10 posts (4×4) set 36 in deep take 42 × 50-lb bags of Fast-Setting concrete (or 27 × 80-lb Concrete Mix), including 10% extra — about 3.8 bags per post.
| Mix | Per post | Total | Est. cost |
|---|---|---|---|
| Fast-Setting 50 lb | 3.8 | 42 | $335 |
| Fast-Setting 60 lb | 3.2 | 35 | — |
| Concrete Mix 80 lb | 2.4 | 27 | $175 |
| Concrete Mix 60 lb | 3.2 | 35 | $170 |
Typical U.S. big-box prices checked October 2026. Tap any price to enter your store's.
| Post in ground | Hole depth | 50-lb Fast-Setting / post | 80-lb Concrete Mix / post | 50-lb bags for 10 |
|---|---|---|---|---|
| 24 in | 30 in | 2.4 | 1.5 | 27 |
| 30 in | 36 in | 3.1 | 1.9 | 35 |
| 36 in ← | 42 in | 3.8 | 2.4 | 42 |
| 42 in | 48 in | 4.5 | 2.8 | 50 |
| 48 in | 54 in | 5.2 | 3.2 | 57 |
The hole is a cylinder: π × (10.5 ÷ 2)² = 86.6 sq in. The post takes up 12.3 sq in, leaving 74.3 sq in of concrete for every inch of depth. Concrete runs 33 in around the post: 74.3 × 33 = 2,453 cu in. 2,453 cu in ÷ 1,728 = 1.42 cu ft.
A 50-lb bag of Fast-Setting makes 0.375 cu ft and an 80-lb bag of Concrete Mix makes 0.6 cu ft, giving 3.79 and 2.37 bags per post. Rounded once for all 10 posts with 10% extra, that is 42 bags — opening fresh bags for every hole would take 50.
If you filled the hole to the brim with no gravel or soil cap — how bag labels and Quikrete's online calculator count — it would take 5 bags of Fast-Setting per post. The base and soil cap save 1 bag per hole.
The full method, material data and tips are on the main post hole concrete calculator page.