Rectangle Calculator

To turn a square into a rectangle, choose what stays constant: area or perimeter. Enter the original square side and target length-to-width ratio. The two constraints generally produce different rectangles.

Calculate a rectangle from length and width, or recover its width from length and diagonal. A square has equal length and width. The diagonal must be longer than either side.

For a missing side, select the known side and provide diagonal, area or perimeter. The other side is A / known side or P / 2 − known side. Every square is a rectangle; a rectangle is a square only when its sides are equal.

d = √(L² + W²); W = √(d² − L²); A = LW; P = 2(L + W)

A 3 by 4 rectangle has diagonal 5, area 12 and perimeter 14. For a layout squaring check, measure the sides and both diagonals: the target diagonals must agree with the calculated value. Equal diagonals alone do not prove every arbitrary quadrilateral is a rectangle.

The result above uses these exact values. This snapshot is included when the page is printed so the output can be checked against the original measurements.

Length unit

m

Known measurements

Length and width

Length

3 m

Width

4 m

Primary result: Diagonal: 5 m.

• Measure from the datum or reference edge described by this tool, and do not mix inside, outside and centerline dimensions.
• Keep inputs in the displayed units and preserve more precision than the final cutting or purchasing tolerance requires.
• When the result is close to a limit, verify it with a test piece, field measurement, manufacturer drawing or qualified project professional.

Assumes straight sides and right angles. The calculation checks ideal geometry, not foundation strength, surveying accuracy or construction tolerances.