How Many Bags of Concrete Do You Need per Fence Post?
For a standard 4×4 post in a 10" hole at 36" deep, you need 1–2 bags of 80-lb concrete. Here's the math and a size chart.
For a standard 4×4 fence post set in a 10-inch diameter hole at 36 inches deep, you need 1 bag of 80-lb concrete — but most people use 2 to be safe and avoid running back to the store mid-job.
Here's the math if you want to check it yourself.
The Math
A 10-inch diameter hole at 36 inches deep has a volume of:
π × (5")² × 36" = 2,827 cubic inches = 1.64 cubic feet
Subtract the post volume (a 4×4 post is actually 3.5"×3.5"):
3.5 × 3.5 × 36 = 441 cubic inches = 0.26 cubic feet
Net concrete needed: 1.64 - 0.26 = 0.38 cubic feet
Wait — where'd the 0.56 number from some guides come from? Those use a slightly wider or deeper hole. Either way, one 80-lb bag yields 0.60 cubic feet of mixed concrete, which covers it. Two bags gives you a comfortable buffer.
Bags by Hole Size and Depth
| Hole Diameter | 24" Deep | 36" Deep | 48" Deep | |---------------|----------|----------|----------| | 8" | 1 bag | 1 bag | 2 bags | | 10" | 1 bag | 1–2 bags | 2 bags | | 12" | 1–2 bags | 2 bags | 3 bags |
These are for 4×4 posts (3.5"×3.5"). For 6×6 posts, deduct slightly more for the post volume, but the numbers barely change at the hole sizes above.
Frost Line: This Is Why Depth Matters
Frost line is the reason people in Minnesota dig 48-inch post holes and people in Georgia dig 24-inch ones. When the ground freezes, it expands. If your post concrete is above the frost line, it heaves — the post pops up, tilts, and eventually fails.
As a general rule:
- Southern climates (USDA zones 7–10): 24 inches is typically enough
- Moderate climates (zones 5–6): 36 inches
- Northern climates (zones 3–4): 48 inches minimum
Check your local building code — many jurisdictions specify minimum post hole depth for fences. A fence permit (if required) will usually spell it out.
Skip the Mixing: Dry-Set Your Posts
Here's a tip that saves a lot of time: for fence posts, you don't need to mix the concrete before pouring it.
Just set the post in the hole, brace it plumb, pour the dry concrete mix straight from the bag, then add water on top. The ground moisture and the water you add will hydrate the concrete from the outside in. It sets just as well as pre-mixed concrete for this application, and the bag directions actually allow for this method — it's called dry setting.
It's faster, less messy, and you don't need a wheelbarrow or mixing tub. Works for fence posts, mailbox posts, basketball hoop anchors, and similar applications.
Planning a Full Fence
Doing the math for a whole run of fence?
A 100-foot fence with posts every 8 feet needs 14 posts. At 2 bags per post, that's 28 bags of 80-lb concrete — about $140–$200 in materials just for the concrete. Add the posts, rails, and pickets and you're looking at a real project.
For anything bigger than a dozen posts, it might be worth pricing out a small ready-mix delivery or a bag of Quikrete Zip & Mix (a self-mixing product). Your lower back will have opinions.
Use our concrete calculator to figure out exact bag counts for any hole size and depth.
Frequently Asked Questions
- How many bags of concrete do I need per fence post?
- Most 4×4 fence posts in a 10-inch diameter hole need 1 bag of 50-lb fast-setting concrete or one 80-lb bag of standard mix. If your posts are 6×6 or your hole is 12 inches wide, use 2 bags per post. Always size the hole to 3 times the post diameter.
- How deep should fence posts be set in concrete?
- Bury posts 1/3 of their total length minimum — so a 6-foot fence uses 9-foot posts set 3 feet deep. In cold climates with hard frost, go below the frost line (typically 36–48 inches in northern states). Shallow posts are the #1 reason fences lean over time.
- What size hole do I need for a fence post?
- The hole diameter should be 3 times the post width. For a 4×4 post, dig a 12-inch diameter hole. For a 6×6 post, go 18 inches wide. Wider holes take more concrete but create better lateral support, especially important in loose or sandy soil.
- Should I use fast-setting or regular concrete for fence posts?
- Fast-setting concrete (Quikrete Fast-Setting or Sakrete Fast-Setting) is popular for fence posts because you pour it dry into the hole and add water — no mixing needed. It sets in 20–40 minutes, so you can move on quickly. Regular concrete is fine too but requires mixing and a longer wait before backfilling.
- How long does fence post concrete take to cure before I can attach panels?
- Fast-setting concrete is strong enough for light work in 4 hours and fully cured in 24–48 hours. Standard mixed concrete needs at least 24–48 hours before you stress the post. Either way, don't hang gates or heavy panels until the concrete is fully cured (7 days for full strength).