How Many Bags of Concrete per Fence Post?
Quick answer: a 4×4 post set in a 10 in wide, 24 in deep hole needs about 0.92 cu ft of concrete: 3 bags of 50 lb fast-setting mix (0.375 cu ft each) or 2 bags of 80 lb concrete mix (0.60 cu ft each). Concrete needed = hole volume − post volume. Bigger holes and deeper posts add up quickly, so use the chart below.
Bag count depends on three numbers: hole diameter, hole depth, and post size. A round hole is a cylinder (π × radius² × depth). Subtract the space the post takes up, convert cubic inches to cubic feet (÷ 1,728), and divide by the bag yield.
For a whole fence line, multiply by the number of posts, or use the post hole concrete calculator to do it in one step.
Bags of concrete per post chart
Concrete fills the whole hole depth around the post, with no gravel base. Results are rounded up to whole bags, with no waste added. Bag yields: 50 lb fast-setting ≈ 0.375 cu ft, 80 lb concrete mix ≈ 0.60 cu ft, 60 lb ≈ 0.45 cu ft.
| Post | Hole width | Hole depth | Concrete (cu ft) | 50 lb fast-set | 80 lb mix | 60 lb mix |
|---|---|---|---|---|---|---|
| 4×4 (3.5 in actual) | 10 in | 24 in | 0.92 | 3 | 2 | 3 |
| 4×4 (3.5 in actual) | 10 in | 36 in | 1.38 | 4 | 3 | 4 |
| 4×4 (3.5 in actual) | 12 in | 24 in | 1.40 | 4 | 3 | 4 |
| 4×4 (3.5 in actual) | 12 in | 36 in | 2.10 | 6 | 4 | 5 |
| 6×6 (5.5 in actual) | 12 in | 24 in | 1.15 | 4 | 2 | 3 |
| 6×6 (5.5 in actual) | 12 in | 36 in | 1.73 | 5 | 3 | 4 |
| 6×6 (5.5 in actual) | 16 in | 24 in | 2.37 | 7 | 4 | 6 |
| 6×6 (5.5 in actual) | 16 in | 36 in | 3.56 | 10 | 6 | 8 |
| 2-3/8 in round steel | 8 in | 24 in | 0.64 | 2 | 2 | 2 |
| 2-3/8 in round steel | 8 in | 36 in | 0.95 | 3 | 2 | 3 |
| 2-3/8 in round steel | 10 in | 24 in | 1.03 | 3 | 2 | 3 |
| 2-3/8 in round steel | 10 in | 36 in | 1.54 | 5 | 3 | 4 |
How to size the post hole
- Width: a common rule is a hole about 3 times the post width. That is 10–12 in for a 4×4 and 12–16 in for a 6×6.
- Depth: Quikrete’s product sheets give between one third and one half of the post height above ground, and below the frost line in cold climates. A 6 ft fence typically means a 2–3 ft deep hole. Check local code for frost depth.
- Gravel base: many builders put 4–6 in of gravel in the bottom for drainage. That reduces the concrete needed. Quikrete’s data sheet, for example, says two 50 lb fast-setting bags set a 4 in post in a 10 in hole 2 ft deep, which matches a hole with a gravel base rather than concrete all the way down.
Worked example: a 100 ft fence
A 100 ft fence with posts every 8 ft needs 13 line posts plus 1 end post = 14 posts. With 4×4 posts in 10 in holes 24 in deep: 14 × 0.92 = 12.9 cu ft of concrete. That is 35 bags of 50 lb fast-setting mix (12.9 ÷ 0.375 = 34.3, rounded up) or 22 bags of 80 lb mix (12.9 ÷ 0.60 = 21.5). Buy one or two extra for spills and uneven holes.
Fast-setting mix or regular concrete?
Fast-setting mix is poured dry into the hole and soaked with water. It sets in about 20–40 minutes, which suits fence posts. Regular concrete mix is mixed first and takes longer to set, but an 80 lb bag yields more concrete for the money. Follow the directions on the bag for water amounts.
Frequently asked questions
How many bags of concrete do I need for a 4x4 post?
For a 10 in wide, 24 in deep hole, about 3 bags of 50 lb fast-setting mix or 2 bags of 80 lb concrete mix. A 12 in hole at the same depth needs about 4 fast-setting bags or 3 bags of 80 lb mix.
How many bags of concrete for a 6x6 post?
In a 12 in hole 24 in deep, about 4 bags of 50 lb fast-setting mix or 2 bags of 80 lb mix. A 16 in hole 36 in deep needs about 10 fast-setting bags or 6 bags of 80 lb mix.
How deep should a fence post hole be?
Usually one third to one half of the post height above ground, and below the local frost line. For a 6 ft fence that is typically 2–3 ft.
How much concrete is in a 50 lb bag of fast-setting mix?
Quikrete lists about 0.375 cubic feet per 50 lb bag of Fast-Setting Concrete Mix.
Should I put gravel at the bottom of a post hole?
Many builders add 4–6 in of gravel for drainage, which helps wood posts last longer and reduces the concrete needed. Follow your local practice and the post manufacturer's guidance.
Related calculators and guides
Sources
- Quikrete: Fast-Setting Concrete Mix data sheet (yield and post-setting example)
- Quikrete: Fast-Setting Concrete spec data (4 in post example)
- Quikrete: Concrete Mix data sheet (bag yields)
Updated: October 17, 2026 by the YardsCalculator editorial team. Figures are planning estimates; confirm weights, truck capacity, and prices with local suppliers and haulers.