How it works
Volume = length × width × thickness
Bag yields are typical: a 25 kg bag makes about 0.012 m³, a 40 kg bag about 0.019 m³, an 80 lb bag about 0.6 ft³ and a 60 lb bag about 0.45 ft³. Check the yield printed on the bag you buy.
For more than about 1 m³ (1.3 yd³), ordering ready-mixed concrete by truck is usually cheaper and easier than mixing bags. A common slab thickness is 10 cm (4 in) for patios and paths and 15 cm (6 in) for driveways.