The old-fashioned mathematical formula way and the shortcut way
Tim Odell, of Odell Complete Concrete, describes a straightforward process for bidding out concrete for the curvy jobs.
Two methods for calculating the area of a circle
- You can do the pi*r2 method, where pi = 3.141592..., r = the radius of the circle, and squared is squared.
- Or, you can shortcut to a ratio: a circle within a square box occupies 78.5% of the space.
For a circle that is 10 feet wide, the box would be 10 feet x 10 feet = 100 square feet, making the circle 785 sq ft.
For sizes other than 10 x 10, multiply the square footage by (.785) to find the area of the circle. The example in the video has a slab that is a quarter-circle occupying an 18 feet x 18 feet box.
- 18 x 18 = 324; 324 x .785 = 254.35 square feet
- (3.14) x (182) = 3.14 x 324 = 1017.36; divided by 4 (because it is a quarter of the circle) = 254.34 square feet
Notice that in the first method, you don't have to multiply the square footage to account for the whole circle and then divide back down to a quarter-circle.
To determine yardage for a concrete order, factor in the thickness of the slab
In the example of the video, the slab will be four inches thick.
- The first step is to convert four inches to feet so that the dimensions will be consistent. Four inches is .3333 foot. (254.34 square feet) x (.3333 foot thickness) = 84.77 cubic feet
- Next, convert cubic feet to cubic yards. There are 27 cubic feet in one cubic yard (3 x 3 x 3 = 27). Divide the cubic feet by 27 to determine cubic yards: 84.77 / 27 = 3.14 cubic yards.
Experienced concrete installers usually add a little to what they calculate because the real world is usually different from a piece of paper. Tim says that he will order about 3.5 or 3.75 yards of concrete for this.
—Odell Complete Concrete is located in southern Calif, but their YouTube channel is everywhere.