What is the formula for the area of a rectangle?

Area = length × width. For example, a rectangle 8 m long and 5 m wide has an area of 40 square meters.

How do I calculate the perimeter of a rectangle?

Perimeter = 2 × (length + width). A rectangle 8 m × 5 m has a perimeter of 2 × (8 + 5) = 26 m.

How do I find the diagonal of a rectangle?

Use the Pythagorean theorem: diagonal = √(length² + width²). For an 8 × 5 rectangle, the diagonal is √(64 + 25) = √89 ≈ 9.43.

What units does this calculator use?

The calculator is unit-agnostic. Enter length and width in any consistent unit (meters, feet, inches, etc.) and the results will be in the same unit (and square unit for area).

How is a rectangle different from a square?

A square is a special rectangle where all four sides are equal (length = width). The rectangle formulas all apply to squares, but squares have the additional property that Area = side².

Can I use this for irregular shapes?

This calculator is for rectangles only. For triangles, circles, or other shapes, use the dedicated calculators for those shapes.

Area of a Rectangle Calculator

Name: Area of a Rectangle Calculator
Availability: InStock
Author: Nham Vu

Enter length and width to instantly get the area, perimeter, and diagonal of a rectangle.

Rectangle Dimensions

Length (l)

units

Width (w)

units

Formulas

Area A = l × w

Perimeter P = 2(l + w)

Diagonal d = √(l² + w²)

Enter length and width to calculate

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Summary

Enter length and width to instantly get the area, perimeter, and diagonal of a rectangle.

How it works

Enter the length of the rectangle in the first field.
Enter the width of the rectangle in the second field.
The calculator instantly computes area, perimeter, and diagonal.
Read off each result along with the formula breakdown shown below.
Click "Copy Results" to copy all values to your clipboard.

Use cases

Calculate the floor area of a rectangular room.
Find the amount of fencing needed around a rectangular yard.
Determine the diagonal of a TV, monitor, or rectangular panel.
Estimate material quantities for rectangular surfaces in construction.
Verify geometry homework answers instantly.
Calculate land area for rectangular plots or fields.
Work out carpet or tile quantities for rectangular floors.
Check aspect ratios and dimensions for design or printing projects.

Frequently Asked Questions

Last updated: 2026-06-13 · Reviewed by Nham Vu