While expenses aren’t always calculated (up until now at least) you do however pretty much know how much you earn (assuming you are employed). Sometimes there are un-accounted sources of income that you wouldn’t really think of as income, that should be accounted for in your budget plan.
These “sources” can be any government stipends you may receive, for instance in my country we receive a child stipend every month. The amount isn’t enough to save me from all my trouble but it adds to my total income and helps me plan my budget more effectively.
Perhaps you are collecting rent from an owned property, or have a big savings account opening up soon that will give you an unexpected flow of cash. Perhaps you have a rich relative who came to visit and gave you some birthday/present money. These should all go into your “income” calculation box.
Once you have this final piece of the puzzle you need to match up your expenses vs. your income and begin with the final stage of trimming off the fat.