Deck Footing Calculator
Calculate concrete bags for deck footings - by post count, footing diameter and frost line depth.
The deck footing calculator returns the concrete bag count for any deck build - residential decks typically use one footing per support post, with 6-9 footings for a 10x12 deck and 12-16 footings for larger spans. Footing diameter depends on post size and load: 10 inch for light-load 4x4 posts, 12 inch standard for most 4x4 deck posts, 16 inch for 6x6 posts carrying heavy decks or roofs. Depth always matches frost line in your region - typically 36 inches in northern US states, 42-48 inches in Canada and the upper Midwest, 24 inches in warm states with no frost. The calculator subtracts post displacement so the bag count matches what you actually pour.
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Frequently asked questions
A 10x12 deck typically needs 6-9 footings. For 9 footings at 12 in diameter, 36 in deep with 6x6 posts (5.5 x 5.5 actual): each footing needs 1.85 cu ft of concrete after displacement, total 18.3 cu ft with 10% waste. That is 31 bags of 80lb or 41 bags of 60lb. For a smaller 6-footing layout with 4x4 posts and 10 in diameter holes, expect about 15 bags of 80lb.
Always below the frost line. In northern US (Minnesota, Wisconsin, North Dakota): 42-48 inches. Upper Midwest and New England: 36-42 inches. Mid-Atlantic and Midwest: 30-36 inches. Southern states: 12-24 inches (or none in frost-free zones). Frost line depths are set by local building code - check your jurisdiction. Footings above frost line will heave each winter and lift the deck out of level.
Usually yes - 12 inch diameter is standard for 4x4 posts, 16 inch for 6x6. The bigger post itself does not need more concrete; the larger footing distributes the heavier load (6x6 posts carry bigger spans or rooflines). Some inspectors require footing diameter at least 2-3x the post width regardless of load. Always check local code.
Yes - Sonotubes (cardboard concrete forms) above grade and a wider bell-shaped footing below frost line is the standard pier-and-tube deck footing. The bell at the bottom (often 24 inch diameter for a 12 inch tube above) distributes load on the soil; the tube above grade keeps the column straight. Pre-cast concrete piers (Diamond Pier and similar) are an alternative that requires no concrete pouring at all.
Three common methods: (1) cast a galvanized post anchor (Simpson ABU or similar) into the wet concrete at the top of the footing, then bolt the post to the anchor after the concrete cures; (2) bury the post directly in concrete - cheaper but the post rots faster; (3) use a post saddle attached to a J-bolt cast into the footing. Method 1 is most common for modern code-compliant decks.