Last updated: 2026-04-01
Quick Answer
A typical 4×4 fence post in an 8" hole at 36" deep needs 1-2 bags of 80-lb concrete. A 6×6 post in a 12" hole at 48" needs 3-4 bags. This calculator does the cylinder math for you - enter your hole diameter, depth, and post size to get exact bag counts per hole and for the whole project. Works for fence posts, deck posts, mailboxes, and signs.
What you'll need to measure
- Hole diameter (inches) - match your auger bit or Sonotube size
- Hole depth (inches) - must be below frost line for structural posts
- Post width (inches) - 4 for a 4×4, 6 for a 6×6, 0 for solid fill
- Number of post holes
- Bag size preference (50, 60, or 80 lb)
Related tools: Concrete Calculator, Concrete Footing Calculator, and Fence Calculator.
How to use this calculator
- Select a preset or enter your hole diameter and depth.
- Enter the post size (width in inches). Use 0 for solid fill.
- Set the number of holes for your project.
- Adjust waste (10% default) and pick your bag size.
- Read bags per hole and total bags needed.
Building a fence? Use our Fence Calculator to figure out post count from your fence length and spacing first, then come back here for the concrete.
Presets are common starting points. Always check local frost depth and code requirements.
Measure the auger or Sonotube diameter.
Must be below frost line for structural posts.
4 for a 4×4, 6 for a 6×6. Set 0 if no post.
10% is typical for post holes (uneven ground, over-dig).
Check your local store price.
Assumptions
- Holes are cylindrical. Post displaces concrete (also treated as a cylinder).
- For square posts (4×4, 6×6), the calculator uses the nominal width as a circular diameter - a slight overestimate that provides a small built-in buffer.
- Bag yields: 50 lb = 0.375 ft³, 60 lb = 0.45 ft³, 80 lb = 0.60 ft³.
- Cost is bags only. Does not include posts, gravel, labor, or delivery.
Post Hole Concrete Results
Total 80-lb bags needed
29
80-lb bags per hole
2
Total volume (ft³)
17.28
Total volume (yd³)
0.64
Detailed Breakdown
12,800+ estimates calculated this month
Quick reference: bags per post hole (80-lb bags, 10% waste)
| Hole size | Post | Concrete (ft³) | 80-lb bags |
|---|---|---|---|
| 8" × 36" | 4×4 | 0.86 | 2 |
| 10" × 42" | 4×4 | 1.76 | 3 |
| 10" × 42" | 6×6 | 1.34 | 3 |
| 12" × 48" | 4×4 | 3.07 | 6 |
| 12" × 48" | 6×6 | 2.59 | 5 |
| 16" × 48" | 6×6 | 5.28 | 9 |
Post Hole Concrete Guide
Bag counts, frost depth, setting methods, and common mistakes for post hole concrete work.
How Much Concrete Goes in a Post Hole?
A standard 8" diameter × 36" deep fence post hole takes 1-2 bags of 80-lb concrete mix after accounting for the post displacement and 10% waste.
- 4×4 fence post (8" hole × 36" deep): 1-2 bags (80 lb)
- 6×6 fence post (10" hole × 42" deep): 2-3 bags (80 lb)
- Deck post (12" hole × 48" deep): 3-4 bags (80 lb)
- Heavy gate post (16" hole × 48" deep): 5-7 bags (80 lb)
The math is straightforward: calculate the cylinder volume of the hole, subtract the cylinder volume of the post, add 10% waste, then divide by the bag yield (0.6 ft³ per 80-lb bag).
Key Takeaways
- 4×4 in 8" hole: 1-2 bags of 80-lb mix
- 6×6 in 10" hole: 2-3 bags of 80-lb mix
- Always add 10% waste for uneven holes
Post Hole Depth Requirements by Region
Structural post holes must extend below the frost line to prevent heaving. Frost depth varies dramatically by location.
- Southern states (FL, TX, AZ): 12"-18" minimum depth
- Mid-Atlantic / Pacific NW: 18"-24" minimum depth
- Midwest / Northeast: 36"-48" minimum depth
- Northern states (MN, WI, MT): 42"-60" minimum depth
For non-structural applications like decorative fences under 4 feet, some jurisdictions allow shallower holes. But for anything load-bearing - deck posts, gate posts, tall privacy fences - going below frost line is not optional. A post that heaves will take the whole fence section with it.
Key Takeaways
- Frost line: 12" (South) to 60" (far North)
- Structural posts must go below frost line
- Check local code before digging
Dry-Set vs. Wet-Mix Concrete for Post Holes
Both methods work, but dry-set (pouring dry mix into the hole and adding water on top) is faster for most fence posts.
- Dry-set method: Pour dry concrete mix around the post, then add water. Ground moisture and added water activate the cement. Fastest for high-volume fence jobs. Quik-Crete and Sakrete both sell fast-setting formulas designed for this.
- Wet-mix method: Mix concrete in a wheelbarrow first, then pour into the hole. Better control over consistency. Preferred for larger holes and critical structural posts.
