The post size barely matters — the hole size decides everything
A 4×4 post displaces about 0.09 square feet of the hole's cross-section. A 12-inch hole has 0.79 square feet. So the concrete you actually pour is the ring around the post, and that ring grows much faster than the post does. Going from an 8-inch hole to a 12-inch hole at the same depth more than doubles the concrete required, while going from a 4×4 post to a 6×6 changes very little.
This is why “two bags per post” advice falls apart. Two bags is right for a shallow, narrow hole. In a 12-inch hole 36 inches deep you'd be short by half.
The formula, so you can verify the chart
Concrete needed is the volume of the hole minus the volume of the buried post:
concrete (cu ft) = (π × r² × depth) − (post width × post depth × depth)
r = hole radius. Keep every measurement in feet: an 8-inch hole has r = 0.333 ft, and 24 inches deep is 2 ft.
Bag yields are the standard mix figures — 40 lb ≈ 0.30 cu ft, 50 lb ≈ 0.375, 60 lb ≈ 0.45, and 80 lb ≈ 0.60. Those are the same numbers the concrete calculator uses. That last one is the useful landmark: since a cubic yard is 27 cubic feet, it takes 45 bags of 80 lb to make a yard. Past about a yard, bags stop being the sensible way to buy concrete.
One note on the charts below: bag counts are calculated from the exact volume, not the rounded figure in the first column. A 12-inch hole 36 inches deep prints as 2.10 cu ft, but the true volume is 2.101 — fractionally over seven bags of 40 lb mix, so the chart shows 8. Rounding the printed number down would leave you a shade short of filling the hole.
Chart: 4×4 posts (actual 3.5 in)
Nominal 4×4 lumber measures 3.5 inches square. The post volume is already deducted below, so these are bags to actually buy.
| Hole dia × depth | Concrete (cu ft) | 40 lb | 50 lb | 60 lb | 80 lb |
|---|---|---|---|---|---|
| 8" × 24" | 0.53 | 2 | 2 | 2 | 1 |
| 8" × 30" | 0.66 | 3 | 2 | 2 | 2 |
| 8" × 36" | 0.79 | 3 | 3 | 2 | 2 |
| 10" × 24" | 0.92 | 4 | 3 | 3 | 2 |
| 10" × 30" | 1.15 | 4 | 4 | 3 | 2 |
| 10" × 36" | 1.38 | 5 | 4 | 4 | 3 |
| 12" × 24" | 1.40 | 5 | 4 | 4 | 3 |
| 12" × 30" | 1.75 | 6 | 5 | 4 | 3 |
| 12" × 36" | 2.10 | 8 | 6 | 5 | 4 |
Buy for the worst post on the run, not the average. Hole diameters wander when you're digging clay or hitting roots, and one oversize hole can eat an extra bag.
Chart: 6×6 posts (actual 5.5 in)
The thicker post removes more volume, so these run slightly under a 4×4 in the same hole — but a 6×6 usually means a bigger, deeper hole, which more than makes up the difference.
| Hole dia × depth | Concrete (cu ft) | 40 lb | 50 lb | 60 lb | 80 lb |
|---|---|---|---|---|---|
| 12" × 24" | 1.15 | 4 | 4 | 3 | 2 |
| 12" × 36" | 1.73 | 6 | 5 | 4 | 3 |
| 14" × 24" | 1.72 | 6 | 5 | 4 | 3 |
| 14" × 36" | 2.58 | 9 | 7 | 6 | 5 |
| 16" × 24" | 2.37 | 8 | 7 | 6 | 4 |
| 16" × 36" | 3.56 | 12 | 10 | 8 | 6 |
How deep and how wide
Two rules of thumb do most of the work, and they're both about stability rather than strength:
- Depth: one-third of the above-ground height. A post standing 6 feet above grade gets 2 feet in the ground. Below 24 inches, fence posts start levering loose in soft soil.
- Diameter: about 3× the post width. A 3.5-inch 4×4 wants a 10- to 12-inch hole. Wider than that mostly buys concrete consumption.
In cold climates, push depth toward 36 inches so more of the footing sits below the active frost layer. Frost heave lifts posts unevenly, and a heaved fence line is expensive to fix.
Fast-set vs regular mix
Fast-setting post mix can go into the hole dry — set the post, pour, add water, done. No wheelbarrow, no mixing paddle, no cleanup. Regular concrete mix costs less per bag but has to be mixed first. On a 20-post fence line, the mixing is the whole job, so most people pay the premium.
Run your own post count
The concrete calculator has a Post hole mode built for exactly this, and that link opens it on the 12-inch hole 3 feet deep around a 4×4 from the chart above — four bags of 80 lb mix, with the buried post deducted. Change the diameter, depth, and post size to your own, then multiply by your post count. It applies the waste factor too, and shows the cubic yards alongside the bags so you can see when it's time to call a ready-mix truck instead of stacking bags.