Cuboid Space Diagonal Calculator

About this calculator

The space diagonal of a rectangular box follows directly from the 3D Pythagorean theorem: d = √(l² + w² + h²) — apply the theorem once across the base to get the floor diagonal, then again between that diagonal and the height to get the true corner-to-corner distance through the box's interior.

Movers and shippers use it to check whether a long item like a pole, mirror, or flat-pack furniture piece will actually fit diagonally inside a box or truck bed that's too short for it to lie flat, and furniture makers and packaging engineers use the same formula when sizing crates around irregular or oversized objects.