- Foam post setting: Two-part expanding foam products (like Sika PostFix) set in 3 minutes. Lighter than concrete but more expensive per hole. Works well for fence posts on flat ground.
For deck posts or anything supporting a structure, wet-mix gives you better compaction and fewer voids. For a 200-foot fence line with 25 posts, dry-set saves hours.
Key Takeaways
- Dry-set: fastest for fence posts
- Wet-mix: better for structural/deck posts
- Foam: 3-min set, more expensive per hole
Common Post Hole Mistakes
The biggest mistake is digging holes too shallow. Second biggest: not accounting for the gravel base.
- Too shallow: Below frost line is the rule. A 24" hole in Minnesota will heave by spring.
- No gravel base: 4"-6" of gravel at the bottom of each hole improves drainage and prevents the post from sitting in standing water. Dig 4"-6" deeper than your concrete depth to allow for this.
- Hole too tight: Leave at least 2" of clearance around the post on each side. An 8" hole for a 4×4 post gives you 2" per side. A 6" hole is too tight to pack concrete properly.
- Over-filling with water (dry-set): Add water slowly. Too much water weakens the mix and makes a soupy mess that takes days to set.
- Not bracing the post: Concrete takes 24-48 hours to set. Brace each post plumb with scrap lumber before you walk away.
Key Takeaways
- Always dig below frost line
- Add 4"-6" gravel base for drainage
- Leave 2"+ clearance around the post
Concrete bags per hole (quick reference)
All values assume 80-lb bags (0.6 ft³ yield) with 10% waste. Post volume subtracted.
| Hole | Post | Bags/hole | For 10 posts | For 25 posts |
|---|---|---|---|---|
| 8" × 36" | 4×4 | 2 | 14 | 34 |
| 10" × 42" | 4×4 | 3 | 22 | 54 |
| 10" × 42" | 6×6 | 2 | 18 | 44 |
| 12" × 48" | 4×4 | 4 | 33 | 81 |
| 12" × 48" | 6×6 | 3 | 26 | 64 |
| 16" × 48" | 6×6 | 6 | 51 | 126 |
Tip: buy 1-2 extra bags per project. Leftover concrete stores fine in a dry garage. Running short means another trip.
Mistakes I see on post hole jobs
- Skipping the gravel base. 4-6 inches of gravel at the bottom prevents water pooling and post rot. Dig deeper to allow for it.
- Holes too shallow. If you are above the frost line, the post will heave. In the PNW where I work, 18-24 inches is minimum. Up in Minnesota or Montana, you are looking at 42-48 inches.
- Flooding the dry mix. Add water slowly. Too much turns the mix into soup that takes forever to set and loses strength.
- Not bracing posts. Even fast-setting concrete takes 20 minutes. Brace each post plumb with scrap lumber or stake braces before walking away.
- Buying too few bags. Holes are never perfectly round or perfectly consistent. That 10% waste factor is real.
From bags to a full estimate
Knowing your bag count is just the first step. EstimationPro builds the full estimate - posts, concrete, hardware, labor - then sends a professional proposal to the homeowner and automatically follows up so you win more of the bids you already send. No more losing fence jobs because someone else replied faster. Try EstimationPro free.
How to Use This Calculator
Pick your hole size
Select a preset (8" for 4×4 fence posts, 12" for deck posts) or enter custom dimensions.
Enter the post size
Type the post width in inches (4 for a 4×4, 6 for a 6×6). The calculator subtracts the post volume from the hole.
Set number of holes
Count your posts. For a fence, divide total length by post spacing (usually 6-8 feet) and add 1.
Read your bag count
The calculator shows bags per hole and total bags needed. Pick 50, 60, or 80-lb size based on what your store carries.
Post Hole Concrete Formula
Hole Volume (ft³) = π × (Diameter ÷ 2 ÷ 12)² × (Depth ÷ 12)
Post Volume (ft³) = π × (Post Width ÷ 2 ÷ 12)² × (Depth ÷ 12)
Concrete per Hole = Hole Volume − Post Volume
Total Concrete = Concrete per Hole × Number of Holes × (1 + Waste%/100)
Bags Needed = Total Concrete (ft³) ÷ Bag Yield (ft³)Where:
- Diameter
- = Hole diameter in inches (auger bit or tube form size)
- Post Width
- = Nominal post width in inches (4 for a 4×4, 6 for a 6×6). Set to 0 for solid fill.
- Bag Yield
- = 50-lb = 0.375 ft³, 60-lb = 0.45 ft³, 80-lb = 0.60 ft³
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Frequently Asked Questions
How many bags of concrete do I need for a fence post?
How deep should a fence post hole be?
Should I use fast-setting or regular concrete for post holes?
Do I need gravel at the bottom of a post hole?
How do I calculate concrete volume for a round hole?
Can I use dry concrete mix in post holes?
How wide should a post hole be compared to the post?
Does this calculator account for the post displacing concrete?
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