Rectangle Calculator

The rectangle is the most common shape in everyday measurement, from rooms and tabletops to screens, sections of land and sheets of material, so being able to read off its key numbers quickly is genuinely useful. This calculator takes the length and the width of a rectangle, along with the unit you are working in, and returns the three figures you most often need: the area, the perimeter and the diagonal. The area is simply the length multiplied by the width, and because you are multiplying two lengths the answer comes out in square units, such as square metres or square feet. The perimeter is the total distance around the outside. A rectangle has two long sides and two short sides, so you add the length and the width and double the result. The diagonal is the straight line joining opposite corners, and it follows from the Pythagorean theorem because the diagonal is the hypotenuse of the right triangle made by one length and one width: it equals the square root of the length squared plus the width squared. You can work in millimetres, centimetres, metres, kilometres, inches or feet, and every result is shown in the matching unit so you never have to convert by hand. This tool is built for students checking geometry homework, tradespeople and renovators estimating materials, gardeners and farmers measuring plots, and anyone who needs the area, perimeter or diagonal of a four-sided shape with right angles. Enter your length and width below, pick a unit, and all three results update instantly as you type.

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