Try...

=5000/(1.5^SUMPRODUCT(--(DAY(ROW(INDIRECT(DATE(2005,3,1)&":"&TODAY())))=1
)))*1.5

....where 5000 is the starting value used as an example, or...

=A1/(1.5^SUMPRODUCT(--(DAY(ROW(INDIRECT(DATE(2005,3,1)&":"&TODAY())))=1) )
)*1.5

....where A1 contains your starting value.

Note that I've used the first day of the current month as a starting
date [DATE(2005,3,1)]. If you're going to use another month as your
starting date, make sure that you use the first day of that month.

Hope this helps!

In article ,
"Mikel" wrote:

I would like to have one cell that will decrease in value by 1.5 every 1st
day of the month