How to Calculate the Square Feet of a Rectangle

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When you calculate square footage of rectangle, you're calculating its area. This is a simple math exercise with any number of real-world applications. A homeowner may want to know the size of a yard to plan a garden, a contractor may need to calculate the size of a room that needs new flooring or a painter may need to determine the size of a wall that needs painting. It's a calculation that requires two measurements: the length of the rectangle and its width, measured in feet. You determine the square footage by multiplying these measurements.

TL;DR (Too Long; Didn't Read)

TL;DR: The area of a rectangle, A, is given by the product of its length, L, and its width, W. The mathematical formula is A = LW.

Making Measurements

Before you calculate the area of a large space, such as your backyard or living room floor, it's important to make sure it's actually a rectangle. If it is, its two longer sides have exactly the same length. So do its two shorter sides. You can usually measure these with a tape measure. If one of the sides is longer or shorter than its opposing side, you can still use the formula to get an approximate figure. If the difference between the measurements of opposing sides is significant, though, you may have to divide the space into a rectangle and another geometric figure, such as a triangle. You can then calculate the areas separately and add them together.

Calculating Area of a Rectangle

Once you're sure the space is a true rectangle, the procedure for calculating the square footage is straightforward:

  1. Measure the Dimensions

  2. Measure the length and width of the rectangle in feet and inches using a ruler or tape measure.

  3. Convert to Decimal Notation

  4. Make the calculation easier by converting the number of inches to a decimal fraction of a foot. You do this by dividing the number of inches by 12. For example, 5 inches = 5/12 = 0.42 feet, so a measurement of 13'5" = 13.42 feet. This step is optional, but it avoids the need to multiply fractions of 12, which can be complicated. It also allows you to do the calculation on a calculator.

  5. Calculate Area

  6. Multiply the length and width together to calculate the square footage, or area, of the space.

    Example: A homeowner wants to calculate the the square footage of the living room floor to determine how much flooring it will take to cover it. The length of the floor is 17'7" and the width is 12'3".

    Converting to decimals, the length and width are 17.58' and 12.25' Multiply to get the area 215.36 square feet.


About the Author

Chris Deziel holds a Bachelor's degree in physics and a Master's degree in Humanities, He has taught science, math and English at the university level, both in his native Canada and in Japan. He began writing online in 2010 with the goal of exploring scientific, cultural and practical topics, and at last count had reached over a hundred million readers through various sites.

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