What is the formula to calculate magnetizing current on a transformer?

Measure the primary inductance with the secondary open. Then use 6.28FL (F= frequency, L=henries) to find the reactance in ohms (unless it's a tiny transformer the dc resistance can be ignored). Once that's found use ohm's law I=V/R. If you don't have an LCR meter simply measuring no-load current will also work. If you have no meters you're in for alot of guess work as you need to know the permeability of the core material at the particular flux density/voltage it normally runs at.