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."