This geometry calculator finds the length or width of a rectangle when the perimeter and the other side are known.
Math
Length and Width of a Rectangle given Perimeter Calculator
Find length or width of a rectangle from its perimeter and the other side.
Calculator
About this calculator
Formula notes
Perimeter = (length + width) x 2; given perimeter and one side, the other side = perimeter / 2 - known side.
Worked examples
- Example: perimeter 20 and length 8 gives width 2.
- Perimeter 20 and length 8 gives width 2.
How to use it
- Enter the perimeter and one side (length or width).
- Leave the unknown side blank.
- Read the calculated length and width.
Frequently asked questions
What if the known side equals half the perimeter?
The calculator rejects it because the other side would be zero, which is invalid.
Can I enter both length and width?
Yes. When both are provided the calculator echoes them back; provide the perimeter to verify consistency.
Limits and interpretation
- The known side must be less than half the perimeter; otherwise the other side would be zero or negative.
- When both sides and the perimeter are provided, the calculator echoes the sides without verifying consistency.