When a loan officer prequalifies you, he works backwards to figure your maximum mortgage amount. You can do the same thing. The first step is to determine your monthly income. It isn't quite as easy as it sounds. Lenders only count income they can document through paperwork.
If you are a salaried employee, and don't earn bonuses, it's easy. Get out your paycheck. If you get paid twice a month, multiply by two. If you are paid every two weeks, then you multiply by 26 (the number of pay periods in a year) and divide by twelve. Unless you're a teacher. Teachers don't always work year round and they have special rules.