Post Hole Concrete Calculator

Calculate how many bags of concrete you need for fence posts and deck footings. Enter your post size, hole dimensions, and number of posts to get an accurate bag count.

iStandard thickness for foot traffic and light furniture. Default thickness: 4"
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Worked Example

A homeowner is setting 20 fence posts (4×4) in 10-inch diameter holes, 24 inches deep, using 80-lb bags of premixed concrete.

  1. 1 Post: 4×4 (actual 3.5″ × 3.5″)
  2. 2 Hole size: 10″ diameter × 24″ deep
  3. 3 Hole volume: π × 5² × 24 = 1,885 cu in
  4. 4 Post volume: 3.5 × 3.5 × 24 = 294 cu in
  5. 5 Concrete per hole: 1,885 − 294 = 1,591 cu in (0.92 cu ft)
  6. 6 Bags per hole (80-lb): 0.92 ÷ 0.6 = ~2 bags
  7. 7 Total for 20 posts: 20 × 2 = 40 bags
Order 40 bags of 80-lb concrete (2 bags per post × 20 posts) to set all fence posts.

How Post Hole Concrete Is Calculated

Hole volume: The hole is a cylinder, so volume = π × radius² × depth. Convert the diameter to radius (half the diameter) and keep all units in inches.

Post volume: The post displaces concrete inside the hole. For a square post (like 4×4 or 6×6), volume = side × side × depth. Use the actual lumber dimensions (a 4×4 is actually 3.5″ × 3.5″).

Concrete needed: Subtract the post volume from the hole volume to get the concrete required per hole. Divide by the bag yield (0.6 cu ft for 80-lb, 0.45 cu ft for 60-lb) to get the number of bags per post, then multiply by the total number of posts.

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Post Depth by Climate Region

Climate / Region Frost Line Min. Post Depth
Mild (Southeast US, coastal) 0–6″ 24″
Moderate (Mid-Atlantic, Pacific NW) 12–24″ 30–36″
Cold (Upper Midwest, Northeast) 36–48″ 36–48″
Very cold (Northern Plains, Canada) 48–72″ 48″+

Frequently Asked Questions

Can I use fast-setting concrete for fence posts? +
Yes. Fast-setting concrete (like Quikrete Fast-Setting) is ideal for fence posts because you can pour it dry into the hole, add water, and the post is set in 20-40 minutes. This eliminates mixing and lets you move to the next post quickly. It reaches full strength in about 4 hours.
Is it better to pour concrete dry or wet into a post hole? +
Both methods work. Dry pour (adding dry mix then water) is faster and easier for most fence posts. Wet pour (pre-mixing in a wheelbarrow) gives more control and is better for structural posts like deck footings or gate posts where you need to ensure no voids. Either way, the post should be plumbed and braced before the concrete sets.
How deep should post holes be below the frost line? +
Post footings should extend at least 6 inches below the local frost line to prevent heaving. Frost lines range from near zero in the deep South to 48 inches or more in northern states and Canada. Your local building department can tell you the exact frost depth for your area.
Is one bag of concrete enough per fence post? +
For a 4×4 post in a 10-inch hole at 24 inches deep, you need about 2 bags of 50-lb fast-setting concrete or 1.5 bags of 80-lb premix. One bag is rarely enough unless the hole is shallow or narrow. Always calculate rather than guess — running short mid-project means inconsistent footings.

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Disclaimer: This calculator provides estimates for planning purposes only. Actual material requirements depend on site conditions, compaction, grading, and local building codes. Always verify measurements on-site and consult with your material supplier before purchasing.
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