Post Hole Concrete Calculator
Enter your hole diameter, depth, and number of posts to instantly get the exact bags of concrete you need — for fence posts, deck footings, mailbox posts, and more.
For a standard 4×4 fence post in a 10-inch diameter hole at 2.5 feet deep, you need 2 bags of 80 lb concrete. For a 6×6 post in a 12-inch hole at 3.5 feet deep: 3 bags. For a 12-inch deck footing sonotube at 4 feet deep: 5 bags. Use the calculator below for any combination of diameter, depth, and post count.
⭕ Post Hole Concrete Calculator
The Post Hole Concrete Formula
Post holes are cylinders. The volume of a cylinder is calculated using the standard formula: π × radius² × height. Here’s how it applies to a post hole, with a full worked example:
(diameter in inches, depth in feet)
80 lb bags: Bags = (ft³ ÷ 0.60) × 1.10 waste factor
60 lb bags: Bags = (ft³ ÷ 0.45) × 1.10
40 lb bags: Bags = (ft³ ÷ 0.30) × 1.10
Example: 10″ dia × 3 ft deep → π × (5/12)² × 3 = 1.64 ft³
→ 1.64 ÷ 0.60 × 1.10 = 3 bags of 80 lb (round up)
Bags of Concrete Per Post Hole — Complete Reference Chart
Use these pre-calculated tables to quickly find the bag count for common hole sizes without any math. All figures use 80 lb bags and include a 10% waste factor. For 60 lb bags, multiply the cubic feet column by 2.44 (÷ 0.45 × 1.10).
Fence Posts (4×4 and 6×6)
| Hole Diameter | Depth | Post Size | ft³ per hole | 80 lb Bags | 60 lb Bags |
|---|---|---|---|---|---|
| 8 in | 2 ft | 2×4, small posts | 0.70 | 2 | 2 |
| 8 in | 2.5 ft | 2×4, small posts | 0.87 | 2 | 3 |
| 10 in | 2 ft | 4×4 post | 1.09 | 2 | 3 |
| 10 in | 2.5 ft | 4×4 post | 1.36 | 3 | 4 |
| 10 in | 3 ft | 4×4 post (frost) | 1.64 | 3 | 4 |
| 12 in | 2.5 ft | 6×6 post | 1.96 | 4 | 5 |
| 12 in | 3 ft | 6×6 post | 2.36 | 5 | 6 |
| 12 in | 3.5 ft | 6×6 post (frost) | 2.75 | 6 | 7 |
| 12 in | 4 ft | Heavy gate post | 3.14 | 6 | 8 |
| 14 in | 3.5 ft | Corner post | 3.74 | 7 | 10 |
| 16 in | 4 ft | Large corner post | 5.59 | 11 | 14 |
Deck Footings (Sonotubes)
| Tube Diameter | Depth | ft³ per footing | 80 lb Bags | 60 lb Bags | Notes |
|---|---|---|---|---|---|
| 8 in | 3 ft | 1.05 | 2 | 3 | Small deck, light load |
| 8 in | 4 ft | 1.40 | 3 | 4 | |
| 10 in | 3.5 ft | 1.91 | 4 | 5 | Standard residential deck |
| 10 in | 4 ft | 2.18 | 4 | 6 | Standard residential deck |
| 12 in | 3.5 ft | 2.75 | 6 | 7 | |
| 12 in | 4 ft | 3.14 | 6 | 8 | Most common deck footing |
| 12 in | 4.5 ft | 3.53 | 7 | 9 | |
| 14 in | 4 ft | 4.27 | 8 | 11 | Heavier deck loads |
| 16 in | 4 ft | 5.59 | 11 | 14 | Large deck, heavy beam spans |
| 18 in | 5 ft | 8.84 | 17 | 22 | Commercial / very heavy loads |
Mailbox Posts & Light Fixtures
| Hole Size | Depth | 80 lb Bags | Typical Use |
|---|---|---|---|
| 6 in dia | 1.5 ft | 1 | Mailbox post, small signpost |
| 6 in dia | 2 ft | 1 | Light post, small flag pole |
| 8 in dia | 2 ft | 2 | Standard mailbox post |
| 8 in dia | 2.5 ft | 2 | Lamp post, basketball hoop |
| 10 in dia | 3 ft | 3 | Swing set, pergola post |
| 12 in dia | 3 ft | 5 | Large pergola, heavy gate |
How Deep Should a Post Hole Be?
Post hole depth is determined by two rules — both must be satisfied:
Rule 1: The One-Third Rule
The buried portion of a post must be at least 1/3 of the total post length. For a 6-foot fence with 8-foot posts, bury at least 2.5–3 feet. For a 4-foot fence with 6-foot posts, bury at least 2 feet. This ensures the post resists lateral wind loads and doesn’t lean over time.
Rule 2: Below the Frost Line (Critical in Cold Climates)
In any region that experiences freezing temperatures, the bottom of your post hole must extend at least 6 inches below the local frost line depth. If the concrete doesn’t reach below the frost line, the ground freezes in winter, expands, and pushes the post upward — a process called frost heave. This destroys fence alignment, tilts posts, and can crack the concrete footing. The frost line is the maximum depth at which soil freezes.
| Fence Height | Post Length | Min. Hole Depth | Recommended Depth |
|---|---|---|---|
| 3 ft | 5 ft total | 20 in | 24 in |
| 4 ft | 6 ft total | 24 in | 28 in |
| 6 ft | 8 ft total | 30 in | 36 in |
| 8 ft | 10 ft total | 36 in | 42 in |
| 10 ft | 13 ft total | 48 in | 54 in |
Frost Line Depth by State — Reference Chart
If you live in a state with winter freezing, your post holes must go below the frost line or frost heave will eventually destroy the fence or deck. Add 6 inches of buffer below the listed depth. Check with your local building department for county-specific requirements.
