Deck Footing Size Calculator

Enter the load per post, soil bearing capacity, number of posts, and footing depth to calculate the required footing size and total concrete needed.

Created by Luis GonzalezLast updated: May 9, 2026

How to Use This Calculator

1
Enter Load per Post (lb)
Input the total weight (dead + live load) that each individual deck post will support. A typical 10x10 deck section might carry 1,500-2,500 lbs per post.
2
Specify Soil Bearing Capacity (psf)
Provide the load-bearing capacity of your soil in pounds per square foot (psf). Clay soils are typically 1,000-1,500 psf; sandy soils 1,500-2,000 psf; gravel 2,000-3,000 psf.
3
Input Number of Posts
Enter the total number of deck posts that require footings. This is used to estimate the total concrete volume.
4
Enter Footing Depth (in)
Specify the depth of each footing hole in inches, ensuring it's below the local frost line (e.g., 42 inches).
5
Review Your Results
The calculator will display the required footing diameter (round), side length (square), bearing area, and total concrete volume.

Example Calculation

A homeowner is building a deck where each post supports 2,000 lbs, the soil bearing capacity is 1,500 psf, there are 4 posts, and footings will be 42 inches deep.

Load per Post (lb)

2,000

Soil Bearing Capacity (psf)

1,500

Number of Posts

Footing Depth (in)

Results

16.5 in

Tips

Confirm Soil Bearing Capacity

The soil bearing capacity is a critical input. If unsure, consult a local geotechnical engineer or your building department for typical values in your area, or consider a soil test.

Verify Post Load

Accurately calculate the load per post by determining the tributary area each post supports. This involves calculating the deck's dead load (materials) and live load (people/furniture).

Consider Square vs. Round

While this calculator provides both, round footings (using concrete tubes) are often easier to excavate and pour. Square footings may be used for specific designs or larger loads.

Calculating Deck Footing Size for Optimal Load Distribution

The Deck Footing Size Calculator is an indispensable tool for ensuring the stability and longevity of your deck's foundation. It precisely determines the required footing diameter or side length based on the load each post carries and your soil's bearing capacity. This calculation, accounting for typical soil capacities ranging from 1,000 to 3,000 psf, prevents settlement and structural issues, ensuring your deck remains safe and level for decades.

IRC Regulations for Deck Footing Sizing

The International Residential Code (IRC) provides specific regulations for sizing deck footings to ensure they can safely transfer the deck's load to the soil without exceeding the soil's bearing capacity. IRC Section R403.1.1 on "Minimum size of footings" dictates that the area of the footing must be sufficient to support the load, based on the presumptive load-bearing values of soil provided in Table R401.4.1. For example, gravelly sand or sand with a high clay content might have an allowable bearing capacity of 2,000 pounds per square foot (psf), while silty clay could be as low as 1,500 psf.

The footing must also be sized to accommodate the total dead load (weight of the deck materials) plus the live load (people, furniture, snow), which is typically 40 psf live load for residential decks. Failure to comply with these sizing requirements can lead to footing settlement, structural damage, and unsafe conditions, making adherence to IRC standards critical for permit approval and long-term deck performance.

The Engineering Behind Deck Footing Size

The Deck Footing Size Calculator applies fundamental engineering principles to determine the minimum required bearing area for each footing. This area is then translated into equivalent round (diameter) and square (side length) footing dimensions.

required bearing area (sq ft) = load per post (lb) / soil bearing capacity (psf)
required bearing area (sq in) = required bearing area (sq ft) × 144
round footing diameter (in) = SQRT((required bearing area (sq in) × 4) / PI)
square footing side length (in) = SQRT(required bearing area (sq in))

The calculator also estimates the concrete volume for a round footing, factoring in the footing depth and the number of posts. This ensures the footing is both adequately sized and has sufficient material.

💡 For calculating the spans of other structural members, our Beam Span Calculator can help determine safe distances for various loads.

