A fence that starts leaning two or three years after installation almost always shares the same two root causes. The holes were dug too shallow to reach stable, frost-free soil, or the crew ran out of concrete bags halfway through and topped off the last few holes with whatever gravel was lying around. Either mistake means the post pivots in its hole every time the wind pushes the fabric, and that slow motion turns a straight fence into a sagging one.
Getting the bag count right on day one avoids both problems. You do not need a degree in cubic yards to do it. Every post sits in a cylindrical hole, the hole has a known diameter and depth, and each eighty pound bag of Quikrete fills a predictable amount of that cylinder. Scale that single hole up by the number of posts and you know exactly how many bags to load onto the truck.
How deep the hole needs to be
Depth matters more than width for holding a post upright. The general rule is to bury about one third of the post length and stop only once you are below the frost line. In much of the United States that lands at 24 inches deep, though northern states want 36 to 48 inches. Shallow holes look fine on day one because the concrete is stiff and new, but a single winter of frost heave will push the post up and out of position.
A corner post or a gate post carries far more leverage than a straight run of line posts. Bury these a few inches deeper, widen the hole by an inch or two, and budget an extra half bag of concrete each. Gate posts in particular flex every time someone swings the gate, and that repeated motion finds any weak spot in the pour.
How wide a 12 inch hole should be
Width follows the post size. Aim for a hole that is 2 to 3 times the width of the post itself. A standard 4x4 post wants a 9 to 12 inch diameter hole, and a 6x6 wants a 14 to 18 inch hole. The 12 inch size comes up so often because it fits most residential 4x4 and 4x6 fence and deck posts while staying narrow enough to dig with a one-man auger.
A 12 inch hole dug 24 inches deep holds about 1.6 cubic feet of concrete. Since one eighty pound bag covers roughly 0.6 cubic feet, that single hole needs close to 3 bags. This is the number worth memorizing, because it covers the common fence-post scenario without any calculation at all.
How the Quikrete math actually lands
You do not have to do geometry by hand, but understanding the result helps you sanity-check the order. A wider hole eats concrete fast. Bumping the diameter from 10 inches to 12 inches at the same depth adds about a quarter more concrete, and dropping the depth from 18 inches to 24 inches adds another third. These small changes are why two people with the same post count can walk away with wildly different bag totals.
Round the final count up to the nearest full bag and then add a couple of spares. Waste comes from over-excavated holes, muddy spots that swallow more mix, and the inevitable bag that tears open in the trailer. Ordering exactly the computed number guarantees a run to the hardware store on Saturday afternoon.
Fast-setting versus regular concrete
Fast-setting concrete and regular concrete sell in the same eighty pound bags, but they behave differently on the job. Fast-setting mix goes into the hole dry, you pour water on top, and it cures rigid in 20 to 40 minutes, which lets you plumb the post once and walk away. It is the standard choice for DIY fence rows because you do not need a mixer or a helper standing by.
Regular concrete needs to be mixed with water to a soupy consistency and poured wet. It takes days to reach full strength, so you have to brace each post and leave the braces on overnight. The payoff is a marginally denser, stronger pour, which matters for heavy decks and pergola footings more than for a light garden fence.
Walking through a real order
Picture six 4x4 posts for a backyard fence, each sitting in a 12 inch hole dug 24 inches deep. Each hole needs about 3 bags, so the whole row calls for roughly 18 bags. Add the couple of spares for waste and the torn-bag tax, and you are at 20 bags. At sixty pounds of load per bag, that is still a manageable trip, and you will not be the person staring into a half-filled hole on a Sunday.
For a deck or pergola with heavier 6x6 posts, the holes widen to 18 inches and the bag count per post climbs past 6. It is always cheaper to over-order two bags than to under-order a whole trip. Leftover bags store fine on a shelf, and a post job rarely comes back to bite you when the concrete runs a little fat.
See also the concrete pad calculator for slabs and the fence calculator for post spacing and line length.
Post Hole Concrete Calculator
A 12 inch hole dug 24 inches deep needs about 3 bags per post, so you can buy Quikrete by the post instead of guessing.
How to use
- Measure the post size and multiply it by 2 or 3 to set the hole diameter
- Dig down to the frost line, usually 24 inches deep for most yards
- Count every post including corner posts and gate posts
- Choose fast-setting or regular concrete and read the bag total
- Add one spare bag per post for muddier than expected soil
Supported formats and limits
- One eighty pound bag equals about 0.6 cubic feet
- Hole diameter should run 2 to 3 times the post width
- Depth must reach below the local frost line
- Does not account for footings that widen at the bottom
Examples
Six posts, each in a 12 inch hole 24 inches deep, need about 18 eighty pound bags total.Common errors
- Hole diameter must be larger than the post itself
- Depth needs to be at least 12 inches
- Post count cannot be zero
Privacy and browser support
All math runs locally in your browser and nothing is uploaded.
FAQ
How many bags of Quikrete do I need per post?
A typical 4x4 fence post in a 12 inch hole 24 inches deep needs about 3 eighty pound bags. Bigger posts or deeper holes need more.
How deep should a fence post hole be?
Bury roughly one third of the post height and never go shallower than 24 inches, below the frost line.
How big should a 12 inch hole be for a 4x4 post?
A 12 inch diameter hole is a solid target for a 4x4 post, leaving room on every side for concrete to lock the post in.
Fast-setting or regular concrete?
Fast-setting Quikrete sets in about 20 to 40 minutes without mixing, which works well for solo fence work. Regular concrete needs a mixer and cures slower.
Can I set posts with gravel instead of concrete?
Light fences may survive on gravel, but decks, pergolas and gate posts really need concrete to resist leveraged loads.
Should I pour the bags dry or mix them first?
Fast-setting bags go straight into the dry hole and you add water on top. Regular bags should be mixed before pouring.