How it is calculated
Concrete is the footing length under all walls × width × depth plus extra. Longitudinal bars run the full length with 10 % added for laps and corners. Stirrups are placed at the chosen spacing; each is the perimeter of the cage with 2 in of cover plus 8 in for hooks. Rebar weight follows the standard bar weights: #3 is 0.376 lb/ft, #4 is 0.668 lb/ft and #5 is 1.043 lb/ft.
Concrete = length × width × depth × (1 + extra)Longitudinal rebar = length × number of bars × 1.1Bar size # = diameter in eighths of an inchExample
A 120 ft footing 16 in wide and 24 in deep needs 12.4 cubic yards with 5 % extra. Four #4 bars come to 528 ft (353 lb, 27 bars of 20 ft). 81 #3 stirrups at 18 in add 487 ft (182 lb). Forms are 240 sq ft and the gravel base 5.3 cubic yards.
Tips from practice
- Footings must bear below the frost line set by your local code.
- Use rebar chairs or dobies so the bars keep their cover; concrete cast against earth needs 3 in.
- Pour each footing in one go and consolidate the concrete.
Questions and answers
What rebar goes in a footing?
Residential footings commonly use two or more continuous #4 bars; your plans and local code govern.
How much concrete for a footing?
Length × width × depth in feet, divided by 27. A 120 ft footing 16 × 24 in is 11.9 cubic yards before extra.
Do I need a gravel base?
A compacted gravel base of 4–6 in gives an even, drained surface under the footing.
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