how to calculate number of cells from OD? - (Aug/07/2006 )

from molecular cloning 1 OD is 2.5*10 to the power 8 cells/ml, short protocols in mocelular biology says OD of 0.1 is 10 to the power 8 cells/ml. i admit im lost. so if i have OD of 0.03 what does it mean?

well for my information
OD 2= 5x10to the power of 8

so if we think of it this way: 0.1OD=ten to the power 8 cells
1OD is 2.5 times 10 to the power 8 cells
and OD2 is 5 times 10 to the power 8 cells....

then if I got 0.6OD it means there are around 1.6 times ten to the power 8 cells.