In order to qualify for a mortgage, most lenders require that you have a debt-to-income ratio of 28/36 (this can vary depending on the down payment and the type of loan you're getting, however). This means that no more than 28 percent of your total monthly income (from all sources and before taxes) can go toward housing, and no more than 36 percent of your monthly income can go toward your total monthly debt (this includes your mortgage payment). The debt they look at includes any longer-term loans like car loans, student loans, credit cards or any other debts that will take a while to pay off.
Here's an example of how the debt-to-income ratio works: Suppose you earn $35,000 per year and are looking at a house that would require a mortgage of $800 per month. According to the 28 percent limit for your housing, you could afford a payment of $816 per month, so the $800 per month this house will cost is fine (27 percent of your gross income). Suppose, however, you also have a $200 monthly car payment and a $115 monthly student loan payment. You have to add those to the $800 mortgage to find out your total debt. These total $1,115, which is roughly 38 percent of your gross income. That makes your housing-to-debt ratio 27/38. Lenders typically use the lesser of the two numbers, in this case the 28 percent $816 limit, but you may have to come up with a bigger down payment or negotiate with the lender.
You also have to think about what you can afford. The lender will tell you what you can afford based on the lower number in the debt-to-income ratio, but that's not taking any of your regular expenses (like food) into account. What if you have an expensive hobby or have plans for something that will require a lot of money in five years? Your lender doesn't know about that, so the $1,400 mortgage it says you qualify for today may not fit your actual budget in five years -- particularly if you don't see your income increasing too much over that period. Take a look at this calculator to see how much house you can afford based on your current income.
In general, it's more difficult to qualify for a mortgage now than it was during the housing boom, when just about any motivated homebuyer could find credit -- even many who couldn't afford to buy a house. In the next section, we'll explain what kind of credit history and income capacity you'll need to pass the lender's background check.