The same design principles apply to the construction of continuous and isolated footing, although the depth of the footing is typically 1’-0” to 2’-0”.

The formula for calculating the volume of concrete for a square or rectangular application is length x width x height = volume

*make sure all the unit measurements are consistent (in., ft., yd., etc.)

Calculations

Assume the length is 6’0”, width is 4’6”, and height is 0’4”
(a common depth for a slab).

The calculation for the quantity of concrete is as follows:

Length x Width x Height = Total cubic feet
6.00 x 4.50 x 0.33 = 8.91 cubic feet

Number of bags required:

40 lb. bag
8.91 cubic feet ÷ 0.30 cubic feet/bag = 29.7
(40 lb. bags of concrete)

80 lb. bag
8.91 cubic feet ÷ 0.60 cubic feet/bag = 14.85
(80 lb. bags of concrete)