The short answer
For a 4×4 post in a 12 in wide, 30 in deep hole, plan on 1.75 cu ft of concrete: 5 bags of 50 lb fast-setting mix or 3 bags of 80 lb concrete mix per post. A 6×6 post in a 16 in × 42 in hole takes about 12 bags of 50 lb fast-setting mix.
Size the hole first
Quikrete’s post-setting instructions call for a hole about three times the width of the post, so about 12 in for a 4×4 and 16 to 18 in for a 6×6. Dig it to a depth of one-third to one-half of the post height above ground, plus 6 in for a layer of gravel at the bottom for drainage. In cold climates, go below the frost line so the post does not heave.
- 6 ft privacy fence on 8 ft 4×4 posts: about 2 ft of concrete plus 6 in of gravel.
- Gate posts and corner posts: go deeper and use 6×6 posts or wider holes, since they carry more load.
- Deck posts: footing size is set by the load and by your local code; see the deck post and footing calculator.
Bags of concrete per post
Concrete volume is the hole minus the part the post fills, using the real size of the lumber: 3.5 in for a 4×4 and 5.5 in for a 6×6. Depth is the part filled with concrete, not counting gravel.
| Post | Hole (wide × concrete depth) | Concrete | 50 lb fast-setting | 80 lb concrete mix |
|---|---|---|---|---|
| 4×4 | 12 in × 24 in | 1.40 cu ft | 4 | 3 |
| 4×4 | 12 in × 30 in | 1.75 cu ft | 5 | 3 |
| 4×4 | 12 in × 36 in | 2.10 cu ft | 6 | 4 |
| 6×6 | 16 in × 36 in | 3.56 cu ft | 10 | 6 |
| 6×6 | 16 in × 42 in | 4.15 cu ft | 12 | 7 |
| 6×6 | 18 in × 48 in | 6.23 cu ft | 17 | 11 |
Bag yields: about 0.375 cu ft for a 50 lb bag of Quikrete Fast-Setting Concrete and 0.60 cu ft for an 80 lb bag of regular Concrete Mix. Round up per post, and buy a spare bag or two for a long fence.
Work it out for your post
Hole volume = 3.14 × (hole diameter ÷ 2)² × depth
Post volume = post width × post width × depth
Concrete = hole volume − post volume
Bags = concrete ÷ bag yield, rounded up
- Convert inches to feet: a 12 in hole has a 0.5 ft radius, and 30 in of depth is 2.5 ft.
- Hole: 3.14 × 0.5 × 0.5 × 2.5 = 1.96 cu ft.
- Post: 3.5 in is 0.292 ft, so 0.292 × 0.292 × 2.5 = 0.21 cu ft.
- Concrete: 1.96 − 0.21 = 1.75 cu ft.
- Bags: 1.75 ÷ 0.375 = 5 bags of 50 lb fast-setting mix, or 1.75 ÷ 0.60 = 3 bags of 80 lb mix.
Fast-setting or regular concrete?
Fast-setting mix is poured dry into the hole around the post and then soaked with water; it sets in 20 to 40 minutes and you can hang rails after about 4 hours. Regular concrete mix costs less per cubic foot but must be mixed first and needs a day or more before the post takes load. For a handful of fence posts, most people choose fast-setting; for many deep holes, regular mix saves money.
Calculators for this job
These calculators do the arithmetic from this guide. Each card shows the result for a typical project; open one to enter your own measurements.
Frequently asked questions
How many bags of concrete for a 4×4 post?
About 5 bags of 50 lb fast-setting concrete or 3 bags of 80 lb mix for a 12 in hole filled 30 in deep. A 24 in deep fill takes about 4 bags of fast-setting mix.
How big should a fence post hole be?
About three times the post width across, and one-third to one-half of the above-ground post height deep, plus 6 in for gravel.
Is one bag of concrete enough for a fence post?
Rarely. One 50 lb bag of fast-setting mix fills only about 0.375 cu ft, which is about 6 in of a 12 in hole around a 4×4 post.
How many bags of concrete for a 6×6 post?
About 10 bags of 50 lb fast-setting mix for a 16 in hole filled 36 in deep, or 6 bags of 80 lb concrete mix.
Sources
- Quikrete, Setting Posts
- Bob Vila, How to Install Fence Posts
- Quikrete, Concrete Mix specification sheet