Concrete Footings for Deck Posts: Diameter, Depth, and Volume Calculation
Every deck post needs a footing, and every footing needs a concrete calculation. But the question most deck builders actually have is not just “how much concrete?”—it is “how big should this footing be?” The diameter and depth depend on the load the post carries, your soil conditions, and your local frost depth. This guide covers the complete concrete footing calculator workflow for deck posts: sizing the footing first, then calculating the volume and converting to bags or cubic yards for ordering.
How to Size Concrete Footings for Deck Posts
Footing size is not arbitrary. The footing must be large enough to spread the deck load over enough soil area that the soil can support it without settling. The International Residential Code (IRC) Table R507.3.1 provides prescriptive footing sizes based on tributary area and soil bearing capacity.
Step 1: Determine Tributary Area per Post
Tributary area is the deck area that each post supports. For a rectangular deck with evenly spaced posts:
$$\text{Tributary area} = \text{Beam spacing} \times \text{Post spacing}$$Two beams at 8 ft from the ledger, posts spaced 8 ft apart along each beam:
$$\text{Tributary area} = 8 \text{ ft} \times 8 \text{ ft} = 64 \text{ SF per post}$$Corner and end posts carry less (half the spacing on the short side), but sizing all footings for the full tributary area is standard practice—it is simpler and provides a safety margin.
Step 2: Determine Required Footing Area
Residential deck design loads are typically 50 PSF total (40 PSF live + 10 PSF dead). Multiply tributary area by total load to get the force on each footing:
$$\text{Post load} = \text{Tributary area} \times \text{Total load (PSF)}$$ $$\text{Post load} = 64 \text{ SF} \times 50 \text{ PSF} = 3{,}200 \text{ lbs}$$Divide by soil bearing capacity to find the required footing area:
$$\text{Footing area} = \frac{\text{Post load}}{\text{Soil bearing capacity (PSF)}}$$- Clay, sandy clay: 1,500 PSF
- Sand, gravel, sandy gravel: 3,000 PSF
- Crystalline bedrock: 12,000 PSF
- Sedimentary rock: 4,000 PSF
If you do not know your soil type, most jurisdictions default to 1,500 PSF (clay)—the most conservative common value.
For our example on clay soil:
$$\text{Footing area} = \frac{3{,}200}{1{,}500} = 2.13 \text{ SF}$$For a round footing, convert area to diameter:
$$d = 2 \times \sqrt{\frac{A}{\pi}} = 2 \times \sqrt{\frac{2.13}{3.14}} = 2 \times 0.824 = 1.65 \text{ ft} \approx 20\text{"}$$where \(A\) is the required footing area in SF and \(d\) is diameter in feet. Round up to the nearest standard sonotube size: 8", 10", 12", 14", 16", 18", 20", 24".
A 20” diameter sonotube provides 2.18 SF of bearing area—just enough. Bumping to 24” (3.14 SF) gives a comfortable margin and is a common choice for standard residential decks on average soil.
Step 3: Determine Footing Depth
Footing depth is driven by two requirements, and you must meet whichever is deeper:
- Frost depth: the footing bottom must be below the local frost line to prevent heaving. Frost depths range from 0” (southern states) to 72” (northern Minnesota, Alaska). Check your local building department or the IRC frost depth map.
- Minimum structural depth: IRC requires a minimum footing thickness of 6” for decks, but most jurisdictions require at least 12” of concrete in the footing.
Concrete Footing Volume Calculation for Deck Posts
Once you have the diameter and depth, the volume calculation for a cylindrical sonotube footing is:
$$V = \pi \times r^2 \times h$$where \(r\) is the radius in feet and \(h\) is the depth in feet. To convert to cubic yards, divide by 27.
Volume per footing:
$$V = \pi \times 1^2 \times 3.5 = 3.14 \times 3.5 = 11.0 \text{ cubic feet}$$Total for 6 footings:
$$V_{total} = 6 \times 11.0 = 66.0 \text{ CF} = \frac{66.0}{27} = 2.44 \text{ CY}$$Convert to 80 lb bags (each yields approximately 0.6 CF):
$$\text{Bags} = \frac{66.0}{0.6} = 110 \text{ bags}$$Convert to 60 lb bags (each yields approximately 0.45 CF):
$$\text{Bags} = \frac{66.0}{0.45} = 147 \text{ bags}$$Add 10% waste factor:
- If ordering ready-mix: 2.44 × 1.10 = 2.7 CY (minimum order applies—most plants charge a short load fee under 5 CY)
- If using bags: 110 × 1.10 = 121 bags of 80 lb mix, or 147 × 1.10 = 162 bags of 60 lb mix
Bags vs. Ready-Mix: When to Switch
The crossover point is roughly 0.75–1 CY. Below that, bags are comparable in cost. Above 1 CY, ready-mix is cheaper per yard and saves significant labor—mixing 100+ bags by hand is brutal work. For our 6-post example at 2.44 CY, ready-mix is the clear choice.
Quick Reference: Sonotube Volume Table
| Tube Diameter | Volume per Foot of Depth (CF) | 36” Deep (CF) | 42” Deep (CF) | 48” Deep (CF) |
|---|---|---|---|---|
| 8” | 0.35 | 1.05 | 1.22 | 1.40 |
| 10” | 0.55 | 1.64 | 1.91 | 2.18 |
| 12” | 0.79 | 2.36 | 2.75 | 3.14 |
| 14” | 1.07 | 3.21 | 3.74 | 4.28 |
| 16” | 1.40 | 4.19 | 4.89 | 5.59 |
| 18” | 1.77 | 5.30 | 6.19 | 7.07 |
| 20” | 2.18 | 6.54 | 7.64 | 8.73 |
| 24” | 3.14 | 9.42 | 11.00 | 12.57 |
To use this table: find your sonotube diameter, read the volume at your depth, multiply by the number of footings, and divide by 27 to convert to cubic yards. Add 10% waste.
Common Deck Footing Mistakes
- Undersizing the footing diameter: a 10” sonotube on clay soil only provides 0.55 SF of bearing—enough for a small, lightly loaded post but not for a main beam support carrying 60+ SF of tributary area
- Ignoring frost depth: footings above the frost line heave. This is a code violation in every cold-climate jurisdiction and will fail inspection
- Forgetting the bell: some codes require the footing bottom to be wider than the tube (a “bell” or “pad”). Check whether your inspector wants a formed pad at the bottom of each hole
- Not accounting for the post bracket: if using a post base bracket set into the wet concrete, the bracket displaces some volume. This is negligible for ordering purposes but matters for your pour sequence
If you are working on the concrete slab that the deck may adjoin, our concrete yardage guide for slabs walks through the volume math for flat work. For projects that include reinforcement in your footings, see the rebar quantity calculation for the bar count and weight conversion. You can also run the numbers quickly with our concrete calculator for standard footing shapes.
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