Post Hole Concrete Calculator
Get the exact amount of concrete needed for your post holes. Enter post type, hole depth, and number of posts — and we will tell you how many bags of concrete mix to buy, including gravel base and cost estimation.
Typically 1/3 of post length
Leave empty to auto-calculate (3× post width)
Gravel at bottom of hole for drainage
Optional: Calculate material cost
Rectangular Hole Volume = Width² × (Depth − Gravel)
Round Hole Volume = π × (Width/2)² × (Depth − Gravel)
Post Volume = Post Width² × (Depth − Gravel)
Bag Yield (cu ft): 50-lb = 0.375, 60-lb = 0.45, 80-lb = 0.6
Bags per Post = Concrete per Post ÷ Bag Yield
How much concrete do I need per fence post?
For a standard 4×4 post with a 12-inch diameter hole 30 inches deep (minus 3 inches gravel): hole volume = 12×12×27 = 3888 in³, post volume = 3.5×3.5×27 = 330.75 in³. Concrete volume = 3557.25 in³ = 2.06 cu ft. That is about 5-6 bags of 50-lb concrete mix per post. For 6×6 posts, expect 7-8 bags each.
What is the best concrete mix for fence posts?
Fast-setting concrete mix (like Quikrete Fast-Setting or Sakrete Fence Post Concrete) is ideal because it sets in 20-40 minutes, supports weight in 4 hours, and requires no mixing in a wheelbarrow — just pour dry into the hole and add water. It expands slightly as it sets, locking the post firmly in place.
How deep should a fence post hole be?
The general rule is to bury 1/3 of the total post length. For a 6-ft fence (8-ft post): bury 30 inches deep. For an 8-ft fence (10-ft post): bury 36 inches. The hole should be 3× the post diameter wide. Add 3-6 inches of gravel at the bottom for drainage. Deeper is better in frost-prone areas.
Should I put gravel at the bottom of post holes?
Yes, always add 3-6 inches of gravel at the bottom of each post hole. The gravel provides drainage so water does not pool around the bottom of the post, which prevents rot in wood posts. It also creates a stable base that resists settling and improves frost heave protection.
🔗 Related Calculators
📐 Formula
Rectangular Hole Volume = Width² × (Depth − Gravel)
Round Hole Volume = π × (Width/2)² × (Depth − Gravel)
Post Volume = Post Width² × (Depth − Gravel)
Bag Yield (cu ft): 50-lb = 0.375, 60-lb = 0.45, 80-lb = 0.6
Bags per Post = Concrete per Post ÷ Bag Yield