Stop Guessing How Much Concrete You Need
Enter your fence details and get exact hole dimensions, concrete volume, and bag counts. No wasted bags. No weak posts.
Calculate NowYour Fence Details
Your Results
Rule of thumb: About 1/3 of your post should be underground. The diagram shows this proportion. If your soil is sandy or you are in a windy area, the calculator adds extra depth for stability.
How to Use This Calculator
Step 1: Pick a Preset or Enter Your Numbers
The presets fill in common fence configurations. A 6-foot privacy fence in average soil with medium wind is the most common setup. If your situation is different, just change the numbers.
Step 2: Check Your Soil Type
Clay holds posts well but drains poorly. Sand drains great but does not grip posts. Loam is the middle ground. If you are not sure, dig a small test hole. If the sides crumble easily, you have sandy soil and need deeper holes.
Step 3: Account for Wind
Open fields and hilltops catch more wind. A fence in a sheltered backyard with trees around it faces less force. When in doubt, choose the higher wind setting. A little extra concrete costs less than a fence that leans after the first storm.
Step 4: Mark Corner and Gate Posts
These posts carry extra weight and force. The calculator doubles the concrete volume for any post you mark as a corner or gate post. Most fences have at least 2 corner posts and 1 gate post.
Example: Planning a Backyard Privacy Fence
Sarah wants to build a 6-foot privacy fence around her backyard. The yard is 60 feet long and 40 feet wide, so she needs about 20 posts at 8-foot spacing. The soil is mostly clay with some sand mixed in. Her yard has a few trees but is fairly open on one side.
She selects the 6-foot privacy preset, changes the soil to loam (a mix), sets wind exposure to medium, and enters 20 posts. The calculator tells her each hole should be 10.5 inches wide and 24 inches deep. She needs 28 bags of 60-pound concrete mix, costing about $154.
She also marks 4 posts as corner posts. Those need double concrete, so the calculator adjusts the total to 32 bags. Sarah rounds up to 35 bags to be safe and adds a note to buy an extra bag for each gate post.
Common Mistakes to Avoid
Digging Holes Too Shallow
The old rule of "2 feet deep" works for short fences in calm areas. But a 6-foot fence in sandy soil with wind needs 2.5 to 3 feet of depth. Shallow holes let posts lean and wobble. The fix is simple: dig deeper. The extra 6 inches of digging saves you from rebuilding the fence in two years.
Making Holes Too Wide
A wider hole does not always mean a stronger post. Holes that are too wide waste concrete and can actually make it harder to keep the post plumb while the concrete sets. The right diameter is 3 times the post width. For a 4x4 post, that is about 10.5 inches.
Ignoring the Frost Line
In cold climates, the ground freezes and thaws. This movement can push posts upward over time. Your local building code specifies a minimum depth below the frost line. This calculator gives general guidance, but always check your local requirements. The frost line in Minnesota is 42 inches. In Georgia, it might be only 6 inches.
Not Bracing Posts While Concrete Sets
Wet concrete is heavy and can push your post out of alignment. Use 2x4 braces to hold the post plumb while the concrete cures. Check with a level on two sides. Let the concrete set for at least 24 hours before attaching fence panels.
Quick Reference: Concrete Bag Yields
Different bag sizes give you different amounts of concrete. Here is what to expect from each bag after mixing.
| Bag Size | Cubic Feet | Fills a 10" hole to depth | Typical Price |
|---|---|---|---|
| 40 lb | 0.30 cu ft | ~4.5 inches | $3.50 - $4.50 |
| 50 lb | 0.375 cu ft | ~5.5 inches | $4.00 - $5.00 |
| 60 lb | 0.45 cu ft | ~6.5 inches | $5.00 - $6.00 |
| 80 lb | 0.60 cu ft | ~9 inches | $6.50 - $8.00 |
What Affects How Much You Need
- Fence height: Taller fences catch more wind and need deeper holes. An 8-foot fence might need 50% more concrete than a 4-foot fence.
- Soil type: Sandy and loose soils need deeper holes for the same stability. Clay holds well but may need wider holes if it is rocky.
- Wind exposure: Open areas, hilltops, and coastal regions face more wind force. The calculator adds depth for high-wind areas.
- Post type: 6x6 posts need wider holes than 4x4 posts. The calculator adjusts diameter based on your post choice.
- Corner and gate posts: These carry extra load. Always use double the concrete volume for these posts.
What This Calculator Assumes
This calculator is designed for standard wood fence posts set in concrete. It assumes:
- You are using pressure-treated wood posts (4x4 or 6x6)
- Your fence is for residential use, not commercial or agricultural
- You are mixing concrete on site from bags, not ordering ready-mix
- Local building codes allow concrete-set posts (some areas require other methods)
Always check your local building codes before starting your fence project. Some areas have specific requirements for post depth, hole diameter, and concrete type. This calculator gives general guidance based on engineering principles, but local rules take priority.
Last updated: 2026 | Version 1.0