round hole minus the volume of the post. Hole = 3.14 x (hole width / 2) x (hole width / 2) x depth. Post = post width x post width x depth. All three are in inches, so divide by 1,728 (12 x 12 x 12) to get cubic feet. [1]
The post is square, so use the width it is dressed to, not its name. A nominal 4 inch width is dressed to at least 3.5 inches when dry, and a nominal 6 inch width to at least 5.5 inches. [2]
Bags per post = the concrete for one post, with any extra you add, divided by the yield of one bag, rounded up. QUIKRETE says an 80 lb bag yields approximately 0.60 cubic feet, and Sakrete's page for its 80 lb mix says .6 cubic feet. [4] [5]
Worked example
A 3.5 in post in a hole 10.5 in wide and 24 in deep. The hole is 3.14 x (10.5 / 2) x (10.5 / 2) x 24 / 1,728 = 1.20 cubic feet. The post takes 3.5 x 3.5 x 24 / 1,728 = 0.17 cubic feet. The concrete for one post is 1.03 cubic feet. One 80 lb bag yields 0.60 cubic feet, so 1.03 / 0.60 = 1.72, which rounds up to 2 bags per post, and 20 bags for 10 posts.
How much concrete for a fence post?
It depends on the hole and the post. For the example above the answer is 1.03 cubic feet per post. A wider or deeper hole takes more, and a wider post takes less: the post table below shows four combinations.
How many bags of concrete per fence post?
Each post rounds up to whole bags, and the calculator multiplies by the number of posts. If you share bags between posts and use the leftovers in the next hole, the whole fence can need fewer. The calculator shows both counts: for the example, 20 bags rounded up for each post, and 18 bags if the posts share.
How much concrete for a 4x4 or 6x6 post?
QUIKRETE's method for setting fence posts says to dig the hole about 3 times the diameter of the post, and that the hole depth should be 1/3 the overall post height. [4] The table applies those two rules to a 4x4 and a 6x6 post at two overall heights. The heights are examples to show the sum, not recommendations.
| Post | Overall height (ft) | Hole width (in) | Hole depth (in) | Concrete (ft³) | 80 lb bags |
|---|---|---|---|---|---|
| 4x4, 3.5 in wide | 6 | 10.5 | 24 | 1.03 | 2 |
| 4x4, 3.5 in wide | 8 | 10.5 | 32 | 1.38 | 3 |
| 6x6, 5.5 in wide | 6 | 16.5 | 24 | 2.55 | 5 |
| 6x6, 5.5 in wide | 8 | 16.5 | 32 | 3.40 | 6 |
Should fence posts be set in concrete?
QUIKRETE gives a method for setting fence posts in its Concrete Mix. [4] A City of Mansfield, Texas standard for wood privacy fences calls for posts set in concrete post footings. [3]
We found no independent guidance we could read that compares concrete with other ways of setting a post. Ask your fence maker or your local building authority which applies to your fence.
How much does concrete for fence posts cost?
We state no price. Ask local suppliers what one bag costs, then enter it under Your bag, extra and price (optional) to see the total for all your posts. The site does not store the prices you type.
Common mistakes and limits
- Using the nominal post width. A nominal 4 inch width is dressed to at least 3 1/2 inches when dry. [2] Use the dressed width.
- Forgetting the post takes up room. The concrete fills the hole around the post, so the calculator takes the post's volume out of the hole's.
- Rounding the total instead of each post. Each post needs whole bags. The calculator rounds each post up, and also shows the count if the posts share bags.
- Ignoring local rules. The City of Mansfield standard gives 24 inches for a fence six feet high and 36 inches for a fence eight feet high. That is one city's rule for one kind of fence. [3]
- What this tool does not cover: bracing, gravel under the post, steel T-posts, the strength of the concrete and local rules.
Frequently asked questions
What concrete mix should I use for fence posts?
QUIKRETE lists pipe and post footings among the uses of its Concrete Mix 1101, and gives a typical 28-day strength of 4000 psi. Check the label of the mix you buy, and follow local building codes for load-bearing footings. [4]
How deep should a fence post hole be?
QUIKRETE's method says the hole depth should be 1/3 the overall post height. One city's standard, in Mansfield, Texas, gives 24 inches for a 6 ft wood privacy fence and 36 inches for an 8 ft fence. Your local frost depth and rules apply. [4][3]
How wide should the hole be?
QUIKRETE's method says to dig the post hole about 3 times the diameter of the post. For a post 3 1/2 inches wide that is 10 1/2 inches. [4]
Do I put dry mix in the bottom of the hole?
QUIKRETE's method says to place 6 inches of dry concrete mix in the bottom of the hole, set the post level and plumb, then mix the concrete with water before it goes into the hole. [4]
How long before I can hang the fence?
QUIKRETE says to wait 24 hours before the post is subjected to any strain. Your own mix may say something else. [4]
Sources and references
- National Institute of Standards and Technology (NIST). NIST Guide to the SI, Appendix B.8: Factors for Units Listed Alphabetically (SP 811, accessed November 2026). Used for: the inch and foot relationship (12 inches in a foot).
- National Institute of Standards and Technology (NIST). Voluntary Product Standard PS 20-25, American Softwood Lumber Standard (PS 20-25, accessed November 2026). Used for: the dressed width of a nominal 4 inch and 6 inch post.
- City of Mansfield, Texas. New or Replacement Wood Privacy Fence Construction Standards (dated 2/11/2020, accessed November 2026). Used for: one city's footing depth and concrete footings for a wood privacy fence.
- The QUIKRETE Companies. Data Sheet, Concrete Mix, Product No. 1101-40, -50, -60, -80, -90 (revised October 2022, accessed November 2026). Used for: the yield of each bag size and the maker's method for setting a fence post.
- Oldcastle APG (Sakrete). Sakrete High-Strength Concrete Mix 80 lb Gray (accessed November 2026). Used for: a second maker's yield for an 80 lb bag.
How we use sources: methodology.
Estimates only. Confirm quantities with your supplier. See the disclaimer. Found a wrong result? Report an error.