Fence Post Hole Calculator
Calculate post hole diameter, depth, cylinder volume, and 60 lb or 80 lb concrete bags needed for sturdy fence footings.
Post hole dimensions & concrete
Enter your measurements and the result updates as you type.
Overview
A fence is only as permanent as the footing underneath it. Digging post holes too narrow or too shallow invites wind damage, frost heave displacement, and premature rotting.
This calculator uses cylinder geometry to determine exact hole volume, concrete requirements, and bag counts for 4x4 and 6x6 post installations.
Common mistakes
- Digging a funnel-shaped hole with a wide top and narrow bottom, which acts like a wedge that frost heave easily jacks upward.
- Digging post holes only 6 inches wide for a 4x4 post, leaving insufficient concrete collar around the wood.
- Throwing dry concrete powder into the hole without proper water mixing, creating brittle honeycomb footings.
- Omitting gravel underneath the post base, sealing the wood bottom in stagnant water.
Related calculators
- Concrete bags calculator Turn hole diameter, depth and the number of holes into a bag count for 50, 60 and 80 pound mixes, with the volume math shown.
- Fence post calculator Calculate line posts, corner posts and end posts from run length and spacing, with concrete for each post included in the take-off.
- Fence Post Length Calculator Find total post length based on fence height, local frost line depth, and drainage gravel base with recommended lumber sizes.
- Gravel calculator Compute gravel volume in cubic feet and yards, then tons by material type, for driveways, pad work and fence post bases.
Related reading
In-depth guides
Frequently asked questions
How wide should a fence post hole be?
The standard building rule is 3 times the width of the post. A 4x4 post (3.5 in wide) requires a 10 to 12-inch hole. A 6x6 post requires a 12 to 14-inch hole.
How many bags of concrete per post hole?
A standard 10-inch diameter, 24-inch deep hole requires 2.5 to 3 bags of 60 lb concrete mix (or 2 bags of 80 lb mix) per hole.
Why should post holes have straight or bell-shaped sides?
Straight or bell-bottomed holes resist frost heave. Inverted conical or V-shaped holes give freezing soil an angled surface to grab and push the post upward.
Can you use fast-setting concrete in post holes?
Yes. Fast-setting concrete sets in 20 to 40 minutes and allows you to pour dry mix around the post and add water directly without wheelbarrow mixing.
How deep should a post hole be for a 6-foot fence?
Dig at least 30 to 36 inches deep: 24 to 30 inches for the post footing plus 6 inches at the bottom for crushed drainage gravel.
What is the easiest way to dig multiple post holes?
For small projects, use a two-handled post hole digger and heavy digging bar. For 10 or more holes, rent a two-person gas or hydraulic power auger.
Should concrete come up to the ground surface?
Fill concrete to within 2 to 3 inches of the ground surface, slope the top collar away from the post to shed water, and cover with topsoil.
How long does concrete take to cure before attaching fence rails?
Wait at least 24 to 48 hours for standard concrete to achieve sufficient structural strength before hanging heavy rails and pickets.