Enter Your Post Hole Details
How the Concrete Calculator Works
This calculator uses the cylinder volume formula (π × r² × depth) to determine the cubic feet of concrete needed per post hole. It then divides the total volume by the coverage of your chosen bag size to give you an exact bag count. We round up to whole bags since you can't buy partial bags at the store.
Choosing the Right Hole Size
The general rule is to dig your post hole 3 times the width of the post and one-third the total post length deep. For a standard 4x4 post on a 6-foot privacy fence, that means a 10-12 inch diameter hole, 24-30 inches deep. Gate posts and corner posts benefit from wider holes (12 inches) for extra stability.
Bag Size Comparison
40 lb bags cover about 0.30 cubic feet each — best for light lifting and small jobs. 50 lb bags cover 0.375 cubic feet and are the most popular choice. 60 lb bags cover 0.45 cubic feet and offer a good balance. 80 lb bags cover 0.60 cubic feet — fewest bags to buy, but heaviest to carry. Quick-setting concrete is recommended for fence posts so you can continue building the same day.
Tips for Setting Fence Posts in Concrete
- Pour 4-6 inches of gravel in the bottom of each hole for drainage before adding concrete
- Use a level to keep posts plumb while the concrete sets
- Crown the concrete above ground level so water drains away from the post
- In frost-prone areas, dig below the frost line (36-48 inches) to prevent heaving
- Brace posts with 2x4s while concrete cures — quick-set takes 20-40 minutes, standard mix takes 24-48 hours
Pouring support piers instead of fence holes? Use our deck footing calculator to estimate deck footing concrete volume, footing count, and minimum diameter from the framing load.
Frequently Asked Questions
How many bags of concrete do I need per fence post?
Most fence posts need 1-2 bags of 50 lb concrete mix. A standard 4x4 post set in a 10-inch-wide, 24-inch-deep hole requires about 0.91 cubic feet of concrete, which is roughly 2-3 bags of 50 lb mix. Larger 6x6 posts or deeper holes will need more.
How deep should fence post holes be?
Fence post holes should be one-third the total post length. For a 6-foot privacy fence using 8-foot posts, dig holes 24-30 inches deep. In areas with frost, dig below the frost line (typically 36-48 inches) to prevent heaving.
How wide should a fence post hole be?
Post holes should be 3 times the width of the post. For a standard 4x4 post (3.5 inches actual), dig a hole 10-12 inches in diameter. This gives enough room for concrete to surround the post and provide solid support.
What size concrete bag should I use for fence posts?
50 lb bags are the most popular for fence posts — they're easy to carry and mix. Use 80 lb bags if you want fewer bags to open. 40 lb bags work well for small repairs or if you prefer lighter lifting. Quick-set concrete is ideal so you can continue building the same day.
Can I set fence posts without concrete?
Yes, in firm clay or rocky soil you can tamp gravel around posts instead of using concrete. However, concrete provides the strongest hold and is recommended for privacy fences, windy areas, and gate posts that take extra stress.