Know the difference between a threedimensional measurement and a twodimensional measurement. Square feet are calculated by only taking the flat surface area into account. That's a twodimensional measurement. Cubic feet and yards are calculated using length, width and depth (or height). This adds the third dimension and makes it a measure of volume. The best way to think of it is as if you are figuring out how much of something it will take to fill up a box.
How to Convert Square Feet to Cubic Yards
In construction and home repair, square feet is a commonly used measurement. From shingles to flooring, it's important to know the entire area of a surface before you begin repairs. It's sometimes necessary to take it one step further and determine the volume of the room as wellthe cubic footage. Since a cubic feet and yards are threedimensional measures and square feet measure two dimensions, there's no simple conversion formula. But if you know the area for which you need the measurement, you can do some quick calculations to figure it out.
Instructions


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Determine how many square feet you are working with. The square footage of a space is essentially its area, meaning you'll need to multiply the length by the width. For example, a room that is 10 feet long and 12 feet wide would be calculated as follows:
10 feet x 12 feet = 120 square feet

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Add the third dimension. Sometimes this will be as easy as measuring the height of a wall or the depth of a flowerbed, but sometimes it's a little more complicated. If you need to know cubic yards to buy items like topsoil, which are measured as such, then determine how deep you want the soil bed to be. This can be your third dimension. For example, if you wanted to fill your aforementioned room with topsoil 2 feet deep then the calculation would be:
10 feet (length) x 120 feet (width) x 2 feet (depth) =240 cubic feet

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Convert the cubic feet to cubic yards. There are 27 feet in a cubic yard. Therefore to determine cubic yards, divide the cubic feet by 27. For example:
240 cubic feet/27 = 8.8 cubic yards.

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Tips & Warnings
 Convert all the numbers to the same measurement. You can't multiply feet by inches and get an accurate measurement. If you wanted to fill the room with topsoil 4 inches deep, then you need to convert 4 inches to feet (.3).
 Photo Credit Hard working construction worker at a construction scene. image by Andy Dean from Fotolia.com