How to calculate unity gain frequency?

In Thomas H. Lee's book,
He mentioned that for bias current = 5mA, in 0.5um technology and
give transistor size in page384, the unity gain frequency is about 35Grps.

Does anyone know how to calculate this value?

Thanks!

I don't know the formula, but that value might be a measured
result that was given by a Fab house.

Actually, in this example, it is given that
Leff = 0.35um, W = 500um, Cox = 3.8mF/m^2, Id = 5mA
I used mobility = 0.0463m^2/V*s and saturation current formula
to get "gm" and use Cgs = (2/3)*Cox*W*L to get "Cgs"
then use gm/Cgs to calculate unity gain frequency.
the result is around 113Grps which is three times of the result
in the book.