BuildCalc Guides · Concrete
How Many Bags of Concrete for a Fence Post?
Almost every fence-building question starts here. The answer depends on your hole, your post, and your frost line — here is the math, a table for common hole sizes, and the trade-offs that actually matter.
The math
A post hole is a cylinder. You calculate the hole volume, subtract the volume of the post itself, then convert to bags. For a 10 inch diameter by 24 inch deep hole with a 4×4 post:
- Hole volume = π × r² × depth. With r = 5″ and depth = 24″ that is π × 25 × 24 = 1,885 in³ (~1.09 ft³).
- Post volume in hole = 3.5 × 3.5 × 24 = 294 in³ (~0.17 ft³). A 4×4 is actually 3.5″ × 3.5″ — that's the nominal-vs-actual lumber rule.
- Net concrete = hole − post = 1.09 − 0.17 = 0.92 ft³ per hole.
- Convert to bags. An 80 lb bag yields about 0.6 ft³, so 0.92 ÷ 0.6 = 1.54 → 2 bags of 80 lb. Or 0.92 ÷ 0.45 = 2.05 → 3 bags of 60 lb.
By hole size (4×4 post)
Hole sizes vary with post type, soil conditions and frost line. Deeper, wider holes are more stable but eat more concrete.
| Hole (Ø × depth) | Net ft³ | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 8" × 24" | 0.53 | 1 | 2 |
| 10" × 24" | 0.92 | 2 | 3 |
| 10" × 30" | 1.15 | 2 | 3 |
| 12" × 30" | 1.75 | 3 | 4 |
| 12" × 36" | 2.10 | 4 | 5 |
| 12" × 48" | 2.80 | 5 | 7 |
Net of 4×4 post volume. Rounded up to whole bags. Add ~10 percent for waste on a multi-post job.
Fast-set vs. standard concrete
The biggest decision on a fence build isn't bag count — it's which mix:
- Fast-set (Quikrete Fast-Setting, Sakrete Fast-Setting) sets in 20 to 40 minutes. Brace the post, dump the dry mix, add water on top, walk away. Best for full-day fence builds with 20+ posts where rebracing is impractical. Costs about 25 to 40 percent more per bag.
- Standard mix needs to be mixed wet in a wheelbarrow or mixer, gives you 30 to 60 minutes of working time, and needs overnight bracing. Best for small jobs, repairs, or when you want to plumb posts with care.
- Gravel only (no concrete) works in well-drained soils for non-load-bearing fence posts. Pack 4 to 6 inches of compacted gravel at the bottom, set the post, then backfill with more compacted gravel. Cheaper, drains better, and the post is easier to replace later — but in freeze/thaw climates or clay soil, concrete is more reliable.
What changes the number
- Frost line. Northern climates require 36 to 48 inch deep holes — that doubles the concrete vs. a 24 inch hole. Check your local frost depth before digging.
- Post size. A 6×6 post (actual 5.5″ × 5.5″) takes up about 3× more hole volume than a 4×4, but the hole grows too. Net concrete typically lands close to the same numbers, just at a larger hole diameter.
- Crown above grade. Many pros dome the concrete 1 to 2 inches above ground at the post base — that sheds water away from the post and adds about 10 percent volume. Cheap insurance against rot.
- Corner and gate posts. These take more lateral load. Bump the hole diameter up one step (12″ instead of 10″) or add 6 inches of depth.
Calculate for your own posts
BuildCalc's concrete calculator handles round footings and sonotube columns directly — punch in your hole diameter and depth, it does the cylinder math, subtracts post volume, and gives you exact bag counts plus a live cost estimate.
Open the concrete calculator →FAQ
How many bags of concrete do I need per fence post?+
For a typical 10 inch diameter by 24 inch deep post hole around a 4x4 wood post, plan on about 2 bags of 80 lb concrete mix (or 3 bags of 60 lb). The exact count depends on your hole diameter and depth — see the table above for common sizes.
How deep should a fence post hole be?+
A good rule is to bury 1/3 of the post — so a 6 ft above-grade fence needs a 3 ft post below grade, total post length about 9 ft. In freeze-prone areas, the hole must also reach below your local frost line (24 to 48 inches depending on region) so frost heave does not lift the post.
How wide should a fence post hole be?+
The standard guidance is 3 times the post width. A 4x4 wood post (actual 3.5 inches) gets a 10 to 12 inch diameter hole. A 6x6 (actual 5.5 inches) gets 12 to 18 inches. Larger diameter = more concrete but also more lateral stability and a stronger fence.
Should I use fast-set concrete or standard mix for fence posts?+
Fast-set (Quikrete Fast-Setting, Sakrete Fast-Setting) sets in 20 to 40 minutes and lets you brace, level and walk away the same hour — ideal when you have many posts in one day. Standard mix is cheaper per bag and gives you working time but requires bracing overnight. For 20+ post jobs the time savings from fast-set usually beat the price difference.
Do I really have to mix the concrete, or can I dump it dry in the hole?+
The "dry pour" method (dump dry mix in the hole, then add water on top) works specifically with fast-setting fence post mixes — the package directions allow it. It will NOT work with standard concrete mix. Even with fast-set, mixing in a wheelbarrow or bucket gives a stronger, more uniform set, so most pros still mix even when not required.
How much waste should I add for a fence project?+
Add at least 10 percent because spilled concrete around fence posts is the rule, not the exception. On a long fence line, plan for one extra bag for every 10 posts to cover slop and uneven holes.
Related guides
- How many bags of concrete for a 10×10 slab? — slab math vs. the cylinder math used here.
- How many cubic yards of mulch for 1,000 sq ft? — for the beds along the fence line.
Related calculators
- Concrete calculator — round footings, slabs and sonotubes in one place.
- Gravel calculator — for the compacted base in the bottom of the hole.