How much concrete for a 6×6 post?
Quick Answer
A 6×6 post set in a 12-inch diameter hole 36 inches deep requires approximately 1.5 bags of 80-lb concrete mix. For a 48-inch deep hole (code minimum for frost-prone areas), use 2 bags. The 6×6 post itself displaces about 0.1 cubic feet of volume in the hole.
Quick Reference Table
| Hole diameter × depth | Bags of 80-lb concrete |
|---|---|
| 10" dia × 30" deep | 1 bag (80-lb) |
| 12" dia × 36" deep | 1.5 bags |
| 12" dia × 48" deep | 2 bags |
| 14" dia × 42" deep | 2.5 bags |
| 16" dia × 48" deep | 3.5 bags |
How to Calculate It Yourself
- 1
Calculate hole volume: π × (radius in ft)² × depth in ft.
- 2
For 12" diameter: radius = 0.5 ft → π × 0.25 × 3 ft = 2.36 cu ft.
- 3
Subtract post volume: 0.5 ft × 0.5 ft × 3 ft = 0.75 cu ft.
- 4
Net concrete needed: 2.36 − 0.75 = 1.61 cu ft.
- 5
One 80-lb bag yields ~0.60 cu ft → 1.61 ÷ 0.60 = 2.7 → round up to 3 bags.
Pro Tip
For 6×6 structural posts (decks, pergolas), dig below the frost line in your area (often 36–48"). Use fast-setting concrete (Quikrete Fast-Setting) — pour dry mix into the hole, add water on top, no mixing required.
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