Width of a Rectangle Calculator
Enter two known measurements of a rectangle — area and length, perimeter and length, or diagonal and length — to instantly calculate the unknown width and all remaining dimensions.
Known measurement
The calculated width of the rectangle
- 1
Width from area and length
w = A ÷ l = 60 ÷ 10 = 6
How does this calculator work?
Choose your known pair of measurements. Width = Area ÷ Length, or Width = Perimeter/2 − Length, or Width = √(Diagonal² − Length²). Enter the two knowns; the calculator returns the width plus all other rectangle dimensions (area, perimeter, diagonal, aspect ratio). All units must be consistent.
Formula
How this is calculated
A rectangle has four sides with opposite sides equal in length: a length l and a width w. Three standard pairs of measurements uniquely determine the width when one side (the length) is already known.
If you know the area A = l × w, rearrange to get w = A ÷ l. If you know the perimeter P = 2l + 2w, rearrange to get w = P/2 − l. If you know the diagonal d (the line connecting opposite corners), use the Pythagorean theorem: d² = l² + w², so w = √(d² − l²). The diagonal formula requires the diagonal to be strictly longer than the length; if d ≤ l the input is geometrically impossible.
Once the width is found, the calculator derives all remaining properties — area, perimeter, diagonal and aspect ratio — using the same relationships. All inputs and outputs share the same length unit (e.g. all in centimetres or all in inches); the area is in that unit squared.
Frequently asked questions
That is perfectly valid — the formulas do not require width < length. Conventionally "length" refers to the longer side, but mathematically the rectangle is symmetric: swapping width and length gives the same shape. If you want to label the longer side as "length", simply swap the two values.
Yes — the calculator is unit-agnostic. Just be consistent: if length is in metres, area must be in square metres, perimeter in metres, and diagonal in metres. Mixing units (e.g. length in cm, area in m²) will give a wrong answer.
If d = l then w = √(d² − l²) = 0, which means the rectangle collapses to a line segment. The calculator requires w > 0. If d < l the expression under the square root is negative — geometrically impossible for a flat rectangle.
Also known as
TG we-Calculate Editorial Team. (2026). Width of a Rectangle Calculator [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/width-of-a-rectangle-calculator
TG we-Calculate Editorial Team. "Width of a Rectangle Calculator." TG we-Calculate. 2026. https://we-calculate.com/calculator/width-of-a-rectangle-calculator.
TG we-Calculate Editorial Team, "Width of a Rectangle Calculator," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/width-of-a-rectangle-calculator
@misc{wecalculate_width_of_a_rectangle_calculator, title = {Width of a Rectangle Calculator}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/width-of-a-rectangle-calculator}}, year = {2026}, note = {TG we-Calculate} }
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