How to Determine Your Mortgage Affordability: A Practical Guide

Understanding the 28/36 rule

The 28/36 rule is a widely used guideline for home affordability. It states that your monthly housing expenses should not exceed 28% of your gross monthly income, and your total monthly debt payments—including housing and other debts—should not exceed 36% of your gross monthly income. This rule helps you gauge a comfortable spending range before you start house hunting.

The 28% portion (front-end ratio) covers your mortgage principal and interest, property taxes, homeowners insurance, HOA fees if applicable, and private mortgage insurance (PMI) if your down payment is less than 20%. The 36% portion (back-end ratio) includes all of those housing costs plus other recurring debts such as credit card minimum payments, auto loans, student loans, personal loans, and child support.

This rule is a useful starting point, but keep in mind that lenders may allow higher ratios with compensating factors like excellent credit, a large down payment, or substantial savings. It's a guideline, not a strict underwriting requirement.

Sources: NerdWallet, Rocket Mortgage, CNBC Select, Zillow, Movement Mortgage, LendingTree

Calculating your gross monthly income

Your gross monthly income is the amount you earn before taxes and other deductions. To calculate it, you can use several methods depending on your pay structure.

If you are salaried, divide your annual salary by 12. If you are paid hourly, multiply your hourly wage by your average weekly hours, then multiply by 52 and divide by 12.

If your income varies, use an average based on your recent pay stubs or tax returns. The key is to use your gross income, not your take-home pay, because the 28/36 rule is based on what you earn before deductions.

Once you have this figure, you can apply the 28% and 36% guidelines to calculate your maximum housing expense and total debt limit.

Sources: Zillow, LendingTree

Estimating your debt-to-income ratio

Your debt-to-income ratio (DTI) is the percentage of your gross monthly income that goes toward debt payments. To calculate it, divide your total monthly debt payments by your gross monthly income and multiply by 100.

The 28/36 rule suggests keeping your total DTI at or below 36%, but lenders may allow different maximums. Conventional loans often permit a back-end DTI up to 43%, while some lenders may accept higher with compensating factors. FHA loans can allow a DTI up to 43% or more in certain cases. VA loans typically cap the back-end ratio at 41%. These limits vary by lender and loan program.

Understanding your DTI helps you see how much of your income is already committed to debts, and it directly affects the maximum mortgage payment you can afford under the 36% rule.

Sources: LendingTree, Zillow

Determining a safe down payment

Your down payment is a key factor in affordability. A larger down payment reduces the amount you borrow and lowers your monthly payment. If you put down at least 20% of the home's purchase price, you typically avoid private mortgage insurance (PMI), which reduces your monthly housing costs.

There is no universal down payment amount that fits every buyer. Consider your savings, monthly budget, and other financial goals when deciding how much to put down. A higher down payment can also help you qualify for a lower interest rate, which further enhances affordability.

Sources: Consumer Financial Protection Bureau, Zillow, