include in formula only if cell is populated?

Is it possible to have cells excluded from a formula if they are not populated. at the moment although not displayed they seem to give a value of 0. this is effecting the calculations being made across the worksheet.
thanks

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Is it possible to have cells excluded from a formula if they are not populated with numerical data. at the moment although all cells are empty they seem to give a value of 0. this is effecting the calculations being made across the worksheet.
Can anyone offer any help?
thanks
James