I have an excel s/s which shows in one column a reading of a central
heating gas usage , In an adjacent column I want it to automatically
show the increment in this gas usage reading when I enter a subsequent
reading here.

I have inserted the formula :-

=IF(C16="","",C16-C15).

This calculates the difference between C16 & C15 entries but does not
do anything for an entry in C17 - the formula does not repeat as I
move down the C column.

I've forgoten how to do this - can someone remind me again , please ?

B.N.