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Fence post concrete

How many bags of concrete your fence posts need, with the right hole size and depth.

Example sizes, change them. The answer updates as you type.

The whole post, including the part that goes in the ground.

Post size
Bag type
= 55 bags Full breakdown below
You need
55 50 lb bags
fast-setting mixincludes 10% extra
5.5 bags per post
12 in hole width
38 in hole depth

Not yet fact-checkedMethod and sources

Your post hole Side view of one post hole: 12 inches wide and 38 inches deep, 6 inches of gravel at the bottom, concrete around a 4x4 post. Not to scale: the hole is drawn wider than true. ground 4x4 post 38 in deep 6 in gravel 12 in wide
Redraws as you typenot to scale

Buy 55 bags for 10 posts: holes 12 inches wide and 38 inches deep.

How we worked it out

  1. Hole width: 3 × 4 in post width = 12 in (Quikrete's rule)
  2. Hole depth: 8 ft × 12 ÷ 3 + 6 in gravel = 38 in (Quikrete's rule), so 32 in of concrete above the gravel
  3. Concrete per hole: (π × (12 in ÷ 2)² − 3.5 in × 3.5 in) × 32 in ÷ 1,728 = 1.87 ft³
  4. 1.87 ft³ × 10 posts + 10% extra = 20.5 ft³
  5. 20.5 ft³ ÷ 0.375 ft³ per 50 lb bag = 55 bags (rounded up)
  6. Gravel: π × (12 in ÷ 2)² × 6 in × 10 posts ÷ 1,728 = 3.9 ft³

Everything else this job needs

  • Bags 55 × 50 lb fast-setting
  • Gravel for the hole bottoms6 inches in each holeoften forgotten 3.9 ft³

Live drawingDraft copy

cross-section of the hole with gravel, concrete and post.

Setting steps from QuikreteDraft copy

(gravel, dry-pour fast-setting, about 1 gallon water per 50 pound bag, sets in 20-40 minutes, wait 4 hours before heavy loads).

Worked examples

Ten 4x4 posts, 8 feet long, 50 pound fast-setting

Number of posts 10Post size 4x4 woodPost length 8 ftBag type 50 lb fast-settingExtra for waste and spills 10%Hole width 0 inHole depth 0 in

Buy 55 bags for 10 posts: holes 12 inches wide and 38 inches deep.

Bags to buy
55
Concrete per post
1.87
Hole width
12.0 in
Hole depth (with 6 inches of gravel)
38.0 in
Gravel for all holes
3.9 ft³

Matches Quikrete's example: 4 inch round post, 10 inch hole, 24 inches deep

Number of posts 1Post size 4 in roundPost length 6 ftBag type 50 lb fast-settingExtra for waste and spills NoneHole width 10 inHole depth 24 in

Buy 2 bags for 1 post: holes 10 inches wide and 24 inches deep.

Bags to buy
2
Concrete per post
0.69
Hole width
10.0 in
Hole depth (with 6 inches of gravel)
24.0 in
Gravel for all holes
0.3 ft³

Method and sources

Each calculator uses the standard formula and the manufacturer's own figures, such as bag yields, and rounds up where running short would cost you a trip. Every step is shown. Cost ranges come from named surveys with the date we checked them. Spot something wrong? Tell us and we will fix it and note the change.

  • Hole width 3x the post's nominal width; depth one-third of post length plus 6 inches of gravel (Quikrete).
  • Concrete fills from the gravel to ground level; the post's own space is taken off.
  • Wood posts use dry dressed sizes (4x4 = 3.5 inches, 6x6 = 5.5 inches, NIST PS 20); green posts are slightly larger.
  • Bag counts are rounded up to whole bags.
  • Check your local frost line and building code; they can require deeper holes.