Key takeaways
- Concrete per post = the round hole's volume minus the post's volume.
- QUIKRETE suggests a hole about 3 times the post's diameter, and depth of 1/3 the post's overall height.
- Contact 811 a few business days before you dig.
Enter the hole size, the post's actual size, and how many posts you're setting. The default is a 4 × 4 post in a 10-inch hole, 32 inches deep.
How the calculator works
- Hole area: 3.14 × (hole diameter ÷ 2)²
- Post area: post width × post depth
- Concrete per post: (hole area − post area) × hole depth ÷ 1,728, in cubic feet
- Total: cubic feet per post × posts × (1 + allowance %)
- Bags: total ÷ yield per bag, rounded up
It assumes the post runs the full depth of the hole. If you use gravel or concrete under the post, your total changes, which is one reason we default to a 10% allowance.
Worked example
Ten 4 × 4 posts, each measuring 3.5 × 3.5 inches, in 10-inch holes 32 inches deep:
- Hole area: 3.14 × 25 = 78.5 square inches
- Post area: 3.5 × 3.5 = 12.25 square inches
- Per post: (78.5 − 12.25) × 32 ÷ 1,728 = about 1.23 cubic feet
- Ten posts with 10% extra: about 13.5 cubic feet
- 23 bags of 80 lb, 31 of 60 lb, or 46 of 40 lb
How big should the hole be?
QUIKRETE's Concrete Mix data sheet suggests:
- Width: about 3 times the post's diameter
- Depth: 1/3 of the post's overall height, so about 32 inches for an 8-foot post
- Finish: taper the top away from the post so rain runs off
- Wait: 24 hours before putting any strain on the post
Its method also places 6 inches of dry concrete mix in the bottom of the hole and uses mixed concrete around the post. For load-bearing uses, it says to follow local building codes for footing specifications. Frost depth and fence rules vary, so check with your building department.
Before you dig
811 is the free national before-you-dig service. Contact it or your state's 811 center a few business days ahead, so buried utilities can be marked. Installing a fence is one of the projects it names.
How hole size changes the concrete
For a 3.5 × 3.5 inch post in a 32-inch-deep hole, before any allowance:
| Hole diameter | Concrete per post | 80 lb bags per post |
|---|---|---|
| 8 in | 0.70 cu ft | 1.17 |
| 10 in | 1.23 cu ft | 2.05 |
| 12 in | 1.87 cu ft | 3.11 |
Digging 2 inches wider than planned adds about half a bag or more per post, and that multiplies across a long fence. Measure your actual holes before you buy for the whole run.
Frequently asked questions
How many bags of concrete per fence post?
For a 4 × 4 post in a 10-inch hole, 32 inches deep, about 1.23 cubic feet, or just over two 80 lb bags. The answer changes quickly with hole size, so run your own numbers.
How do I find my post's actual size?
Measure it. A post sold as 4 × 4 usually measures about 3.5 × 3.5 inches.
Does this work for round posts?
Close enough for planning. Enter a width and depth that give a similar area: multiply the post's diameter by about 0.89 and use that for both.
Sources
We use primary sources where we can: building codes, federal rules and manufacturer documents. Read our editorial standards.
- QUIKRETE: Concrete Mix data sheet, product 1101 (yields and method for setting fence posts)
- 811: What is 811?