This fence post concrete calculator tells you how many bags of concrete each fence post needs, and how many you need for the whole fence, from your hole size, post size and bag weight.
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For a 4×4 post in a 10 inch wide, 24 inch deep hole, about 1.45 bags of 80 lb mix (0.87 ft³), or 1.21 bags with 4 inches of gravel at the bottom. A wider, deeper hole or a 6×6 corner post takes 2 to 3 bags. Enter your own sizes below.
Concrete needed, including waste
14 bags of 80 lb
Inputs
Results are estimates for planning only. Supplier specifications, product yields, site conditions and local codes vary. Confirm quantities with your supplier or a qualified professional before you buy or build.
number of posts 10 pcs, hole diameter 10 in, hole depth 30 in, Square (4×4, 6×6), post width or diameter 4 in, gravel at the bottom 4 in, 80 lb.
Concrete needed, including waste
18 bags of 80 lb
number of posts 1 pcs, hole diameter 12 in, hole depth 36 in, Square (4×4, 6×6), post width or diameter 6 in, gravel at the bottom 0 in, 80 lb.
Concrete needed, including waste
3 bags of 80 lb
number of posts 20 pcs, hole diameter 12 in, hole depth 24 in, Square (4×4, 6×6), post width or diameter 4 in, gravel at the bottom 0 in, 80 lb.
Concrete needed, including waste
50 bags of 80 lb
number of posts 15 pcs, hole diameter 25 cm, hole depth 60 cm, Square (4×4, 6×6), post width or diameter 10 cm, gravel at the bottom 10 cm, 80 lb.
Concrete needed, including waste
19 bags of 80 lb
One post, no gravel base, no waste. Square posts. Yields 0.375 ft³ (50 lb), 0.45 ft³ (60 lb) and 0.60 ft³ (80 lb).
| Hole and post | Concrete | 80 lb bags | 60 lb bags | 50 lb bags |
|---|---|---|---|---|
| 8 in × 24 in, 4×4 post | 0.48 ft³ | 0.79 | 1.06 | 1.27 |
| 10 in × 24 in, 4×4 post | 0.87 ft³ | 1.45 | 1.93 | 2.32 |
| 10 in × 30 in, 4×4 post | 1.09 ft³ | 1.81 | 2.41 | 2.90 |
| 12 in × 24 in, 4×4 post | 1.35 ft³ | 2.25 | 3.00 | 3.60 |
| 12 in × 30 in, 4×4 post | 1.69 ft³ | 2.81 | 3.75 | 4.50 |
| 12 in × 36 in, 6×6 post | 1.61 ft³ | 2.68 | 3.57 | 4.28 |
| 14 in × 42 in, 6×6 post | 2.87 ft³ | 4.78 | 6.37 | 7.64 |
4×4 posts in 10 × 30 inch holes with 4 inches of gravel, 10% waste, 80 lb bags.
| Posts | Concrete | 80 lb bags | 50 lb bags |
|---|---|---|---|
| 5 posts | 5.2 ft³ | 9 | 14 |
| 10 posts | 10.4 ft³ | 18 | 28 |
| 15 posts | 15.5 ft³ | 26 | 42 |
| 20 posts | 20.7 ft³ | 35 | 56 |
| 30 posts | 31.1 ft³ | 52 | 83 |
A post hole is a cylinder with the post sitting in the middle, so the concrete you need is the hole's volume minus the post's volume, above any gravel at the bottom. For a 10 inch hole 24 inches deep around a 4×4 post, that is about 0.87 cubic feet, or roughly 1.5 bags of 80 lb mix. Common advice ranges from one to four bags per post because hole size, depth and bag size all vary.
Sources disagree mostly because they assume different holes. A wider or deeper hole takes more concrete, a gravel base takes some of the depth, and a 50 lb bag yields less than an 80 lb bag. Enter your own sizes here rather than relying on a rule of thumb. Then plan the rest of the fence with the fence calculator, and size larger pours such as footings with the concrete calculator.
Dig the hole about three times the width of the post, so a 4×4 sits in a hole around 10 to 12 inches across. For depth, a common rule is a third to a half of the post's length, and below the frost line where the ground freezes. Line posts, corner and end posts, and gate posts carry different loads. Corner, end, 6×6 and gate posts usually get bigger and deeper holes, which is why they need more concrete.
Put a few inches of gravel in the bottom for drainage, set the post plumb and brace it, then add concrete. Slope the top of the concrete away from the post so water runs off. Let it set as the bag directs before you hang the fence.
For a 4×4 post in a 10 inch hole 24 inches deep, about 1.5 bags of 80 lb mix (0.87 cubic feet), or about 1.2 bags if you put 4 inches of gravel at the bottom. A 12 inch hole 24 inches deep takes about 2.3 bags. Most DIY sources say one to four bags depending on the hole.
A 50 lb bag yields about 0.375 cubic feet. A 10 × 24 inch hole around a 4×4 post needs 0.87 cubic feet, which is about 2.3 bags, so plan on three per post. In a 10 × 30 inch hole it is about 2.9 bags.
An 80 lb bag yields about 0.60 cubic feet. That is about 1.5 bags for a 10 × 24 inch hole and 1.8 for a 10 × 30 inch hole around a 4×4 post. For a 6×6 post in a 12 × 36 inch hole it is about 2.7 bags.
Fast-setting post mix is popular because you pour it dry into the hole and add water on top, with no mixing. Regular mix works too but needs mixing and takes longer to set. Either is fine if you follow the bag instructions.
Fast-setting post mix or standard concrete mix. Both are sold in 50 lb and 80 lb bags. If you mix your own from cement, sand and gravel, a common ratio is about 1 part cement, 2 parts sand and 3 parts gravel by volume.
It depends on fence height and the local frost line. A common rule is to set the post a third to a half of its length into the ground, so a 6 ft fence on 8 ft posts is often 2 to 3 feet deep. Check your local code and frost depth.
Many guides recommend 3 to 6 inches of gravel for drainage, which helps wood posts last longer. Some builders skip it. The calculator lets you enter any depth, including zero.
With a 10 × 30 inch hole, 4 inches of gravel, a 4×4 post and 10% waste, about 18 bags of 80 lb mix, or 28 bags of 50 lb.
About 2.7 bags of 80 lb mix in a 12 × 36 inch hole, which is 1.6 cubic feet before waste. Corner and gate posts are often 6×6 and set deeper.
Background on the materials and units used in this calculator.
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