I want to pour a concrete house slab that is 48 feet long by 26 feet wide. How would I determine how much concrete would be needed?

Thanks for your help, Tom

Concrete is ordered by the cubic yard. One cubic yard of concrete would fill a container that is 1 yard by 1 yard by 1 yard or 3 feet by 3 feet by 3 feet or 27 cubic feet.

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Tom,

In order to determine the amount of concrete that you will need for your project, you will first need to determine the volume of the space that will be filled with concrete. You can do this by:

1. Defining the space as a given shape or combination of shapes.
2. Applying the appropriate formula to find the volume of each shape.
3. Adding or subtracting shapes to find the total volume of all the combined shapes.

The slab you describe is a rectangle, which is 48 feet by 26 feet. The formula used to find the area of a rectangle is:

• Area (of a rectangle) = Length x Width
• A = 48 feet x 26 feet
• A = 1,248 square feet

However, this figure actually only represents the surface area of the concrete - note that the answer is in square feet. To find the volume of concrete, you still need to multiply the surface area by the height or depth of the concrete in order to get cubic feet of concrete.

• Volume (of a rectangular shape) = Length x Width x Height

While you don't mention the thickness of the slab you wish to pour, most residential slabs are about 4 inches thick. So, if you are pouring a typical slab, you will need 1,248 square feet of concrete that is 4 inches thick. With one notable exception, Board Feet of Lumber, like units must be used when multiplying numbers to find volume. So, you need to convert 4 inches into feet and then multiply the square area in feet by the depth in feet.

• 4 inches = 1/3 of 1 foot (12 inches) = .3333 feet
• 1,248 square feet x .3333 feet = 415.9584 cubic feet

So, the volume of concrete contained in the slab is 415.9584 cubic feet. However, since concrete is generally ordered by the cubic yard - in America - that number still needs to be divided by 27 cubic feet (3-feet x 3-feet x 3-feet) in order to arrive at the number of cubic yards.

• 415.9584 cubic feet / 27 cubic feet = 15.4059 cubic yards

Keep in mind that the above number represents the amount of concrete that would be needed without factoring into account any waste, variances in the depth of concrete, pipes, columns, steel reinforcing or other objects that may be taking up space in the area to be poured. While you don't want to have too much concrete left over, it will almost certainly be considerably less expensive to pay for an extra yard that you may not need than to deal with the results of running short.

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