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.
Assumptions to Check
Before ordering materials, confirm the dimensions, product coverage, waste factor, and local installation requirements for your project. Manufacturer coverage tables and local code rules can change the final quantity, especially for structural work, exterior projects, and irregular layouts.
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