Cubic yards and bags for spread footings, piers and strip footings
Bag yields: 0.60, 0.45 and 0.30 ft³ for 80, 60 and 40 lb premix. Waste included.
Footing size comes from the column load divided by the allowable soil bearing pressure. Volume here is only the concrete quantity. For the slab on top, use the slab calculator.
Multiply length × width × depth for one footing, then by the number of footings. Four 2 ft × 2 ft footings 12 in deep need 4 × 4 ft³ = 16 ft³, or 0.59 yd³.
Use π/4 × diameter² × height. A 12 in tube 4 ft deep holds 0.785 × 1 × 4 = 3.14 ft³, about 6 bags of 80 lb premix. Pick Round column in the calculator.
Treat a strip footing as one long rectangle: total length × width × depth. Add the lengths of all runs, without double-counting the corners.
Footings poured against soil often need 10 percent or more, because trench sides are uneven and the bottom is rarely flat.