Length of a Rectangle Calculator
Given a rectangle's width and either its area or perimeter, find the length (the other dimension) — plus the diagonal — instantly.
Known second measurement
sq units
The unknown dimension of the rectangle
- 1
Rearrange A = l × w
l = A ÷ w = 30 ÷ 5 = 6 - 2
Verify: l × w
6 × 5 = 30Multiplying back should equal the original area.
How does this calculator work?
To find rectangle length: from area use l = A ÷ w; from perimeter use l = P/2 − w. Enter the width and one other measurement to get the length, diagonal √(l²+w²), and all remaining rectangle properties.
Formula
How this is calculated
A rectangle has two pairs of equal sides, commonly called length l and width w. If you know the width and the total area, rearrange A = l × w to get l = A / w. If you know the width and the perimeter instead, rearrange P = 2(l + w) → P/2 = l + w → l = P/2 − w.
Select which second measurement you have — area or perimeter — enter its value alongside the width, and the calculator returns the length. It also reports the complete rectangle: area, perimeter, and the diagonal length using the Pythagorean theorem, d = √(l² + w²).
Assumptions: both dimensions must be positive. If the derived length is zero or negative the inputs are geometrically inconsistent (e.g. the area is too small or the perimeter too short for the given width).
Frequently asked questions
By convention, length is the longer side and width the shorter — but formulas are symmetric. The same value l × w equals area regardless of which dimension you label which.
Rearrange A = l × w by dividing both sides by w: l = A / w.
Rearrange P = 2(l + w): divide by 2 → P/2 = l + w, then subtract w → l = P/2 − w.
Also known as
TG we-Calculate Editorial Team. (2026). Length of a Rectangle Calculator [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/length-of-a-rectangle-calculator
TG we-Calculate Editorial Team. "Length of a Rectangle Calculator." TG we-Calculate. 2026. https://we-calculate.com/calculator/length-of-a-rectangle-calculator.
TG we-Calculate Editorial Team, "Length of a Rectangle Calculator," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/length-of-a-rectangle-calculator
@misc{wecalculate_length_of_a_rectangle_calculator, title = {Length of a Rectangle Calculator}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/length-of-a-rectangle-calculator}}, year = {2026}, note = {TG we-Calculate} }
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