Fence Post Concrete Calculator
Calculate exactly how many bags of concrete you need for fence posts. Enter hole diameter, depth, and post size to get instant results.
Post Hole Details
Estimation Results
Total Bags Needed for 1 Post(s):
How to Calculate Concrete for Fence Posts
Calculating the amount of concrete needed for setting a fence post involves basic geometry. You need to calculate the total volume of the cylindrical hole you dug, and then subtract the volume of the wooden or metal post that will be placed inside it.
Fast-setting concrete, which is standard for fence posts because it doesn't require mixing in a wheelbarrow, typically comes in 50lb bags. One 50lb bag of fast-setting concrete yields approximately 0.37 cubic feet. Because post holes flare out and are rarely perfect cylinders, adding a 10% to 15% waste factor is highly recommended.
Hole to Post Size Reference
| Post Size | Recommended Hole Diameter | Estimated 50lb Bags (at 3ft depth) |
|---|---|---|
| 4x4 Wood Post | 10 to 12 inches | 3.5 to 5.5 bags |
| 6x6 Wood Post | 16 to 18 inches | 8 to 11 bags |
| 2" Metal Chainlink Post | 6 inches | 1 to 1.5 bags |
Frequently Asked Questions
How many bags of concrete do I need for a 4x4 post?
For a 4x4 post set in a 12-inch diameter hole that is 3 feet deep, you will need approximately 4 to 5 bags of 50lb fast-setting concrete per hole.
How deep should a fence post hole be?
A standard rule of thumb is to bury at least 1/3 of the post's total length in the ground. For a 6-foot tall fence, dig a hole at least 3 feet deep (using a 9-foot post). You must also ensure the footing extends below the local frost line.
How wide should a fence post hole be?
The diameter of your post hole should be roughly three times the width of the post. For a standard 4x4 wooden post (which is actually 3.5 inches wide), use a 10 to 12-inch auger or hole size.
Should I put gravel under my fence post?
Yes, it is highly recommended to add 2 to 6 inches of crushed gravel at the bottom of the post hole before setting a wooden post. This ensures water drains away from the end-grain of the wood, preventing rot.