Deep (48″+) Moderate (24–48″) Shallow (under 24″)
Always verify frost line depth with your local building department — depths vary within states by elevation and local soil conditions.
What Diameter Post Hole Do I Need?
The general rule is that a post hole diameter should be 3 times the post width. This gives adequate concrete around the post on all sides for proper load transfer and weather resistance.
| Post Size | Actual Width | Recommended Hole Diameter | Min. Hole Diameter |
|---|---|---|---|
| 2×4 lumber | 1.5 in | 6 in | 4 in |
| 4×4 lumber | 3.5 in | 10 in | 8 in |
| 4×6 lumber | 3.5 in | 10 in | 8 in |
| 6×6 lumber | 5.5 in | 16 in | 12 in |
| 3 in round metal post | 3 in | 10 in | 8 in |
| 4 in round metal post | 4 in | 12 in | 10 in |
| Chain-link post (1.66 in) | 1.66 in | 6 in | 6 in |
| Chain-link post (2 in) | 2 in | 8 in | 6 in |
Sandy or loose soil
Increase hole diameter by 2–4 inches. Loose soil provides less lateral support, so more concrete is needed.
Rocky or hard clay soil
Stick to standard diameter. Rocky soil naturally provides excellent lateral support for posts.
High moisture / wet areas
Use pressure-treated posts rated for ground contact (UC4B or UC4C). Always use a gravel drainage layer at the bottom of the hole.
High-wind regions
Increase depth by 6–12 inches beyond frost line. Gate posts and corner posts should be 12–16 inch diameter for stability.
How to Set a Post in Concrete — Step by Step
Proper installation is just as important as the right amount of concrete. Follow these steps for a post that stays plumb and lasts decades:
Dig the hole to the correct depth
Use a post hole digger or gas-powered auger. Diameter should be 3× the post width. Depth: at least 1/3 post length and below frost line. Keep hole walls straight — flared holes waste concrete.
Add 3–4 inches of gravel at the bottom
Pour 3–4 inches of crushed stone or pea gravel into the hole. Tamp it down. This prevents moisture from pooling at the post base and dramatically extends post life.
Set the post and brace it plumb
Place the post and use a level to verify it’s perfectly vertical on two adjacent sides. Brace it with temporary 2×4 diagonal braces before pouring. Double-check plumb after every step — once concrete sets, corrections are extremely difficult.
Mix and pour the concrete
Mix bags per manufacturer instructions. Pour concrete around the post in layers, prodding with a rod or stick to eliminate voids and air pockets. Fill to within 1–2 inches of the surface.
Crown the concrete for drainage
Shape the top of the concrete into a slight mound (crown) sloping away from the post on all sides. This directs water away from the wood-concrete interface — the most common point of rot and failure.
Allow 24–48 hours before loading
Keep braces in place for at least 24 hours. Don’t attach fence rails or deck boards for 48–72 hours. Full strength is reached at 28 days, but the post will hold its alignment after 24 hours of curing.
Dry Pour vs. Mixed Concrete for Post Holes
There are two common methods for setting fence posts in concrete. Both work, but they have different trade-offs:
| Method | How It Works | Pros | Cons | Best For |
|---|---|---|---|---|
| Dry Pour | Pour dry concrete mix directly into hole around post, then add water on top and let it absorb | Faster, no mixing required, minimal tools | Less uniform mix, lower strength, not ideal for structural loads | Fence posts, mailbox posts, non-structural applications |
| Mixed Pour | Mix concrete to proper consistency before pouring around the post | Higher strength, uniform mix, no voids, better for structural loads | More labor, need water source and mixing equipment on site | Deck footings, gate posts, structural posts, any load-bearing application |
How Much Will My Post Hole Concrete Cost?
Concrete bag costs vary by retailer and region, but here are typical 2025 prices to help you budget:
| Bag Size | Cost Per Bag | Cost Per yd³ | Best Use |
|---|---|---|---|
| 40 lb bag | $5 – $7 | $450 – $630 | Small repairs, tiny holes only |
| 60 lb bag | $7 – $9 | $420 – $540 | Light fence posts, mailbox posts |
| 80 lb bag | $7 – $10 | $315 – $450 | Best value — fence and deck footings |
| Quikrete Fast-Set 50 lb | $8 – $12 | $576 – $864 | Quick fence posts (no-mix dry pour) |
Sample Project Cost: 8-Post Wood Fence
Assuming a 6-foot privacy fence, 8×4 panels, 8 posts in 10-inch holes at 3 feet deep:
- Volume per hole: 1.64 ft³ → 3 bags of 80 lb per hole
- Total bags: 8 holes × 3 bags = 24 bags (buy 26 for safety)
- Material cost at $8/bag: $208
Sample Project Cost: 12-Footing Deck
Standard 16×20 ft deck, 12 footings using 12-inch sonotubes at 4 feet deep (below frost line):
- Volume per footing: 3.14 ft³ → 6 bags of 80 lb per footing
- Total bags: 12 × 6 = 72 bags (buy 76 for safety)
- Material cost at $8/bag: $608
- Alternative: order 2.7 yd³ of ready-mix at $125–$175/yd³ = $338–$473 delivered (cheaper for this volume)
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