Concrete Post Hole Calculator
A standard fence post hole (10 in diameter, 36 in deep) needs about 3 bags of 80-lb concrete — 8 posts need roughly 22 bags total. Enter your hole size, depth, and quantity below to get the exact count.
Pro Tips
- →Post holes should be 3 times the post width
- →Depth should be 1/3 to 1/2 the above-ground height
- →Extend below frost line in cold climates
- →Use fast-setting concrete mix for posts
- →Add gravel at bottom for drainage
Includes 10% waste factor
That's typically a professional pour. See costs ↓
4 short emails from Dave: what a fair quote should land at for your slab, the scope changes that swing it ±$500, and whether DIY is actually cheaper at your volume. Reply anytime — he'll review your real quote.
Cost Estimate
Estimated material costs for your project
Recommendation: Bagged Concrete
For smaller projects, bagged concrete gives you more control and less waste.
35 bags × 80lb
0.77 cubic yards + delivery
Prices vary by location and time. Contact local suppliers for accurate quotes.
For general step-by-step instructions, read our complete Concrete Fence Post Guide and Concrete Mailbox Post.
Post Hole Sizing Rules of Thumb
Two simple rules size almost every fence and deck post hole. First, hole diameter should be about 3 times the post width — a 4×4 post (3.5 in actual) takes a 10–12 in hole, and a 6×6 post (5.5 in actual) takes a 14–18 in hole. A wider collar of concrete resists the leverage of wind and frost. Second, hole depth should be at least 1/3 of the above-grade post height, plus enough to clear the frost line. A 6 ft fence (above grade) wants a minimum 24 in hole, deepened to below the frost line in cold regions.
| Post Size | Actual Width | Hole Diameter (≈3× width) | Min Depth (1/3 above-grade) |
|---|---|---|---|
| 4×4 (fence) | 3.5 in | 10–12 in | 1/3 of post height |
| 4×4 (heavy gate) | 3.5 in | 12 in | 1/2 of post height |
| 6×6 (deck/pergola) | 5.5 in | 14–18 in | below frost line |
| 8×8 (structural) | 7.5 in | 20–24 in | below frost line |
These are minimums. Gate posts, corner posts, and anything carrying lateral load (pergola, carport) should go deeper and wider than a line post. For load-bearing deck posts, confirm the footing size against the Deck Footing Calculator and your local code.
Concrete and Bags Per Hole
A post hole is a cylinder, but the post itself sits in the hole and displaces concrete. For a 4×4 (3.5 in) post the displacement is small (about 0.085 sq ft of cross-section); for a 6×6 it is larger and worth subtracting. The table below gives net concrete and 80-lb bag counts per hole (a 60-lb bag yields ~0.45 cu ft, an 80-lb bag ~0.60 cu ft), including a 10% waste allowance and net of a 4×4 post.
| Hole Diameter × Depth | Gross Volume | Net (4×4 post) | 80-lb Bags | 60-lb Bags |
|---|---|---|---|---|
| 8 in × 24 in | 0.70 cu ft | 0.56 cu ft | 1 bag | 2 bags |
| 8 in × 36 in | 1.05 cu ft | 0.84 cu ft | 2 bags | 3 bags |
| 10 in × 36 in | 1.64 cu ft | 1.38 cu ft | 3 bags | 4 bags |
| 10 in × 48 in | 2.18 cu ft | 1.81 cu ft | 4 bags | 5 bags |
| 12 in × 36 in | 2.36 cu ft | 2.09 cu ft | 4 bags | 5 bags |
| 12 in × 48 in | 3.14 cu ft | 2.80 cu ft | 6 bags | 7 bags |
Figures include 10% waste and round up to whole bags — you cannot buy a partial bag. For the full bag-count method behind these numbers, see how many bags of concrete you need.
