Thank you for your quick reply.

That's what I was afraid of. Well, there is only one cell * one cell
on each sheet, so an array would not be needed there.
But that would mean I would do =IF(Sheet1!B1=B1,Sheet1!B2*Sheet1!B3,0)+
IF(Sheet2!B1=B1,Sheet2!B2*Sheet2!B3,0)+
IF(Sheet3!B1=B1,Sheet3!B2*Sheet3!B3,0)+... etc. and I would very
quickly run out of the number of IFs I can use. And it has no
flexibility as to insert more sheets etc.

Do you know of any other way? Or is it possible to create something in
VBA? I'm not really sure how VBA deals with ranges across worksheets.

Ronny