Dimensions Of A Rectangle Calculator

Background.

Area tells you how much surface a rectangle covers but nothing about its shape. Twenty-four square metres could be a 4 by 6 m bedroom, a 2 by 12 m corridor or a 1 by 24 m strip of garden path. Fix one side, though, and the other follows at once, because a rectangle's area is just length times width: divide the area by the side you know and you have the side you don't.

That single division settles a lot of practical questions. A flat listing gives the floor area and you have measured one wall; how long is the other? A plot sold as 500 square metres with 20 m of road frontage runs 25 m deep. A sheet of A4 is 210 mm wide and, as one-sixteenth of the one-square-metre A0 sheet, has an area of 62,500 mm²; dividing gives about 297.6 mm, close to its actual 297 mm length.

Once both sides are known the perimeter follows, twice the length plus twice the width, and that is where shape starts to matter. Of all rectangles with the same area, the square has the shortest perimeter. A 24 m² square room would be about 4.899 m on a side and need about 19.596 m of skirting board; the 4 by 6 room needs 20 m, and the 2 by 12 corridor 28 m. Long, narrow shapes cost more edge for the same area, which shows up in the price of fencing, edging and trim.

Any unit works as long as the area and the side agree: square metres with metres, square feet with feet. The page solves from area and one side only. When what you know is the area and the perimeter, the sides come from a quadratic instead, set out in the questions below.

What is dimensions of a rectangle calculator?

Finding the dimensions of a rectangle means recovering an unknown side from what you already know about it. A rectangle has four right angles and opposite sides of equal length, so its area is the product of its two side lengths, conventionally called length and width. Knowing the area and either side therefore fixes the other: it is the area divided by the known side. The perimeter, the distance all the way round, is then twice the sum of the two sides.

How to use this calculator.

1. 01Under Solve for, choose the side you are missing. The choice only decides which output is labelled length and which width; the division is the same either way, and nothing forces the length to be the longer side.
2. 02Enter the Rectangle area in square units, such as square metres or square feet.
3. 03Enter the Known other dimension in the matching linear unit: metres with square metres, feet with square feet. Convert mixed figures first, remembering that 1 m² is 10,000 cm², not 100.
4. 04Read the missing side, then the perimeter, which comes out in the same linear unit and is the figure to use when buying fencing, skirting or edging.

The formula.

Writing A for area, L for length and W for width, A = L × W. Rearranged, L = A ÷ W when the width is known and W = A ÷ L when the length is known; the page applies whichever you selected. The perimeter is then P = 2 × (L + W). Because the missing side is a quotient, it is inversely proportional to the side you know: halve the known side and, for the same area, the missing one doubles. Units carry through the division, so square metres divided by metres gives metres. A side of zero would make the division meaningless, so both the area and the known side have to be greater than zero. The other common version of the problem, where the area and the perimeter are given and both sides are unknown, is not handled here, because it needs a quadratic rather than a single division.

A worked example.

Frequently asked questions.

References& sources.

1. [1]OpenStax, Rice University. Precalculus 2e, 2021. Trigonometry and analytic geometry chapters. Retrieved 2026-08-06.
2. [2]OpenStax, Rice University. College Algebra 2e, 2021. Algebra and function chapters. Retrieved 2026-08-06.
3. [3]NIST/SEMATECH. e-Handbook of Statistical Methods, 2012. Chapter 1.3 and Chapter 4. Retrieved 2026-08-06.