Post Hole Concrete Calculator
Building a fence or deck? This tool calculates exactly how much concrete you need to set your posts. Enter hole dimensions, post size, and the number of posts for an accurate material list.
Sizing Post Holes Correctly
Depth matters more than diameter for post stability. The standard rule is one-third of the post height underground. For an 8-foot fence, that means 32 inches in the ground plus 6 inches of gravel, totaling a 38-inch hole.
Frost line drives depth in cold climates. Concrete heaves when moisture underneath freezes and expands. Dig below your local frost line to prevent posts from lifting every winter. Check building codes for the minimum depth in your area.
Diameter provides working room. A hole 3 times the post width lets you adjust the post for plumb before the concrete sets. Too narrow and you can't fit a level around the post. Too wide and you waste concrete without gaining strength.
Setting Posts Like a Pro
Start with 6 inches of gravel at the bottom. Tamp it down to create a stable base that drains water away from the post. Set the post on the gravel and brace it plumb in two directions before adding concrete.
Mix concrete on the stiff side, about 4-inch slump. Pour in lifts, tamping with a rod or stick to eliminate air pockets. Fill to within 2 inches of grade, then slope the top away from the post to shed water.
Don't backfill with soil around the concrete. Soil holds moisture against the post and accelerates rot. If you must backfill, use gravel and keep it an inch away from the post to allow airflow.
Fast-Set vs. Standard Concrete
Standard concrete takes 24 to 48 hours to gain enough strength to remove bracing. You'll spend time adjusting posts and checking them for plumb as the mix hardens. On large projects, bracing 20 or 30 posts ties up space and materials.
Fast-setting concrete reaches working strength in 20 to 40 minutes. You can set the post, plumb it, and walk away without braces. The tradeoff is cost: fast-set mixes run 30% to 50% more per bag than standard mixes.
For small projects, the convenience often justifies the extra cost. For large jobs, standard concrete saves money if you have the bracing materials and the time to manage the longer cure cycle.
Frequently Asked Questions
How deep should post holes be?
A general rule is one-third the post height plus 6 inches for gravel. For an 8-foot fence, dig 36 inches deep. Frost line and soil type may require deeper holes.
What diameter should the hole be?
Dig the hole 3 to 4 times the post width. For a 4×4 post, a 12-inch hole works well. Larger holes give you room to plumb the post and ensure good concrete coverage.
Do I need gravel at the bottom?
Yes. Add 6 inches of gravel for drainage. This prevents water from pooling at the post base, which causes rot in wood posts and freeze damage in concrete.
Can I use fast-setting concrete?
Fast-set mixes let you set posts in 20 to 40 minutes without bracing. They cost more per bag but save time on large projects. Standard mixes need 24 to 48 hours to cure before stressing the post.
Should I taper the hole at the top?
Yes. Slope the top of the concrete away from the post to shed water. This simple detail dramatically extends the life of wood posts by keeping moisture away from the grain.