Missing Side From Rectangle Perimeter

Find the missing side of a rectangle

A rectangle has two pairs of equal sides. Its perimeter is the total distance around it, so you add all four sides:

P = length + width + length + width = 2(length + width)

When one side is missing, use the perimeter to build an equation and solve for the unknown.

Method

1. Write the perimeter formula for a rectangle.
2. Substitute the known side lengths and the perimeter.
3. Combine like terms to make the equation simpler.
4. Isolate the missing side by using inverse operations.
5. Simplify the final answer.

Example setup

If the perimeter is known and one side length is given, you can first find the sum of the two different sides by dividing the perimeter by 2. Then subtract the known side to get the missing one.

Check your answer

Use the found side to calculate the perimeter again. If the four sides add to the given perimeter, your answer is correct. Also make sure the missing side is a sensible length, so it should be positive.