Fence Post Concrete Calculator
Calculate exactly how many bags of concrete you need per hole for fence posts, deck posts, and mailboxes.
Post Hole Details
Estimation Results
Total Bags Needed for 1 Post(s):
For estimation purposes only. Verify structural calculations with a licensed engineer.
Post Hole Concrete Calculator
Setting fence posts or deck posts perfectly plumb and secure is critical for the longevity of your structure. However, knowing exactly how much concrete to buy can be incredibly confusing because you have to subtract the volume of the wooden post from the total volume of the hole.
Our fence post concrete calculator takes the guesswork out of the process. By inputting your hole diameter, hole depth, and the size of your wooden post, the tool instantly tells you how many bags to buy at the hardware store. It functions perfectly as both a fence and a deck post concrete calculator.
How to Calculate Concrete for Fence Posts
If you want to know how to calculate concrete for fence posts manually, you have to use a multi-step formula involving Pi ().
1. Find Hole Volume: ( Hole Radius Depth)
2. Find Post Volume: (Post Width Post Depth Depth)
3. Concrete Volume = Hole Volume minus Post Volume
4. Divide Concrete Volume by Bag Yield (e.g. 0.60 cu ft)
Because an auger doesn't dig perfectly smooth holes, you will always end up using slightly more concrete than the exact mathematical formula suggests. Our concrete calculator for post holes accounts for this reality.
Depth of Fence Post Hole & Sizing Rules
How wide and how deep should your post hole be? Contractors follow two universal rules of thumb:
- Hole Diameter: The hole should be 3 times the width of the post. For a standard 4x4 wooden post (which actually measures 3.5" x 3.5"), you should drill a 10-inch to 12-inch diameter hole.
- Hole Depth: The depth of fence post hole should be one-third to one-half of the above-ground height of the post. So, for a 6-foot tall privacy fence, the hole must be at least 2 to 3 feet deep.
Crucial Frost Line Warning: Regardless of the math above, the bottom of your concrete footing MUST sit below your local frost line. If it doesn't, the ground will freeze, expand, and push your entire fence post out of the earth.