Deck Footing Calculator
Deck footings are the most critical structural element of any deck — undersized footings settle, causing the deck to tilt or pull away from the house. This calculator determines the number of footings required, the correct Sonotube diameter based on tributary area and soil bearing capacity (assuming 1,500 PSF — average residential soil), the required depth below frost line, bags of concrete per footing, and total material cost. Post spacing, deck load (40–70 PSF), and frost depth all affect the result. In Midwest and Northeast climates, footings often need to be 42–48 inches deep. Most jurisdictions require permit-approved footings for any deck over 30 inches off the ground. Use this calculator before your permit application to confirm footing size meets your local building department's tables.
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How to Use This Calculator
- 1Enter your measurements in the fields above. Use the units shown next to each field (ft, in, etc.).
- 2Click Calculate to get your material estimate instantly.
- 3Review the highlighted result — it shows the key quantity you need to order.
- 4Use Copy Results to save your estimate, or bookmark the URL — your inputs are encoded in the link.
Pro tip: Always add 10% to your material order to account for waste, cuts, and defects.
Frequently Asked Questions
How deep should deck footings be?
Deck footings must extend at least 6 inches below the local frost depth. In areas with no frost (Southern states), 12 inches is the minimum. Check your local building code — many jurisdictions require footings at 36" or deeper in the Midwest and Northeast.
What size footing tube form do I need?
Most residential decks use 10" or 12" diameter tube forms. For heavier loads (hot tubs, multi-level decks, or spans over 10 feet), use 16" or 18" diameter footings. The footing diameter depends on the tributary area it must support divided by soil bearing capacity.
Do deck footings need to be inspected?
Yes, in most jurisdictions. Most building departments require a footing inspection before you pour concrete — the inspector verifies depth, diameter, and placement. Schedule this inspection after digging but before pouring. Building without an inspection can result in a stop-work order or forced demolition.
How many bags of concrete per deck footing?
A standard 12-inch diameter footing at 36 inches deep requires approximately 5–6 bags of 60lb concrete. A 10-inch diameter footing at the same depth needs about 3–4 bags. Use fast-setting concrete mix (pour dry into the hole, add water) for footings — it's much easier than mixing and sets in 20–40 minutes.