Good evening,

I have to figure out how many yards of concrete I need for a semicircle. I have a radius of 21 feet and the concrete is 4" thick. I took 3.14 x (21)2 then divided by 40 to get how many yards of concrete but that doesn't seem right.

Can you help me?

Thanks in advance.




Since you want the volume in yards (really cubic yards) I think it is easiest to change all the dimensions to yards immediately. There are 3 feet in a yard so 21 feet is 21/3 = 7 yards. There are 36 inches in a yard so 4 inches is 4/36 = 1/9 yards.

The volume of a circular cylinder is

pi x r2 x h

where r is the radius and h is the height. You want half a cylinder so the volume is

 1/2 x pi x 72 x 1/9 = 8.6 cubic yards.