How the post concrete math works
- Area of the ring. The concrete fills the hole around the post: π × (hole diameter ÷ 2)² minus the post's cross-section. A 4×4 is really 3½" × 3½", so it takes up 12¼ square inches.
- Filled depth. Hole depth minus any gravel base and minus the gap you leave at the top for soil.
- Volume. Ring area × filled depth gives cubic inches; divide by 1,728 for cubic feet.
- Bags. Divide by the bag's yield. QUIKRETE lists 0.375 cu ft for a 50 lb bag of Fast-Setting Concrete Mix, and 0.30 to 0.60 cu ft for 40 to 80 lb bags of Concrete Mix. Add your waste margin, multiply by the number of posts, and round up.
Check against the manufacturer: QUIKRETE says two 50 lb bags of fast-setting mix set a 4" round post in a 10" hole 2 feet deep. The ring is π × 5² − π × 2² = 66 sq in; filled to 4" below grade that's 66 × 20 = 1,319 cu in, or 0.76 cu ft. 0.76 ÷ 0.375 = 2.04 bags, which matches their "two bags." (Shopping for one post, the calculator rounds that up to 3 so you're not caught short.)
Bags per post chart
Per post, 50 lb fast-setting mix (0.375 cu ft), filled to 3" below grade, no gravel, no waste margin.
| Post | Hole | Depth | Concrete | Bags per post |
|---|---|---|---|---|
| 4×4 | 10.5" | 24" | 0.9 cu ft | 2.4 |
| 4×4 | 10.5" | 30" | 1.16 cu ft | 3.1 |
| 4×4 | 12" | 36" | 1.93 cu ft | 5.1 |
| 6×6 | 16.5" | 30" | 2.87 cu ft | 7.6 |
| 6×6 | 16.5" | 36" | 3.51 cu ft | 9.3 |
| 6×6 | 18" | 42" | 5.06 cu ft | 13.5 |
Setting posts that stay put
- Brace each post plumb before the concrete goes in; fast-setting mix gives you minutes, not hours, to adjust.
- QUIKRETE says to use about a gallon of water per 50 lb bag of fast-setting mix, and to pour holes deeper than 2½ feet in lifts so water soaks all the way through.
- Slope the top of the concrete away from the post, or cap it with soil, so water drains off rather than pooling against the wood.
- Pouring a pad or footing too? Use the concrete calculator.