Concrete footings transfer the weight of your structure to the ground. Getting the volume right prevents costly short pours and ensures structural integrity. The formula depends on whether you're pouring continuous strip footings or individual pier footings.
Continuous Footing Formula
Volume (cubic feet) = (Width ÷ 12) × (Depth ÷ 12) × Length. Divide by 27 to convert to cubic yards.
Example: A 100 LF footing that's 12" wide × 8" deep = (12/12) × (8/12) × 100 = 66.7 cu ft ÷ 27 = 2.47 cubic yards.
Pier Footing Formula
Volume per pier = π × (Diameter ÷ 24)² × (Depth ÷ 12). Multiply by the number of piers, then divide by 27 for cubic yards.
Example: Six 12" diameter piers at 36" deep = π × 0.25 × 3 × 6 = 14.14 cu ft ÷ 27 = 0.52 cubic yards.
Continuous vs. Pier Footings
Continuous (strip) footings — Run under load-bearing walls; standard for foundations, garage walls, and block walls
Pier (spot) footings — Support point loads like deck posts, porch columns, and fence posts
Continuous footings must be at least twice the width of the wall they support (IRC code)
Pier footings must extend below the frost line — 36"–48" in northern states
Code Requirements & Rebar Placement
IRC Section R403.1 requires minimum 12" wide × 6" deep footings for single-story
Two-story footings typically require 15"–24" width depending on soil bearing capacity
Two #4 rebar continuous in strip footings is standard residential practice
Pier footings often require a single #4 or #5 vertical rebar centered
Rebar must have 3" minimum cover from soil contact surfaces
Always get a soil bearing test for new construction — assumed 2,000 PSF is common but varies widely
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