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Spike's How To

How to Calculate the Amount of Concrete You Will Need

To calculate the amount of concrete needed for a foundation wall or footings and a slab using imperial measurements requires that you convert all the measurements either feet or inches. Using feet in the calculation makes a lot of sense, making the multiplication that much more straightforward.

Formulas:
wall = length*height*thickness
footing = length*width*depth
slab = length*width*depth
To convert cubic feet to cubic yards, multiply cubic feet * 0.037037037037037
To convert cubic feet to cubic metres, multiply cubic feet * 0.028316846592
Example calculation
Wall is 29'-5" long, 7'-8" feet high, and 8 inches thick.
convert 29'-5" to feet by dividing the 5 inches by 12; 
5 ÷ 12 = 0.416666 rounded to 0.42
29'-5" = 29.42 feet

convert 7'-8" to feet;
8 ÷ 12 = 0.6666666666666667 rounded up to 0.7
7'-8" = 7.7 feet

convert  8 inches wall thickness to feet by dividing the 8 by 12;
8 ÷ 12 = 0.6666666666666667 rounded up to 0.7
8 inches = 0.7 feet
29.42 * 7.7 * 0.7 = 158.5738 rounded to 159
To fill this wall, you will need 159 cubic feet of concrete.

In a perfect world, 159 cubic feet of concrete would have been enough, but, more than likely, you will have to decide on a waste factor. Say 10%.
The 159 cubic feet of concrete = 100%
Add the 10%, and you get 110%. Multiply 159 cubic feet by 110%.
You do this by first dividing the 110 by 100;
110 ÷ 100 = 1.1
159 * 1.1 = 174.9
With a waste factor of 10%, you will need 174.9 cubic feet of concrete.

Convert 174.9 cubic feet to cubic yards by multiplying this amount by 0.037; 
174.9 * 0.037 = 6.47
174.9 cubic feet = 6.47 cubic yards of concrete.

Convert 174.9 cubic feet to cubic metres by multiplying this amount by 0.028316846592; 
174.9 * 0.028316846592 = 4.9526 rounded to 4.95
174.9 cubic feet = 4.95 cubic metres of concrete.
Example calculation
a slab is 40 long wide, 20 feet wide, and has a depth of 3.5 inches.
All you need to do before your calculation is convert the 3.5 inches to feet by dividing this amount by 12; 
3.5 ÷ 12 = 0.2916 rounded to 0.29
3.5 inches = 0.29 feet
40 * 20 * 0.29 = 232
You will need 232 cubic feet of concrete for this slab.

Before you pour this slab, you check to see how close the depth measurement is in a few places, and it looks pretty good. But there will be concrete wasted, and you decide on a 6% waste factor.
First divide 6% by 100; 
6 ÷ 100 = 0.06
232 *1.06 = 245.92 rounded to 246
With a waste factor of 6%, you will need 246 cubic feet of concrete.

Convert 246 cubic feet to cubic yards by multiplying this amount by 0.037;
246 * 0.037 = 9.102 rounded to 9.1
246 cubic feet = 9.1 cubic yards of concrete.

Convert 246 cubic feet to cubic metres by multiplying this amount by 0.028316846592;
246 * 0.028316846592 = 6.9659 rounded to 6.97
246 cubic feet = 6.97 cubic metres of concrete.
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