How many bags of concrete for a fence post
A fence post stands up because of the concrete around it, and that concrete is a cylinder you can measure. The sheet below is prefilled with ten 12 in holes, 2½ ft of concrete in each (a 3 ft hole with 6 in of gravel at the bottom), bought as 80 lb bags. Change the count to your number of posts and the diameter to your hole, and it prints the total bags at the manufacturer’s yield per bag.
For this example0.8 cu yd — concrete to order, 10 % extra, about $270 in bagged.The sheet below is filled in for it; change any figure to your own.
Measurements
What you need
Quantity · Amount- 01Column volume12 in round × 2.5 ft × 1019.6 cu ft
- 02Concrete to order, 10 % extra21.6 cu ft0.8 cu yd
- 03Weight, cured150 lb per cu ft1.62 tons
- 04Ready-mix$160 per cu yd, ordered by the quarter yard1 cu yd$160
- 0580 lb bags0.6 cu ft per bag (manufacturer yield) at $836 bags$270
- Order ready-mix by the quarter yard, rounded up; the truck cannot deliver a fraction of what you order back.
The material
Prices — put in what you were quoted
Size the hole
Dig the hole three times the width of the post: 10–12 in for a 4×4 (3½ in actual), 12–18 in for a 6×6. The concrete needs mass on all sides to resist the fence leaning in wind. Keep the walls straight or slightly wider at the bottom; a hole that tapers inward is a plug that frost can lift.
Depth is a third of the above-ground height at minimum, so a 6 ft fence wants 2 ft in the ground and an 8 ft post, and the bottom of the concrete should be below the frost line, which the building department or a fence supplier will tell you; in cold climates that means 36–48 in and a longer post. Gate and corner posts take more load; go a size wider and 6 in deeper.
- 01Post, plumbed and braced before the pour
- 02Hole three times the post width, dug below the frost line
- 03Gravel, 6 in, so the post end drains
- 04Concrete, crowned above grade so water runs off
Gravel, then concrete
Put 6 in of gravel in the bottom of the hole and tamp it. The post end sits on the gravel, not in the concrete, so water that gets down the post drains instead of sitting against the wood. That 6 in is why the sheet is prefilled with 2½ ft of concrete in a 3 ft hole: set the height field to the hole depth minus the gravel. The concrete goes from the gravel to just above grade, crowned so rain runs away from the post.
Bags per hole
The sheet works out the volume of each cylinder, multiplies by the count, adds the spillage allowance and divides by the yield printed on the bag: an 80 lb bag makes 0.60 cu ft, a 60 lb bag 0.45 cu ft, a 50 lb bag 0.375 cu ft. The post displaces some of that volume (a 4×4 post in a 12 in hole takes up roughly a tenth of it), which the sheet doesn’t subtract, so the count is slightly generous. That is the right way round: a bag short with ten holes open is a trip to the store.
Fast-setting mix is poured dry around the braced post and watered in; it holds the post in 20–40 minutes, which matters when you are setting a run of posts alone. Standard mix is mixed in a barrow and shovelled in, sets in hours, and costs less per bag. Both use the same yield table, so pick the one you will use, put its shelf price on the sheet, and read the total.
Setting the posts
- Mark the post positions with string and stakes, 8 ft on center for a standard fence, 6 ft in wind or on a slope.
- Dig each hole to 3× the post width and to depth, with a post-hole digger or a rented auger.
- Drop in 6 in of gravel and tamp it.
- Set the post, brace it in two directions with scrap lumber and screws, and check plumb on two faces with a 4 ft level.
- Fill with concrete to just above grade, rod it to release air, and crown the top away from the post.
- Recheck plumb before it sets. Leave the braces on overnight for standard mix, an hour for fast-setting, before hanging rails.
Common mistakes
- A hole the size of the post. There is nowhere for the concrete to go and the post leans within a year.
- Concrete under the post. The end grain sits in a cup of water and rots first.
- A flat or dished top. Water pools at the post; crown it.
- Setting posts in a line by eye. A string pulled tight at the top and the bottom keeps the run true.
- Buying exactly the calculated bags for a job with many holes. Ground varies, holes cave, and the last one is always bigger.
Open the concrete calculator with these numbers
Questions people ask
- How many bags of concrete per fence post?
- The sheet counts it from the hole volume at the yield on the bag. Set the count to 1 to see one hole, or to your number of posts for the whole run; the total is rounded up to whole bags.
- How deep should a fence post be?
- At least a third of the height above ground, and below the frost line. For a 6 ft fence: 2 ft in the ground in a mild climate, 3–4 ft where the ground freezes deep.
- Fast-setting or regular concrete for posts?
- Fast-setting if you are working alone or need to hang rails the same day; it is poured dry and watered. Regular mix costs less and is fine if you can leave the braces on overnight.
- Should the post sit on gravel or in concrete?
- On 6 in of gravel, with the concrete around it. Water drains out the bottom instead of rotting the end of the post.