You have not given us any clue as to how we know the number of month's of
data that has been entered.
a) if this is based on the current date; let N moths have passed then your
formula
=ytd +(ytd/N)*(12-N) which is the same as 12*ytd/N
If A1 holds the ytd figure then 12*A1/(MONTH(TODAY())-1) will work provided
the data is added the day after the last day of the completed month

b) maybe we could count how many entries have been made
Suppose the value are entered in cells A1:A12 on a monthly basis
The we could use 12*SUM(A1:A12)/COUNT(A1:A12)
Of if A1:A12 has a running total such than the last one is the YTD value
12*MAX(A1:A12)/COUNT(A1:A12)

Give us a bit more info so we can better help
best wishes
--
Bernard Liengme
Microsoft Excel MVP
http://people.stfx.ca/bliengme

"Kelli" wrote in message
...
I have a figure for YTD Sales and am trying to calculate estimated
annualized
sales off of this. And, this will change monthly as sales managers update
their YTD sales to actual for the prior month end. Mathematically, the
formula would be: YTD Sales + ((YTD Sales/# whole months passed) (# whole
months remaining in year)).

Any suggestions?