Post Hole Concrete Calculator
Calculate bags of concrete needed for fence post and deck post holes. Enter values for instant results with step-by-step formulas.
Reviewed by Abdullah, Technical Content Specialist
Formula
Volume = pi * (Rhole^2 - Rpost^2) * Depth
Where Rhole = radius of the hole, Rpost = radius of the post, and Depth = depth of the hole. The concrete fills the annular space between the round hole and the post. Weight is calculated using concrete density of 150 lbs per cubic foot, then divided by bag weight to determine number of bags needed.
Worked Examples
Example 1: Standard Fence Post Project
Problem:Calculate concrete needed for 20 fence post holes, each 10 inches in diameter and 36 inches deep, using 4x4 posts (3.5-inch actual) with 80-lb bags.
Solution:Hole volume = pi * (5/12)^2 * (36/12) = pi * 0.1736 * 3 = 1.636 ft3\nPost volume = pi * (1.75/12)^2 * 3 = pi * 0.02127 * 3 = 0.200 ft3\nConcrete per hole = 1.636 - 0.200 = 1.436 ft3\nWeight per hole = 1.436 * 150 = 215.4 lbs\nBags per hole = ceil(215.4 / 80) = 3 bags\nTotal bags = 3 * 20 = 60 bags\nWith 10% waste = 66 bags
Result:66 bags of 80-lb concrete needed | 3 bags per hole | Estimated cost: $429
Example 2: Deck Post Footings
Problem:Calculate concrete for 6 deck post holes, 12 inches diameter and 42 inches deep, using 6x6 posts (5.5-inch actual) with 80-lb bags.
Solution:Hole volume = pi * (6/12)^2 * (42/12) = pi * 0.25 * 3.5 = 2.749 ft3\nPost volume = pi * (2.75/12)^2 * 3.5 = pi * 0.0525 * 3.5 = 0.577 ft3\nConcrete per hole = 2.749 - 0.577 = 2.172 ft3\nWeight per hole = 2.172 * 150 = 325.8 lbs\nBags per hole = ceil(325.8 / 80) = 5 bags\nTotal bags = 5 * 6 = 30 bags\nWith 10% waste = 33 bags
Result:33 bags of 80-lb concrete needed | 5 bags per hole | Estimated cost: $214.50
Frequently Asked Questions
How deep should a fence post hole be for proper stability?
The standard rule for fence post hole depth is one-third of the total post length should be buried underground. For a typical 8-foot fence post, the hole should be at least 32 inches deep, leaving about 64 inches above ground. In areas subject to frost heave, the bottom of the concrete should extend below the local frost line, which ranges from 12 inches in southern states to 48 inches or more in northern climates. For gate posts and corner posts that bear additional loads, increasing the depth by 6 to 12 inches provides extra stability. Sandy or loose soils may require deeper holes compared to clay soils because they provide less lateral resistance against wind and impact forces.
What diameter hole should I dig for fence and deck posts?
The hole diameter should be approximately three times the width of the post for adequate concrete coverage and structural support. For a standard 4x4 post (actual dimension 3.5 inches), a 10-inch diameter hole is recommended. For 6x6 posts (actual 5.5 inches), use a 12-inch or larger diameter hole. Deck posts that support significant structural loads should have holes at least 12 to 16 inches in diameter depending on the load requirements and local building codes. Larger diameter holes provide more concrete mass around the post, creating greater resistance to lateral forces from wind, impact, and soil movement. Always check local building codes as requirements vary by jurisdiction and application type.
Should I use fast-setting or regular concrete for post holes?
Fast-setting concrete (such as Quikrete Fast-Setting) sets in 20 to 40 minutes and reaches working strength in 4 hours, making it ideal for fence post projects where you want to complete the work quickly. It can be poured dry directly into the hole and then saturated with water, eliminating the need for mixing. Regular concrete (standard mix) takes 24 to 48 hours to set and reaches full strength in 28 days but is generally stronger and more economical for large projects. For structural applications like deck posts, many building codes require standard concrete that is properly mixed to ensure consistent density and strength throughout the footing. Fast-setting concrete costs about 20 to 30 percent more per bag than regular mix.
How many bags of concrete do I need per fence post hole?
The number of bags depends on the hole diameter, depth, post size, and bag weight. For a typical 4x4 fence post in a 10-inch diameter hole that is 36 inches deep, you need approximately 3 bags of 80-pound concrete or 4 bags of 60-pound concrete. A 6x6 post in a 12-inch hole at 36 inches deep requires about 5 bags of 80-pound mix. Always purchase 10 percent extra bags to account for spillage, irregular hole shapes, and variations in hole dimensions. For large projects with many posts, buying a full pallet of concrete bags often provides a significant discount compared to individual bag pricing and ensures you have adequate material on hand.
References
Reviewed by Abdullah, Technical Content Specialist · Editorial policy