The most popular formula is the one from Blaney-Criddle, here in the extended form according to Phelan.

CU = k_c × f(t) × p in mm / month

CU = Consumptive use in mm / month
k_c = plant factor, depending on the typ and the stage of development of the plant (was investigated in experiments)
t = average monthly air temperature in °C
p = monthly percentage of daily hours applied to the year (depend on latitude) in %
f(t) = 0.457 × t + 0.813

The original formula from Blaney-Criddle was much simpler since it calculated with degrees Fahrenheit und inch. With the formula shown above the potential evaporation is not calculated the way other formulas do. It is used to calculate the consumptive use of the plant. For that purpose, Blaney and Criddle have introduced a correction factor k (plant factor)..

Rule of thumb : ET_real = ET_pot × 0.75