Post Hole Concrete Calculator
Setting fence posts, deck footings, or mailboxes? Enter your post size, hole dimensions, and the number of holes to estimate the exact number of concrete bags needed for your project.
Results
How to calculate concrete for post holes
Determining the amount of concrete needed to set fence posts or deck footings requires three simple steps: calculating the volume of the cylindrical hole, calculating the displacement volume of the buried portion of the post, and subtracting the post volume from the hole volume.
Step 1: Calculate Hole Volume (Cylinder)
Hole Volume (cubic inches) = π × radius² × depth
Where radius is half of the hole diameter in inches.
Step 2: Calculate Post Displacement Volume
For a square wood post, multiply its actual (not nominal) dimensions by the depth. For a 4" × 4" post, the actual dimensions are 3.5" × 3.5". For a 6" × 6" post, they are 5.5" × 5.5".
Post Volume (cubic inches) = actual width × actual thickness × depth
Step 3: Subtract & Convert to Bags
Subtract the post volume from the hole volume to get the net concrete volume in cubic inches, divide by 1,728 to convert to cubic feet, add a 10% waste factor, and divide by the yield of your chosen bag size (80lb bags yield 0.60 cu ft, 60lb bags yield 0.45 cu ft, and 50lb bags yield 0.375 cu ft).
The three golden rules of post hole sizing
- The "Rule of Three" for diameter: The diameter of your post hole should always be exactly three times the width of the post. For a standard 4x4 post (which is 3.5" wide), the minimum recommended hole diameter is 10 inches (or 12 inches for extra strength). For a 6x6 post, use a 15-inch or 18-inch hole.
- Depth below the frost line: Post holes should go deep enough that at least one-third of the post height is below ground. More importantly, in cold climates, the bottom of the hole must reach at least 6 inches below your local frost line. If it doesn't, frozen ground will lift the concrete footing (a phenomenon called "frost heave"), throwing your fence or deck completely out of level.
- Leave a gravel base: Before pouring concrete, throw 2 to 6 inches of crushed drainage gravel into the bottom of the hole. This provides a drainage base so rainwater doesn't pool directly under the bottom of the wood post, which would cause rapid rot. Adjust your digging depth to account for this gravel layer!
Deducting post displacement saves money
Many simple concrete calculators forget to subtract the volume occupied by the post itself. This leads to over-ordering. On a large fencing project, the difference is significant!
For example, setting 20 standard 4" × 4" wood posts in 10-inch holes at a 36-inch depth requires about 26 bags of 80 lb concrete. If you don't deduct the posts, you would calculate a need for 31 bags — resulting in 5 wasted bags of concrete sitting on your driveway. That's a 16% over-ordering error that our calculator eliminates.
| Post Type | Concrete per hole (cu ft) | Bags (80 lb) needed for 10 posts | Bags (80 lb) needed for 20 posts |
|---|---|---|---|
| No Post (Footing only) | 1.64 cu ft | 30 bags | 60 bags |
| 2" Round Metal Post | 1.54 cu ft | 29 bags | 57 bags |
| 4" × 4" Wood Post | 1.38 cu ft | 26 bags | 51 bags |
| 6" × 6" Wood Post | 1.01 cu ft | 19 bags | 37 bags |
How to pour concrete for a post
Once your post holes are dug and the gravel base is set, there are two primary ways to set the posts using dry bag concrete:
Method 1: Wet-mixing (Recommended for maximum strength)
Mix the concrete bag with water in a wheelbarrow, tub, or mechanical mixer until it reaches a thick oatmeal consistency. Pour the wet concrete into the hole around your braced and leveled post, and use a shovel or stick to consolidate it and eliminate air pockets. Slope the top surface of the concrete away from the post to shed rainwater.
Method 2: Dry-pouring (Fast but lower strength)
Some fencers pour the dry concrete mix straight from the bag into the hole, then pour water on top. While fast, this method often fails to hydrate the concrete in the center of the hole, leaving dry, crumbly pockets. If using this method, pour in 6-inch lifts, soaking each layer with water before adding the next, and ensure the post is heavily braced as it cures.
Frequently asked questions
How many 80lb bags of concrete do I need for a 4x4 post hole?
For a standard 4x4 wood post (3.5" x 3.5" actual) in a 10-inch diameter hole dug 36 inches deep, you will need approximately 2.5 to 2.6 bags of 80 lb concrete. If you use a larger 12-inch diameter hole for extra strength, you will need about 3.8 bags of 80 lb concrete.
What is the rule of thumb for post hole diameter and depth?
The hole diameter should be 3 times the width of the post (e.g., a 10" hole for a 4" post). The hole depth should be 1/3 to 1/2 of the height of the post above ground, and always at least 6 inches below your local frost line to prevent frost heave.
Should concrete go under the bottom of a fence post?
No, it is highly recommended to place 2 to 6 inches of gravel in the bottom of the hole first, then place the post on top of the gravel, and pour the concrete around the sides. Placing concrete under the post traps water around the end-grain of the wood, causing the post to rot much faster.
Can you pour dry concrete into a post hole and add water?
Yes, but it is not recommended for heavy load-bearing posts. Dry-pouring can lead to uneven hydration, leaving crumbly dry spots inside the footing. For maximum strength, mix the concrete with water in a wheelbarrow first, then pour it in wet, packing it down to remove air pockets.
How long does concrete take to cure before attaching fence rails?
Let the concrete cure for at least 24 to 48 hours before attaching rails or pulling fence lines taut. Concrete continues to gain strength for 28 days, but it is sufficiently solid to support moderate tension and building weight after 2 days. Use temporary bracing to hold the posts plumb as it cures.