Values in several columns

HI ALL,

I have a HUGE excel sheet full of macros and calculations on which we are 3 people working usually. I'm trying to add in 1 more calculation. I'm not good at excel I'm just managing the sheet but this time I would need to add this extra complexity.

I have Column A in which I have up to 650 rows (and growing) the rows contain part descriptions but all of the descriptions start with OB17 or OB18 for example: "OB17 Water ITC - LINX psn" or "OB18 CAC ITC delta".
in Column B I have measured weights for them.

So I would like Excel to add all the numbers, which stand beside an OB18 part description.

I tried it with the IF function but then I would need to add the IF statement for every single row (like =SUM(IF(A10="OB18",B10),IF(A11="OB18",B11) ......) for 650 rows it would take ages and every time we add a new row we would need to add it into this formula, so I don't think this is the right solution for this massive sheet.

Can someone help please?

Thank you that works perfectly!