There are houses you want, and there are houses you realistically can buy. How can you make sure you can afford the house you want? Look to the numbers that qualify you for a mortgage in the first place.

The decisions lenders make about approving a loan are based primarily on numbers such as your assets, income, employment, and credit history. Among the more-important figures are those that tell how much of your income a lender will let you devote to a mortgage payment. For a conventional loan, the most-important percentages are 28 and 36. For a government-backed loan, such as a Veterans Affairs (VA) or Federal Housing Administration (FHA) loan, they are either 29 or 41. Fannie Mae and Freddie Mac, two publicly owned, government-chartered entities, have special programs, similar to the Department of Housing and Urban Development (HUD), that allow for variances.

Lenders look at these general guidelines because they are means for controlling and measuring risks," says Doug Duncan, chief economist for the Mortgage Bankers Association, the industry's trade association in Washington.

For a conventional loan, which is one not guaranteed by a government agency, the principal, interest, taxes, assessments or any other fees added to the monthly payment, cannot total more than 28 percent of your gross -- or pre-tax -- income. The 36 percent includes all your regular monthly debt, such as mortgage expenses, auto loans, credit cards, utilities, and so on. For FHA loans, the numbers are a bit different and easier to meet. To qualify for an FHA loan you can choose between two different numbers. The first figure is 29 percent, or the "front-end ratio." It equals your total mortgage payment divided by your pre-tax monthly income.

If that doesn't work for you, you might be able to qualify by using the "back-end ratio." In this case, all your monthly debts -- mortgage, car, utilities, child support, etc. -- cannot exceed 41 percent of your total pre-tax monthly income. "You can get loans even if your ratios are higher," Duncan adds, "but you'll probably have to pay a higher interest rate."

Now, let's plug in actual numbers and see how the formula works, starting with a conventional loan. Let's say you and/or your spouse make $50,000 a year before taxes. Your total "debt limit" -- mortgage, car payment, utilities, and such -- would be 36 percent of that, or $18,000 a year, which equals $1,500 a month. You can easily figure it yourself: ($50,000 X 0.36) / 12 months. Of that amount, the most your house payment can be is $1,166.66, or ($50,000 X 0.28) / 12 months.

If all your other regular monthly debts total $333.34 or less ($1,500.00 - $1,166.66), you would qualify for a conventional loan. If your regular monthly debts total $500, you would qualify for a mortgage with a regular monthly payment of $1,000, and so on. With a $75,000 family income, the 36 percent figure is $2,250 a month and the limit on the mortgage payment is $1,750. At $100,000 salary, it's $3,000 a month with a mortgage limit of $2,333.33. As you can see, the 36-percent total debt limit is usually more important than the 28-percent figure. So first pinpoint a mortgage payment that is comfortable, and look at homes in that payment range. And remember, these figures apply to the total amount of the loan, not the total cost of the home. The lender is more interested in how much you want to borrow than in how much the house costs.

If you determine that you can make a $1,200-a-month mortgage payment, you have to look at how big a loan that translates into in terms of interest rates, insurance and so on. Depending upon your circumstances you could end up making a $1,200-a-month payment on a house on which you put $5,000 down, or one with a $50,000 down payment. Use this 28/36 mortgage calculator to help figure how large a loan you could receive: http://mortgages.interest.com/content/calculators/afford-borrow2.asp.

Remember, when lenders look at your monthly mortgage payment they are including principal, interest, taxes, insurance and any other fees you have to pay, such as those to a property owners' association. If your down payment is less than 20 percent, you will have to pay private mortgage insurance on top of that.

Now let's look at an FHA "front-end ratio." Your monthly payment can be 29 percent of your gross monthly income. With a family income of $502,000, the maximum monthly payment would be $1,208.33. For a $75,000 income, it's $1,812.50, and if you make $100,000 a year an FHA loan would allow a $2,416.66 payment a month. In this case, the lender still looks at your total regular monthly debts. Although they don't have to fit into a neat formula, the lender will take those debts into consideration.

For an FHA "back-end ratio," the important number is your total monthly debt, including mortgage. It's up to 41 percent, so at $50,000 the monthly payment can be $1,708.33. It doesn't matter how that is split-50-percent mortgage and 50-percent debt or 80-percent mortgage and 20-percent debt. With a family income of $75,000, the monthly figure is $2,562.49 and for $100,000, it would be $3,416.66.

MBA's Duncan added that when figuring out total debt, remember to ignore your current mortgage or rent since that debt will no longer exist once you move. If you take the time to figure out your affordable monthly payment range before you talk to a real estate agent, you can save yourself time and angst by looking only at houses that fall within those parameters. This will help you make sure that the house you fall in love with also is one you can afford.