Rectangle Area, Perimeter and Diagonal Solver

How it is calculated

Three values come from the two sides: Area = length × width, Perimeter = 2 × (length + width) and Diagonal = √(length² + width²), the last one from the Pythagorean theorem because the diagonal is the hypotenuse of a right triangle made by the two sides. The tool has no unit of its own: enter both sides in the same unit. The perimeter and diagonal are then in that unit, and the area is in that unit squared (metres give m², centimetres give cm²). Both sides must be above zero. Results are rounded to two decimals, and the diagonal is calculated from the unrounded sides. A square is just the special case where length equals width.

Common mistakes

• Entering the sides in different units, such as length in metres and width in centimetres. All three results are then meaningless.
• Reading the area in the same unit as the sides. If the sides are in metres the area is in square metres, not metres.
• Adding only length and width for the perimeter. All four sides count, so the sum must be doubled.
• Using a diagonal as one of the sides. If you know the diagonal and one side, the missing side is √(diagonal² − side²); this tool needs both sides.
• Expecting a result for a zero or negative side. Both sides must be positive.