Free Post Hole Calculator — Bags & Concrete (2026)

How to Use This Calculator

1. 1Enter the hole diameter in inches — 8 inches is common for 4x4 posts, 10 to 12 inches for 6x6 posts.
2. 2Enter the hole depth in inches — most fence posts need 24 to 36 inches, deck and mailbox posts often need 36 to 48 inches.
3. 3Enter the total number of holes you need to fill with concrete.
4. 4Click Calculate to get cubic feet per hole, total concrete volume, and the number of 50 lb and 80 lb bags needed.

About This Material

Post holes are cylindrical excavations filled with concrete to anchor vertical posts for fences, mailboxes, deck supports, signs, pergolas, and other outdoor structures. The concrete encases the post base and transfers loads into the surrounding soil, preventing the post from shifting, leaning, or heaving due to wind, frost, or lateral force. For fence posts, the standard rule of thumb is to bury one-third of the total post length underground. A 6-foot fence using 8-foot posts should have 24 to 30 inches of post buried in concrete. For 4x4 fence posts, an 8-inch diameter hole is sufficient. For 6x6 posts or gate posts that bear heavier lateral loads, use a 10 to 12-inch diameter hole to provide more concrete mass and bearing area. Mailbox posts installed to USPS standards typically require a 24-inch deep hole with an 8 to 10-inch diameter. Deck support posts and pergola posts need deeper holes — 36 to 48 inches depending on the frost line in your region. Posts supporting structural loads should always extend below the local frost line to prevent frost heave from lifting the structure. Quikrete fast-setting concrete mix is the most popular product for post holes because it can be poured dry into the hole around the post and then saturated with water — no mixing required. A 50 lb bag of Quikrete fills approximately 0.375 cubic feet, while an 80 lb bag fills approximately 0.6 cubic feet. For large projects with many holes, 80 lb bags are more cost-effective at roughly $6 to $7 per bag versus $4 to $5 for 50 lb bags. Before pouring concrete, add 4 to 6 inches of gravel at the bottom of each hole for drainage. This prevents water from pooling at the post base and accelerating rot in wood posts. Crown the concrete slightly above grade so water runs away from the post rather than pooling against it.

Installation Tips

• •Dig holes 3 to 4 times the width of the post — an 8-inch hole for a 4x4 post, 10 to 12 inches for a 6x6 post.
• •Add 4 to 6 inches of gravel at the bottom of each hole for drainage before setting the post.
• •Plumb the post on two adjacent sides with a level and brace it with 2x4 stakes before pouring concrete.
• •Crown the concrete 1 to 2 inches above grade, sloping away from the post to shed water.
• •Allow the concrete to cure for at least 24 to 48 hours before attaching fence rails, brackets, or loads to the post.

Common Mistakes to Avoid

• ⚠Setting posts too shallow — fence posts should be buried at least one-third of their total length; structural posts must reach below the frost line.
• ⚠Skipping the gravel base — without drainage at the bottom, water pools around the post base and accelerates wood rot.
• ⚠Not bracing posts plumb before the concrete sets — once cured, correcting a leaning post requires breaking out the concrete and starting over.
• ⚠Pouring concrete below grade level — the top of the concrete should crown above ground to prevent water from pooling against the post.
• ⚠Using too small a hole diameter — insufficient concrete around the post cannot resist lateral forces from wind or gate operation.

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