How this calculation works
A rectangle has four right angles. Its diagonal follows the Pythagorean theorem. Use the same units for length and width.
Area = length × width; perimeter = 2(length + width); diagonal = √(length² + width²)A worked example
An 8 by 6 rectangle has area 48, perimeter 28 and diagonal 10.
Good to know
Uses the stated formula and assumptions. Displayed decimal values are rounded; use consistent units for all inputs.
The same perimeter can enclose different areas
A 9 by 1 rectangle has perimeter 20 and area 9. A 5 by 5 square also has perimeter 20 but area 25. Knowing how much border a shape has does not, by itself, tell you how much space it encloses.
Among rectangles with a fixed positive perimeter, the square has the largest area. This follows from keeping length plus width constant while maximizing their product. It is a useful reasonableness check for layout comparisons.
A diagonal is a straight-line shortcut across the shape
The diagonal forms a right triangle with the two sides, so its length is √(length² + width²). It is not found by adding the sides; that sum follows two edges rather than the direct route.
Use matching length units and confirm the shape really has right angles. A slanted parallelogram needs a perpendicular height for area, and an irregular quadrilateral is not determined by just a length and width.
A few questions, answered
Does a square work here?
Yes. Enter the same value for length and width.
Does this include a border or wall thickness?
No. It models the exact dimensions you enter, with no automatic allowance.