  In this work we extend the Zeldovich formula, which was originally derived
for the free electromagnetic field and was interpreted as the number of
photons. We show that our extended formula gives a universal dimensionless
measure of the overall strength of electromagnetic fields: free fields and
fields produced by various sources, in classical and in quantum theory. In
particular, we find that this number (the Zeldovich number) for macroscopic
systems is huge, of the order of $10^{20}$. For the hydrogen atom in the ground
state it is equal to 0.025 and for the xenon atom it is around 50.
