Deck Pier & Concrete Footing Calculator
Calculate Concrete Bags, Footing Volume & Post Pier Sizes
Decks need a strong, solid base to transfer their load into the soil. Cylindrical concrete footings (often poured inside cardboard Sonotubes) anchor deck posts below the frost line to prevent shifting, heaving, and structural collapse. Use this calculator to estimate the volume of concrete and bag counts (60lb or 80lb) required for your posts.
What Is a Deck Footing and Why Does It Matter?
Every wooden deck requires structural support to distribute weight (dead loads of lumber and live loads of people/snow) into the earth. Concrete piers placed beneath post bases prevent the wooden framing from contacting damp soil and sinking. Pouring a bell-shaped footing (a flared bottom) increases the bearing area, spreading the weight over more soil.
To avoid **frost heave**, the bottom of the concrete footing must extend below the local frost line. When water in the soil freezes, it expands; if the footing is too shallow, this freezing action can push the concrete upward, throwing the deck frame out of square.
The standard residential pier diameter is **12 inches** for 6x6 posts and **8 to 10 inches** for 4x4 posts. Spacing between piers is typically 6 to 10 feet depending on deck beams and span widths.
Concrete Pier Sizing & Capacity Specifications
| Tube Diameter | Volume per Foot | 60lb Bags per Foot | Best Post Match |
|---|---|---|---|
| 8 inches | 0.35 cu ft | 0.78 bags | 4x4 Wood Post |
| 10 inches | 0.55 cu ft | 1.22 bags | 4x4 or 6x6 Wood Post |
| 12 inches | 0.79 cu ft | 1.75 bags | 6x6 Wood Post, heavy beams |
| 16 inches | 1.40 cu ft | 3.11 bags | Double post columns, light commercial |
How to Use This Deck Footing Calculator
Enter the number of support posts in your deck plan. Select your pier tube diameter. Provide the total hole depth (measured from ground level) and the depth of the gravel drainage base (typically 6 inches). If your holes are flared at the bottom (bell footing), specify its diameter and height. Click Calculate to output estimates.
Frequently Asked Questions
Why put gravel under concrete footings?
A 6-inch layer of crushed gravel (aggregate) under the concrete footing allows moisture to drain away from the pier bottom, reducing frost heave risks and providing a stable foundation.
Can I pour concrete directly in the dirt hole without a tube?
While you can pour concrete directly into a soil hole, using a cardboard form (Sonotube) is highly recommended. The tube creates a smooth cylinder, preventing rough edges of soil from catching expanding frost and lifting the pier.
How many bags of concrete make a cubic yard?
A cubic yard of concrete is 27 cubic feet. It requires **60 bags of 80lb concrete** or **80 bags of 60lb concrete** to mix a full cubic yard.
- American Concrete Institute (ACI) - Residential building codes and footing concrete specifications.
- North American Deck and Railing Association (NADRA) - Safety standards and deck footing sizing guidelines.