Rectangle Calculator (Find the Missing Side)
Find a rectangle's area, perimeter, diagonal or missing side from any two known values: length and width, a side and the area, a side and the perimeter, or a side and the diagonal.
Last updated: September 2026
Result
80 square units
Formula below · 2 sources (mathworld.wolfram.com, mathsisfun.com) · Updated Sep 2026
Compare with similar
About this calculator
A rectangle is fixed by any two independent measurements. With length l and width w:
area A = l × w perimeter P = 2(l + w) diagonal d = √(l² + w²)
So if you know one side and something else, the other side follows: w = A ÷ l from the area, w = P ÷ 2 − l from the perimeter, or w = √(d² − l²) from the diagonal. The calculator finds the missing side first, then whatever you ask for; the tiles always show both sides.
With the defaults (length 10, width 8): area 80 square units, perimeter 36, diagonal √164 = 12.81.
The first value is always the side you know; the second is the width, area, perimeter or diagonal, as the first menu says. If the numbers cannot form a rectangle (a perimeter shorter than twice the side, or a diagonal no longer than the side), you get a message instead of a result. For a square, enter the same value for length and width, or use the Rectangle and Square Calculator.
How to use
Choose which two values you know, enter them, and choose what to calculate. Example 1 (the defaults): length 10, width 8, area = 80. Example 2: length 12 and area 96: width = 96 ÷ 12 = 8; perimeter = 2 × (12 + 8) = 40. Example 3: width 5 and diagonal 13: length = √(169 − 25) = 12; area = 60. Example 4: length 8 and perimeter 26: width = 13 − 8 = 5; diagonal = √89 = 9.43.
Frequently asked questions
How do I find the width from the area and length?
Divide the area by the length: a 150 ft² room that is 15 ft long is 10 ft wide.
How do I find a side from the perimeter?
Halve the perimeter and subtract the known side: perimeter 30 and length 9 give width 15 − 9 = 6. If the result is 0 or negative, the numbers do not describe a rectangle.
How do I find the diagonal of a rectangle?
Use Pythagoras: d = √(l² + w²). A rectangle 16 by 9 has a diagonal of √(256 + 81) = √337 = 18.36 in the same unit.
Can I find a side from the diagonal?
Yes: the other side is √(d² − known side²). A 13-unit diagonal with a 5-unit width gives a length of 12.
Related calculators
Sources & references
Think a result is wrong? Report a problem with this calculator.