If you know the basics of estimating revenue, you should have no trouble estimating expenditures. Often, the same method is applicable here: past history is a good indication of future performance. If, for the last two years, you haven’t spent as much on office supplies as budgeted, chances are you won’t spend as much in the upcoming year. However, there are always unusual circumstances that must be addressed, but overall, this is a useful method for estimating expenses.