Post Hole Concrete Calculator
Calculate how many bags of concrete you need for fence posts, deck posts, or mailbox posts. See also our Fence Calculator, Concrete Column Calculator, and Concrete Calculator.
How to Use the Post Hole Concrete Calculator
Enter the diameter of your post hole (typically 3x the post width), the depth of the hole, your post size, and the number of posts. The calculator subtracts the post volume from the hole volume to determine the exact amount of concrete needed. Results are shown in bags of 50, 60, and 80 pounds so you can choose the most convenient size for your project. For fence posts, a 10-inch diameter hole works for 4x4 posts, while deck posts (6x6) need 12-inch holes.
When planning your project, remember that hole depth must account for 3-4 inches of gravel at the bottom for drainage. So if you need 30 inches of concrete around the post, dig the hole 34 inches deep. The concrete should extend from the gravel base to 1-2 inches above grade level, crowned to shed water away from the post. In cold climates, holes must extend below the frost line to prevent heaving.
For large projects (20+ posts), consider ordering ready-mix concrete delivered by truck instead of mixing bags by hand. One cubic yard of ready-mix fills approximately 15-20 standard fence post holes and is significantly less labor-intensive than mixing 40+ bags individually.
Post Setting Methods
There are three common methods for setting posts in concrete. The traditional method involves mixing concrete in a wheelbarrow and pouring it around the post. The dry-pour method uses fast-setting concrete poured dry into the hole with water added on top — it sets in 20-40 minutes. The foam method uses expanding polyurethane foam that sets in minutes and is lightweight. Each method has advantages depending on project size, timeline, and soil conditions.
Regardless of method, always brace posts plumb in two directions using temporary 2x4 braces staked to the ground. Check plumb with a level on two adjacent faces. For fence posts, use a string line between end posts to ensure alignment. Allow concrete to cure for 24-48 hours before attaching fence rails or applying lateral loads to the posts.
Tools Needed for Post Hole Projects
Essential tools include: post hole digger (clamshell type) or power auger for digging, level (4 ft preferred) for plumbing posts, string line for alignment, tape measure, wheelbarrow for mixing concrete, garden hose for water, and temporary braces (2x4 lumber with stakes). For rocky soil, a digging bar helps break through stones. A power auger (gas or electric) dramatically speeds up digging — rental cost is typically $50-75 per day and is worthwhile for projects with more than 4-5 holes.
Formula
Hole Volume = π x (Diameter/2)² x Depth
Post Volume = Post Width² x Depth (actual dimensions)
Concrete per hole = (Hole Volume - Post Volume) / 1728 cu ft
Total concrete = Concrete per hole x Number of posts
Bags = Total cu ft / Yield per bag
Example Calculation
8 fence posts: 4x4 post, 10" hole, 36" deep
Hole volume = π x 5² x 36 = 2,827 cu in
Post volume = 3.5² x 36 = 441 cu in
Concrete/hole = (2,827 - 441) / 1728 = 1.38 cu ft
Total = 1.38 x 8 = 11.04 cu ft
80-lb bags = 11.04 / 0.6 = 19 bags
Reference Table: Recommended Hole Sizes
| Post Size | Min Hole Diameter | Min Depth | Bags/Hole (80 lb) | Common Use |
|---|---|---|---|---|
| 4x4 | 10" | 24-36" | 1-2 | Fence posts, mailbox |
| 4x4 | 12" | 36-42" | 2-3 | Tall fence, gate posts |
| 6x6 | 12" | 36-42" | 2-3 | Deck posts, pergola |
| 6x6 | 14" | 42-48" | 3-4 | Tall deck posts, signs |
| 8x8 | 16" | 42-48" | 4-5 | Heavy structural posts |
| Round 4" | 10" | 24-36" | 1-2 | Round fence posts |
Frequently Asked Questions
How deep should fence post holes be?
The general rule is 1/3 of the total post length should be underground. For a 6-foot fence with 8-foot posts, dig 24-30 inches deep. In cold climates, holes must extend below the frost line (36-48 inches in northern states). Gate posts and end posts should be deeper (36+ inches) for extra stability.
Can I use fast-setting concrete for post holes?
Yes, fast-setting concrete (like Quikrete Fast-Setting) is ideal for post holes. Pour it dry into the hole around the post, then add water. It sets in 20-40 minutes, allowing you to continue working the same day. Standard concrete requires mixing and takes 24-48 hours to set. Both achieve the same final strength.
Should concrete be above or below ground level?
Crown the concrete 1-2 inches above ground level and slope it away from the post. This prevents water from pooling around the post base, which causes rot. Some builders prefer to stop concrete 2-3 inches below grade and backfill with soil for aesthetics, but this can trap moisture against the post.
Do I need gravel at the bottom of post holes?
Yes, place 3-4 inches of gravel at the bottom of each hole before setting the post. This provides drainage so water doesn't pool at the post base. Compact the gravel and check that the post height is correct before adding concrete. The gravel layer also helps prevent frost heaving in cold climates.
How wide should a post hole be?
Post holes should be 3 times the width of the post. A 4x4 post (3.5 inches actual) needs a 10-12 inch hole. A 6x6 post (5.5 inches actual) needs a 12-14 inch hole. This provides enough concrete around the post for adequate strength and allows room to plumb the post before the concrete sets.
How many bags of concrete per fence post?
For a standard 4x4 fence post in a 10-inch diameter, 36-inch deep hole: approximately 2 bags of 80-lb concrete or 3 bags of 50-lb concrete. For 6x6 posts in 12-inch holes at 42 inches deep: approximately 3 bags of 80-lb concrete. Always buy 1-2 extra bags for the project in case of deeper holes or wider diameters.