Nothing stalls a fence build like a third trip to the store for concrete at 4 p.m. on a Saturday. The question — how many bags of concrete per fence post — has a real answer you can calculate in advance: for the classic 4×4 post in a properly sized hole, plan on two to three 50-lb bags of fast-setting mix per post, and up to four or five for 6×6 posts or deep gate-post holes. The exact number falls out of simple cylinder math once you know your hole size, and this article gives you the rule, the full table, and a worked whole-project example so you buy once. (Setting technique, bracing, and plumbing the posts is covered in our fence post installation guide — this page is the math.)
Step One: Size the Hole Correctly
Bag count depends entirely on hole volume, so the hole rules come first:
- Diameter: three times the post width. A 4×4 post (actual size 3.5×3.5 inches) gets a 10-to-12-inch hole; a 6×6 (actual 5.5×5.5) gets a 12-to-18-inch hole. Tighter holes leave a concrete collar too thin to grip the soil.
- Depth: one-third of the above-ground height, 24 inches minimum. A 6-foot privacy fence wants posts 2 feet in the ground (8-foot posts); gate posts and corner posts go 6 inches deeper. In frost country, get below the local frost line — 36 to 48 inches across the northern tier — or heave will jack the posts out a little more every winter.
- Add 4 to 6 inches of gravel at the bottom for drainage before any concrete goes in; the post sits on gravel, not sealed in a concrete cup that holds water against the end grain.
The Math Behind the Table
Concrete needed = hole volume minus post volume. A cylinder’s volume is pi × radius² × depth; convert to cubic feet (divide cubic inches by 1,728), subtract the post’s displaced volume, then divide by bag yield: a 50-lb bag of fast-setting mix yields about 0.375 cu ft; 60-lb ≈ 0.45 cu ft; 80-lb ≈ 0.60 cu ft. Example — 10-inch hole, 24 inches deep, 4×4 post: hole = 3.14 × 5² × 24 = 1,885 cu in; post = 3.5 × 3.5 × 24 = 294 cu in; net = 1,591 cu in = 0.92 cu ft. That is 2.5 fifty-pound bags — buy three and you will use the spare.
Bags Per Hole: The Table
Figures assume the gravel base occupies the bottom 4 inches below the listed concrete depth and are rounded to buying quantities:
| Post / hole | Concrete depth | Volume needed | 50-lb bags | 60-lb bags | 80-lb bags |
|---|---|---|---|---|---|
| 4×4 post, 10 in hole | 24 in | 0.92 cu ft | 3 | 2-3 | 2 |
| 4×4 post, 12 in hole | 24 in | 1.40 cu ft | 4 | 3-4 | 2-3 |
| 4×4 post, 12 in hole | 30 in | 1.75 cu ft | 5 | 4 | 3 |
| 4×4 post, 12 in hole | 36 in (frost) | 2.10 cu ft | 6 | 5 | 3-4 |
| 6×6 post, 12 in hole | 24 in | 1.15 cu ft | 3-4 | 3 | 2 |
| 6×6 post, 15 in hole | 30 in | 2.42 cu ft | 7 | 5-6 | 4 |
| 6×6 post, 18 in hole | 36 in | 4.55 cu ft | 12 | 10 | 7-8 |
The pattern worth internalizing: widening a hole costs far more concrete than deepening it (volume rises with the square of diameter), and 6×6 corner or gate posts in big holes consume bags shockingly fast. This is why crews often upsize only the holes that carry load — gates, corners, ends — and keep line posts at the lean spec.
Fast-Setting vs Standard Mix for Posts
For fence posts specifically, fast-setting mix (Quikrete Fast-Setting in the red bag, Sakrete Fast Setting) is the right product, and not just for speed. It is formulated to be poured dry into the hole and hydrated in place: set the post on its gravel, brace it plumb, pour the dry mix around it, then soak roughly a gallon of water per 50-lb bag into the surface. It hardens enough to hold the post unbraced in 20 to 40 minutes, which means you can set a whole fence line in a day without a forest of braces, and hang rails in 4 hours. Standard gray high-strength mix costs about a dollar less per bag ($5-$6 vs $6-$8) and works fine mixed conventionally in a wheelbarrow — the sensible choice if you are already mixing for other work, or on a 40-post project where the per-bag savings buys the beer. Strength is a non-issue either way; a fence post does not challenge 4,000 PSI concrete. What matters more: crown the top of every pour, sloping the concrete up against the post and away at the surface, so water sheds instead of pooling around the wood.
The Dry-Pour Debate, Briefly
Internet fence-building has a standing argument about dry-pouring standard (not fast-setting) concrete and letting soil moisture cure it. The short version: fast-setting products are engineered and labeled for in-hole dry placement with added water — that method is legitimate and manufacturer-backed. Dry-pouring ordinary concrete mix with no added water is a gamble on your soil’s moisture; it often produces a crusted shell around powdery, never-hydrated core material with a fraction of rated strength. Since watering the hole takes ninety seconds per post, there is no scenario where skipping it wins. Pour dry if the bag says you can, but always add the water.
Worked Example: 100-Foot Privacy Fence
Put it together for a real project — 100 feet of 6-foot wood privacy fence, posts 8 feet on center:
- Post count: 100 ÷ 8 = 12.5 sections → 13 line/end posts + 1 gate post pair upgrade; call it 14 posts total, with 2 gate posts and 2 end posts treated as heavy-duty.
- Line posts (10): 4×4 in 12-inch holes, 24 inches of concrete → 4 fifty-lb bags each = 40 bags.
- End and gate posts (4): 4×4 (or 6×6 at the gate) in 12-inch holes, 30 inches deep → 5 bags each = 20 bags.
- Total: 60 fifty-lb bags (about 1.7 tons — plan the truck accordingly), or roughly 38 eighty-lb bags if you prefer fewer, heavier lifts.
- Cost: 60 bags × ~$6.50 = about $390 in concrete, plus $40 of gravel (roughly a third of a ton) for the bases. Add 10% overage — holes never dig to theoretical dimensions, especially where an auger wallows in soft spots.
Two buying tips from the field: bags are cheaper by the pallet (often 10 to 15% off at contractor desks), and returns of unopened bags are painless at the big boxes — so err generous on the order rather than precise. Past roughly 80 bags, price out a ready-mix short load instead; at 2 yards and up, delivered concrete beats bag math on both cost and your lower back.
When to Skip Concrete and Use Gravel Instead
Concrete is not automatically the right backfill. Crushed gravel-set posts — the hole filled with 3/4-inch minus in compacted 4-inch lifts — drain water away from the wood instead of trapping it in the slight gap that opens between shrinking concrete and shrinking post, which is exactly where buried posts rot first. Gravel setting is standard practice for chain-link, common for farm and ranch fencing, favored in well-drained soils, and makes future post replacement a shovel job instead of a demolition project. Concrete keeps the advantage where the fence fights leverage and load: solid privacy fences catching wind like sails, gate posts absorbing daily swing loads, corner and end posts under tension, and any post in loose sandy soil that gravel cannot lock. A pragmatic hybrid many pros use: concrete at gates, corners, and ends; compacted gravel at line posts. However you fill the holes, the bag math above stays the same — measure the hole, do the cylinder arithmetic once, and buy the whole fence’s concrete in a single trip.
