Cuboid Space Diagonal Calculator

How it is calculated

The space diagonal runs from one bottom corner to the opposite top corner through the inside of the box: d = √(l² + w² + h²), where l, w and h are the length, width and height. It comes from applying Pythagoras twice: first the floor diagonal √(l² + w²), then that diagonal with the height. All three sides must be above zero. The tool does not convert units; it simply adds the label cm to the answer, so enter the sides in centimetres for the label to be right (any other unit gives a number in that same unit). The result is rounded to two decimals. The value is for the inside dimensions: wall thickness, padding and the object's own width are not considered.

Common mistakes

• Measuring the outside of the box and expecting the inside diagonal. Cardboard walls and padding reduce the usable space.
• Mixing units, for example length in metres and height in centimetres. All three sides must be in one unit, and the output is labelled cm.
• Believing a pole as long as the diagonal fits easily. It only fits when it lies exactly corner to corner, with no room for the thickness of the object.
• Using only the floor diagonal. It leaves out the height and gives a shorter figure than the true space diagonal.