Only if you do not like BJ's perfectly fine method:

The reason your formula fails when you add the 31st argument is Excel's
limit of 30 arguments in any function.

But this will work
B2=SUM('01'!B3,'02'!B3,'03'!B3,'04'!B3,'05'!B3,'06 '!B3,'07'!B3,'08'!B3,'09'!B3,'10'!B3,'11'!B3,'12'! B3,'13'!B3,'14'!B3,'15'!B3,'16'!B3,'17'!B3,'18'!B3 ,'19'!B3,'20'!B3,'21'!B3,'22'!B3,'23'!B3,'24'!B3,' 25'!B3,'26'!B3,'27'!B3,'28'!B3,'29'!B3,'30'!B3)
+'31'!B3)

or any other breakdown, say 1 to 15 and 16 to 21
B2=SUM('01'!B3,'02'!B3,'03'!B3,'04'!B3,'05'!B3,'06 '!B3,'07'!B3,'08'!B3,'09'!B3,'10'!B3,'11'!B3,'12'! B3,'13'!B3,'14'!B3,'15'!B3)
+SUM('16'!B3,'17'!B3,'18'!B3,'19'!B3,'20'!B3,'21'! B3,'22'!B3,'23'!B3,'24'!B3,'25'!B3,'26'!B3,'27'!B3 ,'28'!B3,'29'!B3,'30'!B3,'31'!B3)

best wishes
--
Bernard V Liengme
Microsoft Excel MVP
www.stfx.ca/people/bliengme
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"kmaki" wrote in message
...
I have a monthly total worksheet linked to multiple worksheet for each day
of
the month. The sum fomula works for a 30-day month but it will not allow
me
to add data for a 31-day month.

B2=SUM('01'!B3,'02'!B3,'03'!B3,'04'!B3,'05'!B3,'06 '!B3,'07'!B3,'08'!B3,'09'!B3,'10'!B3,'11'!B3,'12'! B3,'13'!B3,'14'!B3,'15'!B3,'16'!B3,'17'!B3,'18'!B3 ,'19'!B3,'20'!B3,'21'!B3,'22'!B3,'23'!B3,'24'!B3,' 25'!B3,'26'!B3,'27'!B3,'28'!B3,'29'!B3,'30'!B3)

B3 =SUM(B2,'31'!B3)

Is there a better way to perform this caculation?