This is my formula for adding up numbers in a column.

=SUM(IF(ISERR(FIND("-",J20:J137)),,REPLACE(J20:J137,1,FIND("-",J20:J137),"")-LEFT(J20:J137,FIND("-",J20:J137)-1)+1))

I have extended the length of the column to J20 to J183 as I now have more
entries, if I my manually change the J137 to J183 then the formula no
longer works.
Why is this? I dont understand the formula as it was done for me.

What am I doing wrong.

Because there is a range of cells being covered, you have an array
formula... you need to commit the formula using Ctrl+Shift+Enter. To do
this, select the cell **then** put the cursor into the formula bar... then
press Ctrl+Shift+Enter and the formula should work again.

Rick