Excel formula to Sum every 5 rows

Question

i'm struggling with an excel formula:

NEED: I need a formula to sum every 5 rows AND place the value on the respective row. (see table below)

Here's what i have so far but it isn't quite there:

=SUM(OFFSET($A$2,(ROW()-ROW($B$2))*5,,5,))

    | A   |  B | C
-----------------------
1   | 5   | 37 | 37  
2   | 10  | 24 | 37
3   | 7   | 17 | 37
4   | 7   | 0  | 37
5   | 8   | 0  | 37  
6   | 1   | 0  | 24
7   | 3   | 0  | 24  
8   | 5   | 0  | 24  
9   | 5   | 0  | 24
10  | 10  | 0  | 24
11  | 5   | 0  | 17
12  | 1   | 0  | 17
13  | 8   | 0  | 17
14  | 2   | 0  | 17
15  | 1   | 0  | 17

COLUMN "A" = VALUES

COLUMN "B" = CURRENT OUTPUT (w/Formula Above)

COLUMN "C" = DESIRED OUTPUT

Thanks for your assistance.

Answer 1

Enter this formula in B1:

 =SUM(OFFSET($A$1,5*INT((ROW()-1)/5),,5))

and drag/copy it down.

Tested on ~10K rows of random numbers, it works reasonable.

Answer 2

Another way using INDEX instead of OFFSET

=SUM(INDEX(A:A,FLOOR(ROW()-1,5)+1):INDEX(A:A,FLOOR(ROW()-1,5)+5))

Explanation

THE FLOOR function takes the value of the first argument and rounds it down to the nearest multiple of the second argument. In the first INDEX statement, for rows 1-5, in the FLOOR statement one is subtracted from the row number to give 0-4. The FLOOR statement rounds all these values down to zero, so after adding one you get a value of 1 for each of the first 5 rows.

The second INDEX and FLOOR statements work the same way, except that you add 5 at the end to give a value of 5 for the first 5 rows.

INDEX returns a reference, so it is valid to put INDEX(A:A,1):INDEX(A:A,5) to get the first 5 cells of column A, then you can just SUM them.

For rows 6-10, the FLOOR statement yields 5 in the same way so after adding 1 and 5 respectively you get cells 6-10 of column A and so on.

Row()     Row()-1    FLOOR(Row()-1)   FLOOR(Row()-1)+1   Floor(Row()-1)+5
1         0          0                1                  5
2         1          0                1                  5
3         2          0                1                  5
4         3          0                1                  5
5         4          0                1                  5
6         5          5                6                  10
7         6          5                6                  10
8         7          5                6                  10
9         8          5                6                  10
10        9          5                6                  10

Answer 3

This is cheating, but in B5 enter:

=IF(MOD(ROW(),5)=0,SUM(A1:A5),B6)

then copy this cell both upwards and downwards:

The formulas in B1 through B4 are invalid, but yield the correct result.