Quickly calculate how many bags of concrete you need for your fence post project. Supports all common bag sizes.
How to Use This Fence Post Concrete Calculator
Select your post size (4×4, 6×6, or custom), then enter your hole diameter and depth. The calculator uses the volume of a cylinder formula: π × (diameter ÷ 2)² × depth. Enter the number of posts you're setting, choose your preferred bag weight, and set a waste factor (10% is standard).
Results show concrete volume per post, total volume in both cubic feet and cubic yards, and the number of bags to buy — always rounded up to the next whole bag. Check "Include cost estimate" to calculate your total concrete cost.
How Much Concrete Per Fence Post: A Simple Formula
Setting fence posts in concrete is one of the most common DIY projects — and one where people consistently buy the wrong amount of concrete. Too little means a second trip to the store mid-project. Too much means wasted money and bags sitting in your garage for years. This guide covers exactly how to calculate your concrete needs and avoid the most common mistakes.
The Formula
A fence post hole is essentially a cylinder. The volume formula is:
Volume = π × (hole diameter ÷ 2)² × hole depth
This gives you cubic inches. Divide by 1,728 to get cubic feet. Most concrete bag labels list their yield in cubic feet:
- 40 lb bag yields approximately 0.30 cubic feet
- 50 lb bag yields approximately 0.45 cubic feet
- 60 lb bag yields approximately 0.45 cubic feet
- 80 lb bag yields approximately 0.60 cubic feet
How Big Should the Hole Be?
The diameter rule of thumb is simple: 3 times the post width. For a standard 4×4 post (actual size 3.5" × 3.5"), that gives you a hole about 10-12 inches across. This provides enough room to properly tamp concrete around the post and ensures adequate lateral support.
For depth, there are two critical rules:
- Below the frost line: In cold climates, the hole bottom must be below your region's frost depth — typically 24" in moderate zones, 36-48" in northern states, and 12-18" in the South. If concrete sits above the frost line, freeze-thaw cycles will gradually push your post upward (frost heave).
- One-third rule: At least one-third of the post's total length should be underground. For a 6-foot-tall fence, posts should be at least 2 feet in the ground — meaning you need 8-foot posts.
Do You Need a Gravel Base?
Yes — add 4-6 inches of gravel at the bottom of each hole before pouring concrete. This serves three purposes:
- Drainage: Prevents water from pooling around the post bottom, which causes rot in wood posts.
- Frost protection: Provides a drainage path that reduces frost heave pressure.
- Post alignment: Gravel allows you to fine-tune post height before the concrete sets.
Remember to subtract the gravel depth from your hole depth when calculating concrete volume. If you want a 24-inch-deep hole with 4 inches of gravel, you're pouring 20 inches of concrete.
50 lb vs 80 lb Bags: Which to Choose?
The choice comes down to weight you can handle and cost per cubic foot:
- 50 lb bags are easier to carry, lift, and pour — ideal if you're working alone or don't have strong help. You'll need more bags and mixing sessions.
- 80 lb bags offer the best cost per cubic foot and fewer mixing cycles for larger projects. But each bag is heavy, especially when you're pouring dozens of posts in a day.
- 60 lb bags are a middle ground that many pros prefer. They're manageable and cost-effective.
Common Mistakes to Avoid
- Hole too shallow: The #1 reason fence posts lean over time. Go below the frost line, period.
- Hole too narrow: A hole barely wider than the post leaves almost no concrete around it. That's just a post in dirt with expensive fill.
- Forgetting drainage: No gravel base = water trapped against the post = rot within 5-10 years. Always gravel first.
- Mixing concrete too wet: Overly wet concrete is weak. Follow the water ratio on the bag — it should be the consistency of thick oatmeal, not soup.
- Not bracing posts while curing: Concrete needs 24-48 hours to set properly. Bracing posts with temporary 2×4 stakes prevents wind and gravity from shifting them before the concrete hardens.