Post Hole Concrete Calculator
Work out exactly how many bags of concrete for fence posts, deck posts and mailbox posts - accounts for post displacement.
How many bags of concrete for a fence post? The standard answer is 1-2 bags of 60lb concrete per post, but the exact count depends on hole diameter, depth and the post size that displaces the concrete. This post hole concrete calculator factors in everything: number of posts, hole dimensions, post cross-section (4x4, 6x6 or round) and your chosen bag size. Use it for fence posts, deck posts, mailbox posts or any pier footing. The result includes a 10% waste factor and rounds bag counts up per post so you do not stop mid-job to buy more.
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Frequently asked questions
A standard 4x4 post hole (10 inches diameter, 30 inches deep) needs about 1.05 cubic feet of concrete after post displacement. That is 3 bags of 60lb concrete or 2 bags of 80lb concrete per post with 10% waste. For a 12-post fence, that is 36 bags of 60lb or 24 bags of 80lb.
Dig at least 1/3 of the post height into the ground - so a 6 ft above-ground post needs a 2 ft deep hole minimum. In cold climates, go below the frost line (24-48 inches depending on region) to prevent frost heave. Wider holes (3x post width) give better lateral stability than deep narrow ones.
Fast-setting concrete (Quikrete Fast-Setting, Sakrete Fast-Setting) is ideal for fence posts. Pour the dry mix into the hole, add water and it sets in 20-40 minutes. The 50lb fast-set bag yields about 0.375 cubic feet, slightly less than a standard 60lb bag. You can typically hang the fence the same day.
Yes - 4-6 inches of gravel at the bottom improves drainage and prevents the post from sitting in water, which is the #1 cause of post rot. The gravel base does not affect concrete volume because it is below the concrete itself. Some climates allow gravel-only setting without concrete, but concrete is more rigid.
The post takes up space inside the hole that does not need to be filled with concrete. A 4x4 post (3.5 inches actual width) in a 30-inch deep hole displaces about 0.21 cubic feet of concrete. For a 6x6 post in the same hole, the displacement is 0.53 cubic feet - significant savings on bag count for big jobs like deck footings.