Estimating concrete the right way
Concrete is sold by the cubic yard (US) or cubic meter (rest of world). 1 cu yd = 27 cu ft = 0.7646 m³.
A 60-lb bag yields about 0.45 cu ft cured; an 80-lb bag yields about 0.60 cu ft.
Always order 5–10% extra — over-excavation, spillage and uneven subgrade eat material fast.