How To Calculate How Much Concrete Do I Need For My Project?

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

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

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

Square or Rectangle Slab

Following are the formulas to calculate how many cement bags you will require for a square or rectangular shaped pour:

Length x Width x Thickness divided by 12 = cubic feet of concrete (CF)

Number of bags of cement needed
CF divided by 0.45 = number of 60 lb. bags
CF divided by 0.6 = how many 80 lb. bags

Number of cubic feet of concrete needed
CF divided by 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 divided by 12 = 40 cubic feet of concrete
40 divided by 0.45 = 89 (88.8) 60 lb bags of concrete
40 divided by 0.6 = 67 (66.7) 80 lb. bags of concrete
40 divided by 27 = 1.5 cubic yards of concrete

Circular Slab

Following are the formulas to calculate the concrete you will need for a circular slab:

3.14 x r x r x T divided by 12 = cubic feet of cement (CF)

Number of bags of cement needed
CF divided by 0.45 = number of 60 lb. bags
CF divided by 0.6 = number of 80 lb. bags

Number of cubic yards of concrete needed to fill circle
CF divided by 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 divided by 12 = 47.5 cubic feet of concrete
47.5 divided by 0.45 = 106 (105.6) 60 lb bags of concrete
47.5 divided by 0.6 = 80 (79.2) 80 lb. bags of concrete
47.5 divided by 27 = 1.8 cubic yards of concrete

Tube or Column

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

3.14 x r x r x H divided by 1728 = cubic feet of cement (CF)

Number of bags of concrete needed
CF divided by 0.45 = number of 60 lb. bags
CF divided by 0.6 = total quantity of 80 lb. bags

Number of cubic yards of concrete needed
CF divided by 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 and the column has a radius of 5" (10" diameter tube)
3.14 x 5 x 5 x 96 divided by 1728 = 4.4 cubic feet of concrete
4.4 divided by 0.45 = 10 (9.8) 60 lb bags of concrete
4.4 divided by 0.6 = 8 (7.3) 80 lb. bags of concrete
4.4 divided by 27 = 0.16 cubic yards of concrete

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