I am having a problem whereby I want a cell to -1 from another cell unless that cell = 0. If 0 then I want it to remain at 0 (i.e. not go into negative numbers) For example: If A1= greater than 0 then -1, if A1= 0 then return 0. I hope that makes sense and someone can help. Thanks

I’d use =IF($A1>0, $A1-1, $A1) Depends whether you want to force 0 or copy the value I.e. this will allow a negative value.

OK I am closer to a solution but I think my explanation wasn't great. The answers above leave me with zero because there is nothing in the Feb '18 box to minus 1 from. The cell is blank, I need the formulae to provide the number. I will try to explain better: I have sheet Jan'18 which shows installment payments remaining, so in Jan there are 6 payments remaining, in Feb '18 there will be 5 and so on until zero payments remaining. What I am trying to do is get a formula that automatically reduced the number of installments across the sheets until zero and when it reaches zero it remains at zero and doesn't go to -1.

The easiest way of doing this is probably to use the MAX function - i.e. = MAX (A1-1,0) will reduce A1 by one but the lowest it'll go is 0.