Bags Per Hole: 4×4 vs. 6×6 Posts
The post you set changes the bag count at the same hole size, because a 6×6 displaces noticeably more concrete than a 4×4. At small hole diameters the larger post can cut a full bag off the total; at larger diameters the difference shrinks as a share of volume.
| Hole × Depth | 4×4 Post (80-lb bags) | 6×6 Post (80-lb bags) |
|---|---|---|
| 10 in × 36 in | 3 bags | 3 bags |
| 12 in × 36 in | 4 bags | 3 bags |
| 12 in × 48 in | 6 bags | 5 bags |
| 14 in × 42 in | 6 bags | 5 bags |
| 16 in × 48 in | 8 bags | 7 bags |
| 18 in × 60 in | 13 bags | 11 bags |
For large 6×6 deck or pergola holes you are approaching the point where mixing bags by hand becomes a chore — that is the dry-pour-vs-wet-set decision below.
Total Bags by Number of Posts
Fence and deck projects multiply fast. Use your per-hole bag count from the tables above, then scale by post count. The table below assumes a standard 10 in × 36 in fence hole with a 4×4 post (3 bags of 80-lb mix each).
| Number of Posts | 80-lb Bags Total | 60-lb Bags Total | Approx. Volume |
|---|---|---|---|
| 4 posts | 12 bags | 16 bags | 0.20 cu yd |
| 6 posts | 18 bags | 24 bags | 0.31 cu yd |
| 8 posts | 22 bags | 30 bags | 0.41 cu yd |
| 12 posts | 33 bags | 44 bags | 0.61 cu yd |
| 20 posts | 55 bags | 73 bags | 1.02 cu yd |
Once a job crosses roughly 1 cubic yard (around 20+ standard holes, or fewer large deck footings), ready-mix delivery or a short-load trailer can beat the cost and labor of stacking 50-plus bags. Compare the math with the Concrete Cost Calculator before you buy a pallet of bags.
Fast-Setting vs. Standard Mix
For fence posts and most deck posts, fast-setting concrete (Quikrete Fast-Setting or equivalent) is the standard choice. Per the manufacturer's directions you pour the dry mix into the hole around a braced post, add water, and it sets hard in 20–40 minutes — no premixing, and you can run a whole fence line in a day.
Standard mix (general-purpose 4,000 PSI concrete) takes 24–48 hours to reach working strength and must be mixed first. Choose it when post positions may still shift, when a structural deck footing calls for a known PSI, or when you simply want longer working time. See how to mix concrete for batch ratios.
| Factor | Fast-Setting | Standard Mix |
|---|---|---|
| Set time | 20–40 min | 24–48 hr to load |
| Mixing | Often dry-pour, no premix | Must mix first |
| Best for | Fence lines, mailbox posts | Structural deck footings |
| Working time | Very short | Long |
Wet-Set vs. Dry-Pour, Gravel Base, and Frost
The wet-set vs. dry-pour debate is specific to fast-setting mix. Wet-set (mix, then place) gives the most reliable strength and is what most pros use for load-bearing posts. Dry-pour (dump dry mix in the hole, then add water on top) is faster and convenient for light fence posts, but it relies on water reaching the bottom of the hole — fine for a 4 ft fence post, riskier for a deep structural footing. The trade-offs are laid out in the dry-pour concrete guide.
Regardless of method, add 3–4 inches of gravel at the bottom of each hole before setting the post. Gravel drains water away from the post base, which is the single biggest cause of premature rot in wood posts and rust in steel ones. Then extend the hole below the frost line — footings that stop above it heave each winter and tilt the post out of plumb.
| Region | Frost Line Depth |
|---|---|
| Deep South (FL, LA, TX coast) | 0–6 in |
| Mid-Atlantic, Pacific Coast | 12–24 in |
| Midwest, Upper South | 30–42 in |
| Northern states (MN, WI, ME) | 42–60 in |
Always verify the required depth with your local building department — code frost depths are authoritative and fence/deck inspections check them. To set posts correctly: gravel the base, plumb and brace the post in two directions, pour concrete to ~2 in below grade, dome the top to shed water, and hold braces 4 hours (fast-set) or 24 hours (standard) before loading. For a full walkthrough see the concrete fence post guide. Ready to size your job? Enter your hole diameter, depth, and post count in the calculator above for an exact bag count.