Bag yields, published
Every bag of concrete mix lists its yield — how much concrete it makes. Per QUIKRETE's published specs: an 80-lb bag yields 0.60 ft³, a 60-lb bag yields 0.45 ft³, and a 40-lb bag yields 0.30 ft³. Divide your total cubic feet by the yield and always round up: you can't buy half a bag, and running short mid-pour is the expensive mistake.
The 45-bag rule
One full cubic yard (27 ft³) takes 45 eighty-pound bags (27 ÷ 0.60), 60 sixty-pound bags (27 ÷ 0.45), or 90 forty-pound bags (27 ÷ 0.30). Memorize 45 — it's the fastest sanity check in the aisle: if your pour is about a yard, you're buying about 45 bags.
Worked examples
A 10×10 slab at 4 inches: 33.3 ft³ ÷ 0.60 = 56 eighty-pound bags (62 with 10% waste). A 4×4 pad at 4 inches: 5.3 ft³ ÷ 0.60 = 9 bags. A single 12-inch × 4-foot sonotube: 3.14 ft³ = 6 bags. The calculator above runs all three bag sizes live, with waste included.
When bags make sense
Bags win under about half a cubic yard — small pads, repairs, setting posts. Above that, a ready-mix truck is cheaper per yard and saves enormous labor: 45 bags is over 3,600 lb of lifting and mixing. If you're committed to bags for a bigger job, rent a mixer; your arms will thank you.