Post Hole Concrete Calculator
Calculate exactly how many bags of concrete you need for fence posts, deck posts, mailbox posts, or any other post installation. Enter your hole dimensions, post size, and number of posts for an accurate bag count.
Enter Post Hole Details
Full Breakdown
Hole Volume (each)
Post Displacement
Concrete per Hole
Bags per Post
Total Posts
Total Bags Needed
Total Concrete Volume
How to Use This Calculator
- Select or enter your hole diameter — typically 3× the post width.
- Select or enter the hole depth — at least ⅓ of total post length or below frost line.
- Choose your post size to subtract its volume from the hole.
- Enter the number of posts for your project.
- Select your bag size and click calculate.
The Formula
Hole Volume = π × (Hole Radius)² × Depth
Post Volume = Post Width² × Depth (square) or π × r² × Depth (round)
Concrete Needed = Hole Volume − Post Volume
Volume is in cubic inches, converted to cubic feet (÷ 1728). Bag yields: 40 lb = 0.30 cu ft, 50 lb = 0.375 cu ft, 60 lb = 0.45 cu ft, 80 lb = 0.60 cu ft.