Calculating the amount of concrete you require for a square or rectangular slab can be confusing.
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Notes:
- Pre-mix concrete comes in bags that are 60 lbs. and 80 lbs.
- A 60 lb. bag provides 0.45 cubic feet of cured concrete.
- A 80 lb. bag provides 0.6 cubic feet of cured concrete.
- If you have concrete delivered it is sold by the yard (which is a cubic yard = 27 cubic feet).
Following are the formulas to calculate how much concrete you will require for a square or rectangular shaped pour:
L (feet) x W (feet) x T (inches) ÷ 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
10 x 8 x 6 ÷ 12 = 40 cubic feet of concrete
40 ÷ 0.6 = 67 (66.7) 80 lb. bags of concrete
40 ÷ 27 = 1.5 cubic yards of concrete