  From the standard TEM00 Gaussian beam profile equations, we have derived the
equations for beam waist at lens focus as a function of variable spot size. In
this process, we obtained two equations for beam waist and the validity of
these equations is studied with respect to Rayleigh length. The physical
validity of the equations has been theoretically checked and also
experimentally verified with He-Ne laser at 632 nm. The derived equations are
useful for the estimation of the spot size of high peak power lasers (those
spot sizes less than few mm). In complicated experimental setup by inserting a
beam splitter, we can walk out the some of the beam and the spot size of this
beam at the particular position can be measured by normal methods (pinhole and
knife-edge techniques). By using our derived equations we can know the spot
size of the beam at any position in the experimental setup form the measured
spot size.
