A rectangle has two sets of congruent sides. The width is the shorter dimension of a rectangle and the measurement of one of the two longer sides is the length. Width is also used informally to refer to the circumference of a person's waist.
How to Calculate the Width
Determine which sides of the rectangle are the shorter pair. If it is too close to tell, you will have to measure both.
Measure one of the two shorter sides. Because a rectangle has a two sets of congruent sides, it does not matter which side you measure. This is the width.
If you know the area and the length but not the width, you can also find the width by dividing the area by the width. For example, if you know you want a room to be 105 square feet and you know it will be 15 feet long, divide 105 by 15 to get 7 so the width of the room would be 7 feet.
To find the width of a person, place the start of the measuring tape just above one of the hips. Circle the measuring tape around the person just above the hips to find the measurement.
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