What is the formula for the area of a rectangle?

Area = length × width. Make sure both are in the same unit.

Which units can I use?

You can enter length and width in mm, cm, m, in, ft, or yd. Area can be reported in mm², cm², m², in², ft², yd², hectares, or acres.

Can I calculate diagonal and perimeter too?

Yes. This calculator also outputs perimeter and diagonal using the same input units.

How accurate are the results?

Results are computed using exact formulas and precise unit conversion factors from NIST. You can control the displayed decimal precision.

How do I share my calculation?

The URL updates as you type. Copy and share the link to preserve your inputs and selected units.

What happens if I enter negative numbers?

Negative dimensions are invalid. The tool flags an error and explains how to correct it.

Rectangle Area Calculator

Quickly compute the area of a rectangle from its length and width. Built for students, educators, engineers, and DIYers, this accessible tool also returns perimeter and diagonal, supports multiple units, and creates a shareable link with your inputs.

Measurement unit for length and width *

Length *

Width *

Area output unit

Decimal places: 2

Results

Area

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Perimeter

—

Diagonal

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Aspect ratio (L:W)

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Data Source and Methodology

Authoritative Data Source:
1) Wolfram MathWorld — Rectangle (accessed 2025-09-14): https://mathworld.wolfram.com/Rectangle.html
2) NIST Special Publication 811 — Guide for the Use of the International System of Units (SI), 2008 Edition (updated 2023), for unit conversion factors: https://www.nist.gov/pml/special-publication-811

All calculations are strictly based on the formulas and data provided by this source.

The Formula Explained

Area formula: \\( A = L \\times W \\)

Perimeter: \\( P = 2\\,(L + W) \\)

Diagonal (Pythagorean theorem): \\( d = \\sqrt{L^2 + W^2} \\)

Aspect ratio: \\( r = \\dfrac{L}{W} \\quad (W \\ne 0) \\)

Glossary of Variables

L (Length): The longer side of the rectangle.
W (Width): The shorter side of the rectangle.
A (Area): The surface enclosed, measured in square units.
P (Perimeter): The total distance around the rectangle.
d (Diagonal): The straight-line distance between opposite corners.
Units: Supported inputs: mm, cm, m, in, ft, yd. Area outputs: mm², cm², m², in², ft², yd², ha, ac.

How It Works: A Step-by-Step Example

Inputs: Length = 8 ft, Width = 5 ft, Area unit = ft², Precision = 2 decimals.

Use the area formula: \\( A = L \\times W = 8 \\times 5 = 40\\,\\text{ft}^2 \\).
Perimeter: \\( P = 2(L+W) = 2(8+5) = 26\\,\\text{ft} \\).
Diagonal: \\( d = \\sqrt{8^2 + 5^2} = \\sqrt{64+25} = \\sqrt{89} \\approx 9.43\\,\\text{ft} \\).
Aspect ratio: \\( r = \\dfrac{8}{5} = 1.6:1 \\).

Frequently Asked Questions (FAQ)

Do length and width need the same units?

Yes. This tool assumes both are in the unit you selected in the “Measurement unit” field. You can change the area output unit independently.

Can I enter zero?

Yes. A dimension of zero results in an area of zero. Negative values are not allowed.

How precise are unit conversions?

We use authoritative conversion factors from NIST SP 811. Calculations are exact; you control only the displayed decimal places.

What is the diagonal used for?

The diagonal is often useful in material planning, television/monitor sizing, and verifying squareness.

How do I share or bookmark my calculation?

The URL updates with your inputs. Copy the address bar to share a link that reproduces your values.

Will this work offline?

Yes. The calculations run entirely in your browser. External links (e.g., sources, LaTeX rendering) may require internet to load initially.

Strumento sviluppato da Ugo Candido. Contenuti verificati da CalcDomain Editorial Team.
Ultima revisione per l'accuratezza in data: September 14, 2025.