By Carter McBride, Updated Aug 30, 2022

Rectangles have four sides, and adjacent sides are typically of different lengths. Knowing the measurements of two sides lets you determine the rectangle’s aspect ratio—the proportional relationship between the length and width. This concept is foundational in geometry and helps students understand rectangle properties. Once you know the ratio and one side’s length, you can compute the missing side.

Step 1: Measure the sides

Measure the rectangle’s length and width. For example, suppose one side measures 8 inches and the adjacent side measures 4 inches.

Step 2: Set up the ratio

Place the longer side in the numerator and the shorter side in the denominator. In this example, the ratio is 8 inches ÷ 4 inches.

Step 3: Simplify the ratio

Divide the numerator by the denominator to reduce the fraction to its simplest form. 8 ÷ 4 equals 2, so the rectangle’s aspect ratio is 2 : 1.

TL;DR (Too Long; Didn’t Read)

Rectangles that share the same length‑to‑width ratio are considered similar.