Hi, Daniel, thank you so much for your response. I just tried your
formula and it works great!

Thanks again!
On Aug 16, 4:08*am, DanielCo <dcolardelle...@free.fr> wrote:
> HI,
> A1 : 1
> A2 : =IF(LEFT(B2,10)<>LEFT(B1,10),MAX($A$1:A1)+1,"")
> Copy downwards.
> Daniel
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> > Hi, I’m trying to number some Excel spreadsheets with thousands of
> > rows. *Basically, what I’m trying to do is get the numbering to
> > increase by one every time the first part of the data (the first 10
> > characters) in column B changes. *The numbers after the decimal point
> > in column B don’t matter as far as the sequential numbering in column
> > A is concerned. *This is how I would like the numbering to look:
>
> > Column A * * * * * * Column B
> > 1 * * * * * * * * * * * * AB00015365.001
> > 2 * * * * * * * * * * * * AB00095763.002
> > 3 * * * * * * * * * * * * AB00037241.003
> > * * * * * * * * * * * * * *AB00037241.004
> > * * * * * * * * * * * * * *AB00037241.005
> > * * * * * * * * * * * * * *AB00037241.006
> > 4 * * * * * * * * * * * * AB00028195.007
> > * * * * * * * * * * * * * *AB00028195.008
> > 5 * * * * * * * * * * * * AB00036378.009
> > 6 * * * * * * * * * * * * AB00063493.010
>
> > I’ve been trying different formulas using various combinations of IF,
> > COUNTIF, and LEFT but so far I haven’t been successful. *Can this even
> > be done with a formula? *Thanks in advance for any help or
> > information.