Calculating the amount of concrete you require for a slab in the shape of a circle 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 circular slab:
3.14 x r (feet) x r (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
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