How Much Concrete Do You Need for a Fence Post?
A 4x4 fence post in a 12-inch hole 30 inches deep, with a 4-inch gravel base, takes about 1.52 cubic feet of concrete, or 2.53 bags of 80-lb mix. Ten posts with 10 percent extra need 28 bags, so most people buy three bags per post and use the rest elsewhere.
Hole size comes first
The amount of concrete depends on how big you dig the hole, so decide that first. Bob Vila's fence post guide gives two widely repeated rules: the hole diameter should be about three times the width of the post, and the depth should be about one-third to one-half of the post's height. By that guidance a 4-inch post needs a 12-inch hole, and a 6-foot post needs a hole at least 2 to 3 feet deep. The same article calls for a layer of gravel in the bottom of each hole for drainage.
Those are rules of thumb, not code. Frost depth, soil, fence height and wind load can all call for deeper holes, so check your local requirements and the fence manufacturer's instructions. Call 811 a few days before you dig so buried utilities are marked.
The formula
A post hole is a cylinder, and the post occupies part of it. Subtract the post from the hole and you have the concrete.
concrete = hole − post width² × concrete depth
80-lb bags = concrete (ft³) ÷ 0.60
Concrete depth is the hole depth minus any gravel base, since the gravel is not filled with concrete. Work in inches and divide by 1,728 to get cubic feet. The post hole concrete calculator does all of this and also converts to 60 and 50-lb bags.
Bags per post by hole and post size
All rows below assume a 4-inch gravel base, so the concrete depth is 4 inches less than the hole depth.
| Post | Hole | Concrete per post | 80-lb bags per post |
|---|---|---|---|
| 4×4 | 10 in × 30 in | 1.00 ft³ | 1.66 |
| 4×4 | 12 in × 30 in | 1.52 ft³ | 2.53 |
| 4×4 | 12 in × 36 in | 1.87 ft³ | 3.11 |
| 6×6 | 16 in × 36 in | 3.16 ft³ | 5.27 |
Notice how fast bags climb with hole size. Going from a 10-inch to a 12-inch hole at the same depth raises the concrete per post by about half, which is why digging an oversized hole by habit gets expensive across a long fence.
Worked example: ten 4×4 posts
You are setting ten 4×4 posts in 12-inch holes, 30 inches deep, each on 4 inches of gravel. Each post takes 1.52 cubic feet of concrete, or 2.53 bags. Ten posts is 15.2 cubic feet; adding 10 percent for spills and uneven holes brings that to 16.7 cubic feet, which is 28 bags of 80-lb mix after rounding up. Plan for three bags per post if you want a cushion.
For long runs the math changes. Once the total approaches a cubic yard (45 bags of 80-lb mix), a delivered truck can be cheaper and faster than hauling and mixing bags. The cubic yard calculator converts your cubic feet so you can compare. For the fence materials themselves, use the fence calculator.
Bags, pour-in-dry mix or ready-mix?
An 80-lb bag yields about 0.60 cubic feet, a 60-lb bag about 0.45 and a 50-lb bag about 0.375, so the same hole takes more of the smaller bags. For a handful of posts, bagged mix is the practical choice because there is no minimum order and no delivery. Pour-in-dry fast-setting products save mixing time but still need the water quantity on the bag. Ready-mix only pays off when you are setting dozens of posts or several yards in total, and the truck needs access close to the fence line.
Whatever you choose, shape the top of the concrete so it slopes away from the post. Water that pools around the base is what rots wood posts early, and a small crown at the surface costs nothing.
Common mistakes
- Forgetting to subtract the post. A 4×4 displaces roughly a tenth of the concrete in a 12-inch hole, so ignoring it overbuys slightly.
- Counting the gravel as concrete. The drainage layer at the bottom is stone, not mix.
- Digging too narrow. A hole barely wider than the post leaves no room to brace and fill around it.
- Burying too shallow. Skimping on depth to save concrete costs you a leaning fence. Check frost depth locally.
- Using the same count for every post. Gate posts and corner posts are often larger and deeper, so calculate them separately.
What else to order or consider
Besides concrete, plan on drainage gravel for each hole, braces or stakes to hold posts plumb while the mix sets, and water if you are using a pour-in-dry fast-setting product. Follow the bag directions for water and cure times, and wait before hanging heavy gates or panels. Posts set in unusually soft or wet soil, or tall privacy fences that catch wind, may need deeper or wider holes than the rules above suggest; ask your fence supplier or local building department.
Calculators for this guide
Frequently asked questions
How many bags of concrete for one fence post?
A 4x4 post in a 12-inch hole 30 inches deep, with 4 inches of gravel at the bottom, takes about 2.53 bags of 80-lb mix. Most people plan on three bags per post.
How big should a fence post hole be?
The diameter should be about three times the post width, so 12 inches for a 4x4. The depth should be about one-third to one-half of the post height, and deeper where your frost line or local rules require it.
How much concrete do 10 fence posts need?
Ten 4x4 posts in 12-inch holes 30 inches deep need about 28 bags of 80-lb concrete, including 10 percent extra.
Sources and further reading
- How Much Concrete Per Fence Post? — Bob Vila
- Call 811 before you dig — Common Ground Alliance
More guides
Estimates only. Always confirm quantities with your supplier or a qualified contractor before ordering, and add extra for waste and site conditions.