Fence post concrete calculator: how many bags of concrete do I need per fence post?
38 bags
1.4 cu ft per hole · 14 cu ft in all · a bag yields 0.375 cu ft · for 10 posts in 12 × 24 in holes
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3/8 or 10' 6".
38 bags
1.4 cu ft per hole · 14 cu ft in all · a bag yields 0.375 cu ft
Source: Quikrete Fast-Setting Concrete Mix data sheet, No. 1004 (revised November 2025), read October 1, 2026. Data sheet (PDF)
Ten 4 × 4 posts in 12 inch holes with 24 inches of concrete take 38 bags of 50 lb fast-setting mix: each hole holds 1.4 cubic feet around the post, and a 50 lb bag yields 0.375 cubic feet (Quikrete data sheet).
A worked example
Seven 6 × 6 posts in 16 in holes with 30 in of concrete, in 60 lb bags needs 47 bags of 60 lb. A 16 in hole is 201 sq in, less the 30.25 sq in of the post, times 30 in: 2.97 cu ft a hole and 20.8 cu ft in all. At 0.45 cu ft a bag that is 46.1, so 47.
How big and how deep should a fence post hole be?
Quikrete's setting guide digs the hole 3 times the width of the post, so about 12 inches across for a 4 × 4, and 1/3 to 1/2 of the post's height above ground deep: a 6 ft fence gets a hole at least 2 ft deep. Six inches of gravel go in the bottom for drainage, the post is braced plumb, the dry mix is poured to 3 or 4 inches below ground level, and about a gallon of water per 50 lb bag is poured on and left to soak. It sets hard in 20 to 40 minutes and takes load after 4 hours. The depth to enter above is the concrete, not the whole hole. A post that carries a deck or other structure is a footing: the same guide puts it at least 6 inches below the frost line, which your building department sets.
What else it is called
Fence post concrete is also searched as post hole concrete, post footing or “how much Quikrete per post”. Fast-setting mix is the one poured dry into the hole; regular concrete mix works too but is mixed first, and its yields are on the concrete slab page.
Common sizes
| Hole (diameter × concrete depth) | 4 × 4 post, cu ft | 4 × 4 post, 50 lb bags | 6 × 6 post, cu ft | 6 × 6 post, 50 lb bags |
|---|---|---|---|---|
| 8 × 24 in | 0.53 | 1.4 | 0.28 | 0.7 |
| 10 × 24 in | 0.92 | 2.5 | 0.67 | 1.8 |
| 12 × 24 in | 1.40 | 3.7 | 1.15 | 3.1 |
| 12 × 30 in | 1.75 | 4.7 | 1.44 | 3.8 |
| 12 × 36 in | 2.10 | 5.6 | 1.73 | 4.6 |
| 16 × 36 in | 3.93 | 10.5 | 3.56 | 9.5 |
| 18 × 36 in | 5.05 | 13.5 | 4.67 | 12.5 |
Per hole, the cylinder less the post. Bags are shown unrounded so you can multiply by your posts; the calculator rounds up once on the total.
Where the numbers come from
- Bag yields: Quikrete Fast-Setting Concrete Mix technical data sheet, product No. 1004, Division 3, 03 31 00 Structural Concrete, revised November 2025. A 50 lb bag yields approximately 0.375 cu ft and a 60 lb bag 0.45 cu ft; there is no 80 lb bag of this mix. The sheet's own example, two 50 lb bags for a 3 in square post in a 9 in hole 30 in deep, comes out of this arithmetic once the 6 in gravel base and the 3 to 4 in of fill on top are taken off the 30 in. Read October 1, 2026. Data sheet (PDF)
- Hole size: Quikrete “Setting Posts” project guide. Dig the hole 3 times the width of the post (a 12 in hole for a 4 in post) and 1/3 to 1/2 of the post's height above ground deep (at least 2 ft for a 6 ft fence), put 6 in of gravel in the bottom, pour the mix in dry to 3 to 4 in below grade and add about a gallon of water per 50 lb bag. Read October 1, 2026. Source
- Actual post sizes: Wood Handbook, Wood as an Engineering Material (USDA Forest Service, Forest Products Laboratory, FPL-GTR-190, 2010), Table 6-6, American Standard Lumber sizes, which reproduces the minimum dressed sizes of the American Softwood Lumber Standard PS 20: a nominal 4 in dresses to 3 1/2 in dry and a nominal 6 in to 5 1/2 in. So a 4 × 4 post takes 12.25 sq in of the hole, a 4 × 6 19.25 and a 6 × 6 30.25. Read October 1, 2026. Wood Handbook (PDF)
- The volume is geometry: a cylinder of the hole diameter and the concrete depth, less the post's own cross-section, in cubic feet (÷ 1,728). Bags are the total for all holes divided by the yield, rounded up once, because the mix is poured dry and a part bag goes on to the next hole.
Bag counts are rounded up over the whole run, not hole by hole. This is the concrete only: it does not count the 6 in of gravel under each post, and the depth to enter is the depth of concrete, which in Quikrete's method is the hole less the gravel base and the 3 to 4 in of soil put back on top. A post that carries a deck or a roof is a footing, and goes below the frost line.
By Javier Miralles · Last updated . How this site is made.
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