Bags Needed:
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Post Hole Digger — Fiberglass Handle
Quikrete Fast-Setting Concrete Mix, 50 lb
Magnetic Post Level — Fits 4×4 & 6×6
Tools worth having for this job. We don’t currently earn commission on these — they’re suggestions, not sponsored links.
How Much Concrete Per Fence Post?
The amount of concrete you need per fence post depends on the post size, hole diameter, and hole depth. Here is a quick reference:
| Post Size | Hole | Depth | Concrete | 50 lb Bags |
|---|---|---|---|---|
| 4×4 | 10" dia | 24" | 0.35 cu ft | ~1 bag |
| 4×4 | 10" dia | 36" | 0.52 cu ft | ~1.5 bags |
| 6×6 | 12" dia | 24" | 0.47 cu ft | ~1.5 bags |
| 6×6 | 12" dia | 30" | 0.59 cu ft | ~1.5-2 bags |
| 6×6 | 12" dia | 36" | 0.71 cu ft | ~2 bags |
The Formula
Concrete per post hole is the volume of the cylindrical hole minus the volume taken up by the square post:
All measurements are converted to feet before calculating. A 50 lb bag of concrete fills approximately 0.375 cubic feet, and an 80 lb bag fills approximately 0.6 cubic feet.
On this page
Fence Post Installation Tips
Hole Size Guidelines
- Diameter: The hole should be about 3 times the post width. For a 4×4 post, a 12" hole is ideal and 10" is the minimum. For a 6×6 post, use a 12" hole at minimum.
- Depth: Set posts at least 1/3 of the total post length underground, or a minimum of 24 inches. For a 6-foot fence with an 8-foot post, that means 24" deep. For taller fences or windy areas, go 30-36" deep.
Fast-Setting vs Regular Concrete
- Fast-setting concrete (like Quikrete Fast-Setting) is ideal for DIY fence posts. Just pour the dry mix into the hole around the post, add water, and it sets in 20-40 minutes. No mixing required.
- Regular concrete must be pre-mixed with water in a wheelbarrow or tub before pouring. It takes 24-48 hours to set but can be stronger for structural applications.
Crown the Concrete
Shape the top of the concrete so it slopes away from the post on all sides. This "crowning" prevents water from pooling against the post, which causes rot and shortens the lifespan of wood posts.
Fence Post Pro Tips & Mistakes to Avoid
Pro Tips
- Dig deep enough. Set posts one-third of their above-ground height into the ground (plus a few inches), and always below the local frost line.
- Size the hole right. Make the hole about 3× the post's width so there's a solid concrete collar around it.
- Gravel at the bottom. Add 4–6 inches of gravel under the post for drainage so the base doesn't sit in water and rot.
- Crown the top. Slope the concrete surface away from the post so water sheds off instead of pooling against the wood or metal.
- Brace and plumb. Level the post in two directions and brace it before the concrete sets.
Mistakes to Avoid
- Setting posts above the frost line. Shallow footings heave in winter and push the fence out of alignment.
- A flat or dished top. Concrete that traps water at the post is the leading cause of premature post rot.
- Skipping the gravel base. Wood posts encased directly on soil wick moisture and fail early.
Frequently Asked Questions
How many bags of concrete per fence post?
For a standard 4×4 post in a 10" diameter hole at 24" deep, you need about 1 bag (50 lb) of concrete per post. For a 6×6 post in a 12" hole at 30" deep, plan on 1.5 to 2 bags per post. Use the calculator above for an exact count based on your specific setup.
Should I use fast-setting or regular concrete?
For most DIY fence post installations, fast-setting concrete is the best choice. You pour the dry mix directly into the hole, add water, and the post is set in about 20-40 minutes. No mixing tub or wheelbarrow needed. Regular concrete works fine but requires pre-mixing and 24-48 hours of cure time before you can work on the fence.
How long before I can attach fence rails?
With fast-setting concrete, you can typically attach rails and fence panels after about 4 hours. With regular concrete, wait at least 24-48 hours before putting any load on the posts. In cold weather, add extra cure time regardless of the concrete type.
Can I set fence posts without concrete?
Yes, in firm, well-drained soil you can set fence posts using compacted gravel or crushed stone. This method actually improves drainage around the post base and can extend the life of wood posts. However, concrete provides significantly more rigidity and holding strength, especially for tall fences, gates, or areas with soft or sandy soil.
How deep should fence post holes be?
Bury one third of the post's total length or 2 feet, whichever is deeper, and always below the local frost line. A 6-foot fence typically uses 8-foot posts set 2.5–3 feet deep.
How wide should a fence post hole be?
About three times the post width — so 10–12 inches for a 4x4, and 12 inches for a 6x6. Too narrow and there isn't enough concrete to resist the leverage the fence puts on the post.
Should concrete cover the top of the post hole?
Crown it above grade and slope it away from the post so water sheds off. Finishing the concrete flush or dished holds water against the post, which is exactly where rot starts.
How long does fast-setting concrete take for fence posts?
It sets in 20–40 minutes and holds the post firmly within about 4 hours. Wait 24 hours before attaching rails or panels, and longer before hanging a gate — full strength still takes the usual 28 days.
Can I set fence posts in gravel instead of concrete?
Yes, and for some soils it outperforms concrete. Compacted gravel drains freely so the post never sits in water, and it's far easier to replace a post later. It gives less lateral rigidity, so it suits post-and-rail better than tall solid privacy fences that catch wind.
Sources & Standards
- American Concrete Institute — Concrete strength classes, curing, and control joint spacing
- International Residential Code (ICC) — Residential construction requirements adopted by most US jurisdictions
Estimates for planning only. This calculator sizes material quantities. It does not size structural members, verify load paths, or confirm compliance with the code in your jurisdiction. Local requirements — frost depth, footing sizes, permits, and inspections — override anything here. For structural or safety-critical work, have the design checked by a licensed engineer or contractor. See our full method and assumptions.