A handy utility for everybody that works with lasers. ULO Optics Spot Calculator calculates laser beam spot size and Rayleigh length (depth of focus) for a given beam by taking into account beam quality, wavelength, beam diameter and focus length.
Version 2.0 also offers a rupture calculator to determine the minimum thickness of a cutting lens, taking into account cutting gas pressure.
Version 3.0 includes iPhone 5 support and a power density calculation.