Concrete Calculator

How the math works

Everything gets turned into cubic feet first, then divided by 27, since there are 27 cubic feet in one cubic yard (3 ft x 3 ft x 3 ft).

Slab or footing:  cubic yards = Length(ft) x Width(ft) x (Thickness in / 12) / 27
Round column:     cubic yards = 3.1416 x (Diameter in / 24)^2 x Height(ft) / 27
With waste:       order = cubic yards x (1 + waste% / 100)
Bags:             bags = cubic feet with waste / bag yield, rounded up

Bag yields used here are the common numbers printed on premix bags: 80 lb = 0.60 cu ft, 60 lb = 0.45 cu ft, 40 lb = 0.30 cu ft. Check your brand's bag, since mixes vary.

Example: a 20 x 10 ft slab at 4 inches is 20 x 10 x 0.333 = 66.7 cu ft. Divide by 27 and you get 2.47 cubic yards. With 10% waste, order about 2.72 yards.

Most ready-mix plants sell by the quarter or half yard, and many charge extra for small loads. Ask yours before you order.