Formula Which Changes If Cell Is Blank

I have a spreadsheet which projects the amount of work on hand each week. It runs up to the end of the year and shows the estimated figures and then each week I input the actual figures. The way it works at the moment is that it looks at the estimated work on hand, adds the work on which has been received and then takes off the work that has been cleared...this then gives an estimated level of on hand work.

What I want it to do is to look at the cell which would show the actual work on hand and if thats empty then go on to look at the estimated work on hand. In other words - I want a formula that looks at a cell to see if there's a figure in it and if not uses another cell instead. I have tried: =If(u5=0,r5+y5-z5)

but when I input a figure into U5 it inputs FALSE into the cell. I don't want it to do that, I want it to look in cell U5 and if there's something there then do U5+Y5-Z5.....and if U5 is empty then do R5+Y5-Z5