Post Hole Concrete Calculator
✓ Your estimate
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Cu Ft / Hole
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Total Cu Ft
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Total Cu Yards
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Bags
⚠ Hole depth requirements vary by frost line and local code. Check requirements in your area before digging. Confirm bag yields with your supplier.
What the Result Means
This calculator treats each post hole as a cylinder and computes the concrete volume needed to fill it. For a fence project, you also need to estimate the posts, panels or pickets, and rails -- the fence material calculator covers those quantities. For a general understanding of concrete bag yields, see the concrete slab calculator.
Fast-setting concrete (Quikrete or similar) is popular for posts because you pour the dry mix directly into the hole and add water on top -- no mixer needed. Yields are slightly lower per bag than standard concrete, so confirm the label before purchasing.
How the Calculation Works
Radius = Diameter / 2 (in inches)
Volume per hole = pi x Radius² x Depth (cubic inches)
Cubic feet per hole = Volume in cubic inches / 1,728
Total cubic feet = Cu ft per hole x Number of holes
Bags = Total cu ft (with waste) / Yield per bag
80 lb = 0.60 cu ft | 60 lb = 0.45 cu ft | 40 lb = 0.30 cu ft | 50 lb fast-set = 0.375 cu ft
Worked Example
Example: 10-inch diameter holes, 24 inches deep, 10 posts
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Radius: 10 / 2 = 5 inches
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Volume per hole: 3.1416 x 5² x 24 = 1,885 cubic inches
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Cu ft per hole: 1,885 / 1,728 = 1.09 cubic feet
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Total: 1.09 x 10 = 10.9 cubic feet
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80 lb bags (0.60 cu ft each) with 10% waste: 10.9 x 1.10 / 0.60 = 20 bags
Ordering Notes
Assumptions Used by This Calculator
- Hole diameter and depth are entered in inches.
- Holes are treated as perfect cylinders -- actual hand-dug holes vary.
- Volume is calculated using the cylinder formula: pi x radius squared x depth.
- Cubic inches are divided by 1,728 to convert to cubic feet.
- Bag yields are approximate manufacturer averages and vary by mix and water amount.
- Frost depth, soil conditions, and local building code may require deeper holes than estimated.
- This calculator does not account for the post volume displacing concrete inside the hole.
Frequently Asked Questions
How much concrete do I need for fence posts?
A typical 6-inch diameter hole at 24 inches deep holds about 0.33 cubic feet of concrete -- roughly 1 bag of 40 lb or half a bag of 80 lb. For 10 posts at that size, you need about 3.3 cubic feet total, or 6 bags of 80 lb.
How deep should a post hole be?
A general rule is one-third of the post length should be in the ground, with a minimum of 24 inches below grade. In cold climates, holes must extend below the local frost depth -- check your local building code. Frost-prone areas typically need 36-48 inches.
How wide should a post hole be?
Post holes should be 3 times the width of the post. For a 4x4 post (3.5 inches actual), dig a 10-12 inch hole. For a 6x6 post (5.5 inches actual), dig a 16-18 inch hole. A wider base adds stability in soft soil.
How many bags of concrete per post?
For a standard 6-inch diameter x 24-inch deep hole, about 1 bag of 80 lb concrete fills the hole. For larger or deeper holes, use the calculator above. Always check manufacturer yield on the bag label.
Should fence posts be set in concrete?
Yes for most applications. Concrete holds posts firmly and prevents movement. For temporary fences, gravel-tamped posts are an option. Wood posts set in concrete can rot at the concrete line over time -- use pressure-treated lumber rated for ground contact.
How much extra concrete should I buy?
Add 10% to account for variation in hole diameter -- hand-dug holes are rarely perfectly cylindrical. For machine-bored holes, 5% extra is usually enough.
Can I use fast-setting concrete for posts?
Yes -- Quikrete Fast-Setting Concrete and similar products are designed for posts. Pour dry mix directly into the hole, add water on top, and it sets in 20-40 minutes. No mixing required. Yields are slightly lower per bag than standard concrete -- check the bag label.
Disclaimer: This calculator provides estimates only. Hole depth requirements vary by frost line, soil conditions, post load, and local building code. Always verify requirements with your local building department before installation. See all outdoor calculators.