The flagship question
Hole size in, bag count out — 50, 60, and 80 lb, with the spare already counted. Arriving from a state page? Your frost-line depth is filled in.
| Bag size | Exact | Buy (with spare) |
|---|
The buy column adds a 10% spare margin and rounds up. Running out mid-pour is the one mistake you can't fix.
A post hole is a cylinder. Diameter squared, times pi over four, times depth — minus the post that sits in the middle of it, because wood displaces concrete. Divide by a bag's yield and that's your count. This page does it in inches and cubic feet so the numbers match the bag.
Two site rules feed the inputs. The hole runs three times the post's width, and its bottom reaches below your frost line — that's what stops the freeze from lifting the post. If you don't know your frost line, it's one click away, and the hole depth here fills itself when you arrive from your state's page.
The stock answer: a 4×4 post in a 12″ hole, 30″ deep, takes about 1.75 cubic feet — four 80 lb bags with the spare. But the honest answer depends on your hole, which is why the calculator is at the top of this page.
Per the manufacturer data sheets: a 50 lb bag yields about 0.375 cubic feet, a 60 lb bag about 0.45, and an 80 lb bag about 0.6. Big bags are cheaper per cubic foot; your back gets a vote too.
Yes. The buy column here already adds 10% and rounds up. Coming up half a bag short with a post braced and the hole open is the classic Saturday failure — return an unopened spare instead.
Fast-setting mixes are made for exactly that: pour dry around the post, add the labeled amount of water on top, no mixing. Standard mix should be mixed wet. Either way, follow the bag — the bag is right.