Calculating the amount of concrete you require for your project can be a very confusing exercise.

Pre-mix concrete comes in 60 lb. and 80 lb. Bags. A 60 lb. bag provides 0.45 cubic feet of cured concrete and an 80 lb. bag provides 0.6 cubic feet of cured concrete.

If you have mixed concrete delivered, it is sold by the yard (which is a cubic yard = 27 cubic feet).

Square or Rectangle Slab

Following are the formulas to calculate how much concrete you will require for a square or rectangular shaped pour:

L x W x T ?12 = cubic feet of concrete (CF)

Number of bags of concrete needed
CF ? 0.45 = number of 60 lb. bags
CF ? 0.6 = number of 80 lb. bags

Number of cubic yards of concrete needed
CF ? 27 = cubic yards

L = Length in feet
W = Width in feet
T = Thickness in inches
CF = Cubic feet of concrete

Example
A patio 10' long by 8' wide - 6" thick
10 x 8 x 6 ? 12 = 40 cubic feet of concrete
40 ? 0.45 = 89 (88.8) 60 lb bags of concrete
40 ? 0.6 = 67 (66.7) 80 lb. bags of concrete
40 ? 27 = 1.5 cubic yards of concrete

Circular Slab

Following are the formulas to calculate how much concrete you will require for a circular slab:

3.14 x r x r x T ? 12 = cubic feet of concrete (CF)

Number of bags of concrete needed
CF ? 0.45 = number of 60 lb. bags
CF ? 0.6 = number of 80 lb. bags

Number of cubic yards of concrete needed
CF ? 27 = cubic yards

r = radius of the circle in feet (1/2 of the diameter)
T = Thickness in inches
CF = Cubic feet of concrete

Example
A circle 6" thick where the circle has a radius of 5.5' (11' diameter circle)
3.14 x 5.5 x 5.5 x 6 ? 12 = 47.5 cubic feet of concrete
47.5 ? 0.45 = 106 (105.6) 60 lb bags of concrete
47.5 ? 0.6 = 80 (79.2) 80 lb. bags of concrete
47.5 ? 27 = 1.8 cubic yards of concrete

Tube or Column

Following are the formulas to calculate how much concrete you will require for a circle, tube or column shaped pour:

3.14 x r x r x H ? 1728 = cubic feet of concrete (CF)

Number of bags of concrete needed
CF ? 0.45 = number of 60 lb. bags
CF ? 0.6 = number of 80 lb. bags

Number of cubic yards of concrete needed
CF ? 27 = cubic yards

r = radius of the circle in inches (1/2 of the diameter)
H = Height in inches
CF = Cubic feet of concrete

Example
A column 8' high where the column has a radius of 5" (10" diameter tube)
3.14 x 5 x 5 x 96 ? 1728 = 4.4 cubic feet of concrete
4.4 ? 0.45 = 10 (9.8) 60 lb bags of concrete
4.4 ? 0.6 = 8 (7.3) 80 lb. bags of concrete
4.4 ? 27 = 0.16 cubic yards of concrete

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