How Many Bags of Concrete per Fence Post? Charts and Math
The short answer
A 4×4 wood post set in a 10 inch hole 36 inches deep takes about 3.4 bags of 50 lb fast-setting concrete, or 2.1 bags of 80 lb standard concrete mix. That assumes you stop the concrete 3 inches below the ground and finish with soil. For a whole fence, round up once at the end, not post by post. Rounding each post up separately can leave you with a dozen spare bags.
The number swings a lot with hole width. Going from a 10 inch to a 12 inch hole adds about half again as much concrete. The charts below cover the common combinations.
Where the numbers come from
The concrete fills a ring: the round hole minus the post. Here is the math the fence calculator runs for every post:
Concrete per post (cu ft) = (3.1416 × (hole diameter ÷ 2)² − post area) × (hole depth − soil on top) ÷ 1,728
All measurements are in inches. Dividing by 1,728 turns cubic inches into cubic feet. Then divide by what one bag makes:
- Quikrete Fast-Setting Concrete, 50 lb: about 0.375 cu ft per bag, according to Quikrete’s spec data sheet.
- Quikrete Concrete Mix, 80 lb: about 0.60 cu ft per bag, according to its data sheet. The same sheet gives 0.30 cu ft for a 40 lb bag and 0.45 for a 60 lb bag.
Use the actual post size, not the name. A 4×4 measures 3½ × 3½ inches (12.25 sq in). A 6×6 measures 5½ × 5½ (30.25 sq in).
Quikrete gives one figure you can check the formula against. Its sheet says two 50 lb bags set a 4 inch round post in a 10 inch hole 2 ft deep, with the mix poured to about 3 to 4 inches from the top. The formula gives 0.76 cu ft for that hole with 4 inches of soil left on top. That is 2.04 bags, so the two numbers agree.
Bags per post: chart
All rows assume 3 inches of soil on top. The bag counts are for one post and are not rounded, so you can multiply by your post count and round once.
| Post | Hole diameter | Hole depth | Concrete per post | 50 lb fast-setting bags | 80 lb Concrete Mix bags |
|---|---|---|---|---|---|
| 4×4 | 10 in | 24 in | 0.81 cu ft | 2.1 | 1.3 |
| 4×4 | 10 in | 30 in | 1.04 cu ft | 2.8 | 1.7 |
| 4×4 | 10 in | 36 in | 1.27 cu ft | 3.4 | 2.1 |
| 4×4 | 10 in | 42 in | 1.50 cu ft | 4.0 | 2.5 |
| 4×4 | 12 in | 24 in | 1.23 cu ft | 3.3 | 2.0 |
| 4×4 | 12 in | 30 in | 1.58 cu ft | 4.2 | 2.6 |
| 4×4 | 12 in | 36 in | 1.93 cu ft | 5.1 | 3.2 |
| 4×4 | 12 in | 42 in | 2.28 cu ft | 6.1 | 3.8 |
| 6×6 | 12 in | 24 in | 1.01 cu ft | 2.7 | 1.7 |
| 6×6 | 12 in | 30 in | 1.29 cu ft | 3.5 | 2.2 |
| 6×6 | 12 in | 36 in | 1.58 cu ft | 4.2 | 2.6 |
| 6×6 | 12 in | 42 in | 1.87 cu ft | 5.0 | 3.1 |
| 6×6 | 16 in | 24 in | 2.08 cu ft | 5.5 | 3.5 |
| 6×6 | 16 in | 30 in | 2.67 cu ft | 7.1 | 4.4 |
| 6×6 | 16 in | 36 in | 3.26 cu ft | 8.7 | 5.4 |
| 6×6 | 16 in | 42 in | 3.86 cu ft | 10.3 | 6.4 |
Two things stand out.
A 6×6 in a 12 inch hole uses less concrete than a 4×4 in the same hole. The bigger post takes up more of the hole. But it also leaves only about 3¼ inches of concrete around each face. Both Quikrete sheets say to dig the hole about three times the post width. For a 6×6, that means roughly a 16 inch hole, and the bag count jumps.
Depth matters less than width. Adding 6 inches of depth to a 4×4 in a 10 inch hole adds about 0.6 bag of fast-setting mix. Going from a 10 to a 12 inch hole at 36 inches adds 1.7 bags.
Worked example: 100 ft straight fence with a heavy gate
Take a 6 ft privacy fence in one straight run: 48 ft of fence, a 4 ft walk gate, then 48 ft more. Posts are 8 ft apart.
Posts. Each 48 ft section is 6 bays. The gate is 1 bay. That makes 13 bays and 14 posts: 12 line posts and 2 gate posts.
Line posts. These are 4×4s in 10 inch holes 36 inches deep. From the chart, each takes 1.266 cu ft. Twelve posts need 15.19 cu ft, which is 40.5 bags of fast-setting mix. Buy 41.
