How many E. coli = 1 O.D. unit?

[P. Friedl]

In article <3sb250$nbu at riscsm.scripps.edu>, "Edward K.L. Chan" <echan at scripps.edu> says:
>
>Hi netters,
>   I wonder if someone may be able to provide a reliable formula to calculate
>the number of DH5-alpha or XL1-Blue based on O.D. measurement.  Thanks in advance.
> 

Hello,
with E.coli an OD (420nm) of 0.06 corresponds to 2x10E7 cells per ml
Bye
P.F.>