Follow these steps to determine your footing requirements:

1. Calculate Tributary Area: This is the total area of the deck that is supported by a single post. A common way to calculate this is by multiplying half the joist span by half the beam span that the post supports.
2. Select Soil Type: Choose the soil type that best matches your location. If you are unsure, selecting "Average Soil" (1500 PSF) is a common and conservative starting point. For an exact value, you may need a soil test.
3. Enter Frost Line Depth: Look up the mandated frost line depth for your specific county or municipality. This is the most critical factor for footing depth.
4. Calculate & Interpret Results: The calculator provides the minimum *diameter* for a round concrete footing and the minimum *depth* from the ground surface to the bottom of the footing.

• Footing: A concrete base that supports a foundation or post, spreading the load over a larger area.
• Tributary Area: The portion of a structure's floor or roof area that is supported by a specific structural member (in this case, a single post).
• Soil Bearing Capacity: The maximum pressure the soil can safely support, measured in pounds per square foot (PSF).
• Frost Line / Frost Depth: The depth to which the ground freezes in a particular region. The bottom of a footing must be below this line to prevent 'frost heave,' which can lift the footing and post.

"Never cheat the frost line. I've seen entire decks destroyed because the footings were too shallow and frost heave lifted the posts unevenly. If your local code says 42 inches, dig to 42 inches—no exceptions. It's the most important part of the foundation." - Foundation Contractor

"When in doubt about your soil, assume it's worse than you think. Using a lower soil bearing capacity (like 1500 PSF) will result in a wider footing, which provides a larger margin of safety. A little extra concrete is cheap insurance against settling."

Example 1: 64 sq ft tributary, average soil (1500 PSF), 36 in frost line.
At 50 PSF total load, the post carries 64 x 50 = 3,200 lb. Divided by 1500 PSF soil that needs about 2.13 sq ft of bearing, roughly a 19.8-inch circle — rounded up to a 24-inch diameter footing. Depth is the 36-inch frost line plus the 8-inch footing, so dig to 44 inches.

Example 2: 120 sq ft interior post, same soil, 42 in frost.
A heavier 120 sq ft tributary area means 6,000 lb, or 4 sq ft of bearing — a 27-inch circle, up-sized to a 30-inch diameter pier. With a 42-inch frost line the required depth jumps to 50 inches, showing why both load and geography drive the hole you dig.

• Digging Above the Frost Line: The bottom of the footing must sit below the local frost depth, not merely start there. A shallow pier heaves in winter and cracks the deck above.
• Guessing the Tributary Area: Overlooking roof or beam load carried by the post undersizes the bearing area. Add every joist and rafter that frames into the post before you calculate.
• Assuming the Best Soil: Choosing 3000 PSF gravel when you actually have soft clay can nearly halve your required diameter. When uncertain, default to the weaker 1500 PSF figure.
• Betting on a Minimum 8-Inch Diameter: Many codes set an 8-inch minimum pier regardless of math. Never pour a footing smaller than your local minimum even if the calculation says so.
• Bearing on Undisturbed Fill: A footing only works on compacted native soil. Set it into loose backfill and it settles, no matter how wide the base is.