Concrete Fence Post Calculator
Setting fence posts is the one job where you really don't want to run out of concrete mid-pour. This calculator figures the volume of each post hole, subtracts the space the post itself takes up, and converts the rest into ready-mix bags β so you know exactly how many to throw in the truck.
How this calculator works
The calculator computes the cylinder volume of each hole (Ο Γ radiusΒ² Γ depth), subtracts the post's volume over the buried depth, multiplies by your number of posts, and adds a 10% waste allowance for spillage and uneven holes. It then divides by the yield of your chosen bag size. Yields used: 40 lb = 0.30 cu ft, 50 lb = 0.375 cu ft, 60 lb = 0.45 cu ft, 80 lb = 0.60 cu ft.
Frequently asked questions
How many bags of concrete do I need per fence post?
For a typical 4Γ4 post in a 12"-wide, 24"-deep hole, you need about 1.5 cu ft of concrete β that's roughly three 60 lb bags or two and a half 80 lb bags per post. Bigger holes eat concrete fast: going to a 16" hole nearly doubles the volume.
How deep should a fence post hole be?
Bury at least one-third of the post's above-ground height, with a 24" minimum for a 6 ft privacy fence. In cold climates, go below the frost line (30β48" in the northern U.S.) so freeze-thaw cycles don't heave the post.
Should I use fast-setting or regular concrete for fence posts?
Fast-setting mix is the DIY favorite: you pour it dry into the hole, add water on top, and it sets in 20β40 minutes with no mixing. Regular mix is slightly cheaper and stronger, but for fence posts the difference rarely matters β convenience usually wins.