What formulas does this rectangle calculator use?

It uses the three core rectangle relationships: area = length × width, perimeter = 2 × (length + width), and diagonal = √(length² + width²). Reverse-solve modes simply rearrange those same relationships.

Can it solve a missing side from area and one known side?

Yes. If you know the area and one side, divide area by that side to recover the missing dimension. The calculator then fills in perimeter and diagonal from the solved rectangle.

Why would diagonal plus a side ever be invalid?

Because the diagonal is the longest line in a rectangle. If the diagonal is shorter than or equal to the side you entered, no real rectangle can match those measurements.

Does it also work for squares?

Yes. A square is just a rectangle with equal length and width. When both sides match, the calculator keeps the same formulas and simply makes the square case visible in the result note.

What unit does the area use?

Area is shown in the squared form of the length unit you selected: m² for meters, cm² for centimeters, ft² for feet, and in² for inches.

Rectangle Calculator

Solve a rectangle from the measurements you already have. Start with length and width for the full geometry, or switch to area, perimeter, or diagonal modes to reverse-solve the missing side.