Assuming your data in B1:B18

In C2: =IF(AND(B2=1,B3<1),SUMIF($B$1:B2,1)-SUM($C$1:C1),"")
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"Katie" wrote:

I need a formula to subtotal colums of sequential 1s as shown:
e.g. B is the column of data, C has the subtotals.

1 )
1 )
1 )
1 )
1 ) subtotal = 5
4
5
6
7
1 )
1 )
1 ) subtotal = 3
3
4
5
1
1 = subtotal = 2
5

Any help?