Worked Example: Sizing Footings for a 2,000 lb Post Load

Let's calculate the footing size for a deck where each post supports 2,000 lbs, the soil bearing capacity is 1,500 psf, and there are 4 posts with 42-inch deep footings.

Calculate Required Bearing Area (sq ft): 2,000 lb / 1,500 psf = 1.333 sq ft.
Convert to Square Inches: 1.333 sq ft × 144 sq in/sq ft = 191.95 sq in.
Calculate Round Footing Diameter: SQRT((191.95 sq in × 4) / PI) = SQRT(244.3) = 15.63 inches. Round to 15.6 inches.
Calculate Square Footing Side Length: SQRT(191.95 sq in) = 13.86 inches. Round to 13.9 inches.
Calculate Concrete per Footing: For a 15.6-inch diameter, 42-inch deep footing, this would be approximately 3.73 cu ft.
Calculate Total Concrete (All Posts): 3.73 cu ft/footing × 4 posts = 14.92 cu ft, or 0.55 cu yd.

The calculator determines a required round footing diameter of 15.6 inches or an equivalent square footing side of 13.9 inches, ensuring the load is safely distributed onto the soil.

💡 For other masonry projects, our Block / CMU Calculator can help estimate the materials needed for walls and foundations.

Practical Considerations for Footing Installation

Beyond calculation, the practical aspects of footing installation are crucial for ensuring structural integrity. Once the required size is determined, excavation must be precise: holes should be dug to the correct depth (below the frost line) with a flat, undisturbed bottom. Using concrete forms (Sonotubes for round footings) ensures consistent dimensions. For optimal performance, concrete should be mixed to the correct consistency and allowed to cure properly, typically for 7-14 days before placing significant loads. Reinforcing steel (rebar) may be required for larger footings or in areas with poor soil, as per local building codes. Finally, proper backfilling and compaction around the cured footings provide lateral stability and prevent future settlement. These steps, combined with accurate sizing, create a robust foundation for your deck.

IRC Regulations for Soil Bearing Capacity

The International Residential Code (IRC) provides crucial guidance on soil bearing capacity, which directly informs deck footing size calculations. IRC Table R401.4.1, "Presumptive Load-Bearing Values of Foundation Materials," is a key reference for builders and engineers. This table assigns conservative load-bearing values to various soil types:

Clay, Sandy Clay, Silt, Silty Clay: Often rated at 1,500 pounds per square foot (psf).
Sand, Silty Sand, Clayey Sand, Clayey Silt: Typically rated at 2,000 psf.
Gravel, Sandy Gravel, Silty Gravel, Clayey Gravel: Can achieve 3,000 psf.
Sedimentary Rock: May support 4,000 psf.
Crystalline Bedrock: Rated at 12,000 psf.

These values represent the maximum allowable pressure the soil can safely support. Footing designs must ensure that the total load transferred from the deck post, divided by the footing's area, does not exceed these presumptive values. In cases where soil conditions are unknown or questionable, or for very large structures, a geotechnical investigation by a qualified engineer is mandated to determine site-specific bearing capacities in 2025.

Frequently Asked Questions

What is soil bearing capacity and why is it important for deck footings?

Soil bearing capacity is the maximum pressure the soil can withstand without excessive settlement. It's crucial for deck footings because the footing must be large enough to distribute the deck's weight over a sufficient area of soil. If the footing is too small for the soil's capacity, the deck can settle, crack, or become uneven, compromising its structural integrity.

How does load per post affect footing size?

The load per post directly influences the required footing size. A higher load means the footing needs a larger bearing area to safely transfer that weight to the soil without exceeding the soil's capacity. For example, doubling the load on a post will roughly double the required footing area.

What is a typical safety factor for footing bearing area?

While this calculator focuses on the minimum required area, engineers often design footings with a safety factor of 2 to 3. This means the actual bearing area provided is 2 to 3 times the calculated minimum, offering an extra margin of safety against unexpected loads, soil variations, or future settlement.