Post holes are cylinders, not rectangles — the volume math is π × r² × height. This calculator handles single posts, multi-hole batches (a 100-foot fence line in one calculation), and converts to fast-setting bag counts.
How post hole concrete is calculated
total = volume per hole × number of holes × 1.10 (10% waste)
The calculator handles single holes or batches — enter the count and it multiplies. For a 30-post fence line at 10" diameter × 30" deep, it’ll give you the total cubic yards in one shot.
Post hole depth by climate
| Region | Frost depth | Minimum hole depth |
|---|---|---|
| Florida / Gulf Coast | 0" | 24" (still need depth for stability) |
| Mid-Atlantic | 18-24" | 30" |
| Ohio Valley | 24-36" | 36" |
| Upper Midwest / New England | 36-48" | 42-48" |
| Northern Plains / Mountain West | 48-60" | 60" |
For decks, pergolas, or any structural use, always check local code — many jurisdictions require deeper holes than the IRC minimum.
Hole diameter rules
| Post size | Minimum hole diameter | Concrete per hole (30" deep) |
|---|---|---|
| 4×4 fence post (3.5") | 10" | 0.96 cu ft (~2.5 bags Fast-Setting) |
| 4×6 corner post (3.5×5.5") | 12" | 1.4 cu ft (~3.5 bags) |
| 6×6 structural (5.5") | 16-18" | 2.5-3.1 cu ft (~7 bags) |
| 8×8 heavy structural (7.5") | 20-24" | 4-6 cu ft (~10-15 bags) |
| Mailbox post (4×4) | 10" | 0.96 cu ft (~2.5 bags) |
Fast-setting concrete is the right choice for posts
Standard 50 lb bags of Quikrete Fast-Setting or Sakrete Fast-Setting are designed specifically for posts. The technique:
- Dig the hole, place 6" of compacted gravel at the bottom (for drainage)
- Set the post, plumb it, brace it
- Pour the dry concrete mix around the post (fill to ~3" below grade)
- Pour water on top per the bag instructions (typically 1 gallon per 50 lb bag)
- Wait 30 minutes — initial set
- Top off with dirt, mounded slightly to shed water away from the post
Don’t pre-mix fast-setting concrete in a wheelbarrow — it’ll set before you can pour it.
Estimating for fence projects
For a typical residential fence:
- 6 ft fence, 8 ft post spacing, 100 linear ft: 13 posts × 30" deep × 10" diameter = ~12.5 cu ft = ~33 × 50 lb fast-setting bags
- 6 ft fence, 8 ft post spacing, 200 linear ft: 26 posts × 30" deep × 10" diameter = ~25 cu ft = ~67 bags
At 1 cu yd (27 cu ft = ~72 bags), it’s worth comparing against ready-mix delivery. For a 200+ ft fence, ready-mix is cheaper IF you can pour all posts in one day.
Common mistakes
- Hole too shallow — post heaves in winter, fence racks
- Hole too narrow — not enough concrete around post for lateral stability
- No drainage gravel — water pools at base of post, accelerates rot
- Mounded backfill below grade — water collects against post
- Using regular concrete dry-in-hole — produces weak, crumbly concrete; only Fast-Setting works that way
Frequently asked questions
How deep should a post hole be?
1/3 of the post above ground, plus below frost line for your area. Common minimums: 30 inches in mid-Atlantic, 36 inches in northern US, 42 inches in upper Midwest. For a 6 ft fence post, that's a 30-36" hole. Always check local code.
How wide should a post hole be?
3× the post width. A 4×4 post (3.5" actual) needs a 10-12" diameter hole. A 6×6 post (5.5" actual) needs a 16-18" diameter hole. Wider holes give more concrete around the post for stability.
Do I need concrete or can I just tamp gravel?
Concrete: Required for any structural post, gates, or load-bearing fence. Gravel only: Acceptable for split-rail or non-structural decorative posts. Concrete + gravel: Best practice — 6" of compacted gravel at the bottom for drainage, then concrete around the post.
Fast-setting vs regular concrete for posts?
Fast-setting (Quikrete or Sakrete) is the standard choice for fence and mailbox posts. Pour the dry mix into the hole around the post, then pour water on top — sets in 30 minutes, full strength in 4 hours. Regular concrete requires pre-mixing in a wheelbarrow.
How many bags of fast-setting concrete per post?
For a standard 10" × 30" hole (0.96 cu ft after waste): about 2.5 × 50 lb bags of Quikrete Fast-Setting per post. For 6×6 posts in 16" × 36" holes: 6-7 bags. The calculator gives you total cubic feet — divide by 0.375 for 50 lb fast-setting bag count.