How do you calculate rectangle area?

Multiply length by width. Use the same length unit for both inputs.

Can I find rectangle area from a side and diagonal?

Yes. The calculator recovers the missing side using the Pythagorean relationship, then multiplies the two sides.

Can perimeter alone determine rectangle area?

No. Many rectangles can share the same perimeter. You need another measurement, such as a side or the area for reverse checking.

Rectangle Area Calculator

Calculate rectangle area from length and width, or recover rectangle measurements from side plus diagonal, area, perimeter, diagonal angle, or perimeter plus area.

Known values

Unit

Length (m)

Width (m)

Main workflow: enter length and width in the same unit. Unit changes convert entered values and the current result.

Rectangle area from length and width

The dominant English intent is a fast length-by-width area calculator. The calculator keeps that main workflow first and adds reverse modes for diagonal, perimeter, known area, and diagonal angle.

A = l w

A is rectangle area; l is length; w is width.

P = 2 (l + w)

P is perimeter.

d = l^{2} + w^{2}

d is diagonal by the Pythagorean theorem.

Supported known values

Known values	What is calculated
Length and width	Area, perimeter, diagonal, circumradius, and diagonal angle.
Side and diagonal	The missing side, then area.
Side and area	The missing side from area divided by the known side.
Side and perimeter	The missing side from half the perimeter.
Perimeter and area	The two positive rectangle sides, when possible.

This is a geometry calculator, not a construction estimate. Material waste, cutouts, layout, and coverage rates are separate practical inputs.

Frequently Asked Questions

Sources and References

Calculations are based on the listed reference sources. Links open in a new tab.

Updated: June 2, 2026

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