Gate posts. A gate hangs its weight off one post and swings it every day, so these are 6×6s. Following the three-times rule, they go in 16 inch holes, dug 6 inches deeper at 42 inches. Each takes 3.855 cu ft, so the pair needs 7.71 cu ft. That is 20.6 bags of fast-setting mix. Buy 21.
Total. 22.9 cu ft of concrete:
| Bag type | Line posts | Gate posts | Total to buy | Weight to haul |
|---|---|---|---|---|
| 50 lb fast-setting | 40.5 → 41 | 20.6 → 21 | 62 bags | 3,100 lb |
| 80 lb Concrete Mix | 25.3 → 26 | 12.9 → 13 | 39 bags | 3,120 lb |
Either way it is about a ton and a half of material. Check your vehicle’s payload rating before you load it, or have it delivered. Fast-setting mix uses about a gallon of water per 50 lb bag (Quikrete’s figure), so 62 bags means roughly 62 gallons of water at the site.
The fence calculator uses one post size and one hole size for the whole fence. So run it twice. First run the full layout with 4×4 posts and 10 × 36 inch holes to get the post count and the line-post concrete. Then switch to 6×6 posts and 16 × 42 inch holes and read the “per post” figure for the two gate posts.
Pour it dry or mix it first?
This depends on the product, and the two Quikrete bags give different directions.
- Fast-Setting Concrete (red bag): pour it dry around the braced post until it is 3 to 4 inches below the top. Then add water until the powder is soaked, about 1 gallon per bag. In holes deeper than 2 ft 6 in, place it in layers (lifts) of 2 ft 6 in or less so the water reaches the bottom. It sets in 20 to 40 minutes. Quikrete says to wait 4 hours before putting heavy loads on it.
- Concrete Mix (standard bag): its data sheet tells you to put about 6 inches of dry mix in the bottom of the hole. Then mix the rest with water before placing it, and slope the top away from the post so rain runs off. Wait 24 hours before putting any strain on the post.
Pouring standard mix dry and wetting it from the top is a common shortcut. It is not what the data sheet says to do, and dry pockets can stay deep in the hole.
Hole depth and the frost line
The two Quikrete sheets use different rules of thumb for depth. The fast-setting sheet says to dig half the above-ground height (3 ft for a 6 ft fence). The Concrete Mix sheet says one third of the post’s overall length. Treat both as minimums.
The number that really controls depth is your local frost line. Posts set above it can be pushed up by frozen soil. The International Residential Code leaves frost depth to each jurisdiction, which fills it in on its own climate table (Table R301.2). Michigan’s state table, for example, lists 42 inches. Warm-climate areas list far less. Fences themselves are often covered by local zoning or fence rules rather than the residential building code. Call your building department and ask two questions: what frost depth they use, and whether fences need a permit.
If frost depth pushes your holes deeper, update the depth in the chart math. A 4×4 in a 10 inch hole 42 inches deep takes 4.0 bags of fast-setting mix instead of 3.4.
When to use the concrete calculator instead
The concrete calculator has a round-column mode. It fills the whole hole and does not subtract a post. That makes it the right tool when nothing sits in the concrete, for example a pier poured in a cardboard form tube with a metal post base set on top.
For a post set directly in concrete, it over-orders. For a 4×4 in a 10 inch hole, the hole-only volume is about 18% more than the ring of concrete the post actually needs. Its default 10% waste allowance adds more on top. Use the fence calculator or the chart above for set posts.
Common mistakes
- Rounding up per post. Fourteen posts at 3.4 bags each is 47.3 bags. Round once and buy 48. Rounding each post up to 4 bags first means buying 56.
- Using nominal lumber sizes. A “4×4” is 3½ inches. Entering 4 inches understates the concrete in a 10 inch hole by about 6%.
- Leaving no soil cap. Concrete poured flat to the surface collects water against the post. Quikrete’s fast-setting directions stop the concrete 3 to 4 inches below grade and finish with soil; its Concrete Mix directions taper the top away from the post so rain runs off.
- Sizing gate posts like line posts. A gate post carries a hanging, swinging load. A bigger post in a wider, deeper hole costs a few extra bags and saves re-hanging a sagging gate.
- Holes that bell or narrow. An auger in rocky soil can leave a hole wider at the top or bottom than you think. Measure a couple of finished holes and adjust the diameter before you buy.
- Skipping 811. Installing a fence is on 811’s list of projects that need a utility locate. The service is free. Request it a few business days before you dig.
When to call the building department or a pro
Call your local building or zoning office before buying materials if any of these apply:
- The fence is taller than your city allows without a permit. Height limits are set locally.
- It sits on or near a property line, or you are in an HOA.
- It is part of a pool barrier, which has its own local rules.
- It is a retaining structure, holds back soil, or carries a heavy gate wider than a standard walk gate.
A fence contractor or engineer is worth paying for long driveway gates and fences in high-wind areas, where post size and footing depth matter more than any rule of thumb.