TopToolAIRectangle Calculator
ⓘ About this tool
🔄 Last updated 25 Sep 2026
✓ Reviewed by TopToolAI Team
Enter the length and width of a rectangle to get its area, perimeter and diagonal, all worked out step by step.
How to use Rectangle Calculator
Step-by-step guide to the boxes below. Open it only if you need help.
How to use Rectangle Calculator
Step-by-step guide to the boxes below. Open it only if you need help.
-
1
Length Text
The longer side of the rectangle. Use the same unit for both sides.
Example: 8
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2
Width Text
The shorter side of the rectangle, in the same unit as the length.
Example: 5
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3
Press “Calculate”
Click the button under the boxes. If something is missing or not valid, a short message tells you which box to fix.
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4
Read your result
Your answer appears under the button, with a short note in plain words where it helps. Change any box and press again to compare different numbers.
Rectangle Calculator FAQs
How is the diagonal worked out?
With the Pythagorean theorem: the diagonal is the hypotenuse of a right triangle formed by the length and width, so diagonal = √(length² + width²).
Does this work for a square?
Yes, just enter the same value for both length and width.
How do I calculate the area of a rectangle?
Length × width. A 5 by 3 rectangle has an area of 15.
How do I calculate the perimeter?
2 × (length + width). For 5 by 3, the perimeter is 16.
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