Rectangle calculator
Area, perimeter and diagonal, with each side in whatever unit you have it in. The multiplication is not the interesting part; converting the answer is, because an area does not convert with a length factor. Nothing you type here leaves your browser — it is worked out on your own device, so there is nothing to send and nothing to log.
An area does not convert with a length factor
There are 3.2808 feet in a metre. There are 10.7639 square feet in a square metre — the square of that, not the same number. Using the length factor on an area is one of the most common unit errors there is, and it is out by the factor itself:
| A 4 m by 5 m room | 20 m² |
|---|---|
| Correctly, in square feet | 215.28 |
| With the length factor | 65.62 |
Out by a factor of 3.28, which is 228% — enough to order a third of the flooring you need and not notice until it arrives. This page always squares the factor, and shows you what the other answer would have been so the difference is visible.
And it is invisible if the unit does not change. Ask for the area of a metric rectangle in square metres and the factor is 1; one squared is also 1, and both methods agree perfectly. A converter tested only in its own units passes.
The multiplication really is simple, and I checked
This page exists because the inventory had “area of a rectangle” on it, and the honest thing is to say that length × width is not where the difficulty lies. It was measured rather than assumed: 0.1 × 0.3 is exactly 0.03, and 2(l + w) gives the identical double to 2l + 2w. There is no floating-point story in the arithmetic itself.
What there is, is the unit conversion above, and the fact that perimeter and area do not scale together. Make both sides 10% longer and the perimeter grows 10%; the area grows 21%. Double them and the area quadruples. That catches people out with carpet, paint and baking tins, which is why this site has a baking tin converter that does the same arithmetic for a different reason.
Every imperial length here is exact
The inch has been exactly 25.4 millimetres by international agreement since 1959, so a foot is exactly 0.3048 m and a yard exactly 0.9144 m. None of the imperial lengths on this page is an approximation, and the checks behind it compare against exact rational arithmetic rather than against another floating-point sum.