How do I find the diagonal of a rectangle from its length and width?

Use the Pythagorean theorem: diagonal = sqrt(length^2 + width^2).

Can a rectangle have equal length and width?

Yes. When length equals width the rectangle is a square. The same formulas apply.

I need to tile a floor. Which value do I use?

Use the area to determine how many tiles you need. Use the perimeter to calculate baseboards or border trim.

Rectangle Area and Perimeter Calculator

Calculate the area and perimeter of a rectangle from length and width. Instant results using Area = l x w and Perimeter = 2(l + w).

By AbraCalc Editorial Team · Reviewed by AbraCalc Methodology Review · Last reviewed: 2026-06-25

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How to use this tool

Enter length and width in the fields above.
Results update instantly as you type — or click Calculate.
Read your area and the full breakdown beneath it.

Formula

Area = length x width; Perimeter = 2 x (length + width)

How it works

A rectangle has two pairs of equal sides. Multiply length by width for area. Add all sides (or double the sum of length and width) for perimeter.

Worked example

Room 8 m long by 5 m wide

Area = 8 x 5 = 40 m^2
Perimeter = 2 x (8 + 5) = 2 x 13 = 26 m

Area = 40 m^2, Perimeter = 26 m

Common mistakes to avoid

Computing perimeter as length + width instead of 2 x (length + width), missing the two pairs of equal sides.
Confusing area (square units) with perimeter (linear units); area fills the interior, perimeter traces the boundary.
Entering diagonal length instead of side lengths; the formula requires the two side lengths, not the corner-to-corner distance.

Key terms

What is a rectangle?: A quadrilateral with four right angles and opposite sides equal.
Difference between area and perimeter?: Area measures the surface enclosed (m^2); perimeter measures the boundary length (m).

Frequently asked questions

How do I find the diagonal of a rectangle from its length and width?: Use the Pythagorean theorem: diagonal = sqrt(length^2 + width^2).
Can a rectangle have equal length and width?: Yes. When length equals width the rectangle is a square. The same formulas apply.
I need to tile a floor. Which value do I use?: Use the area to determine how many tiles you need. Use the perimeter to calculate baseboards or border trim.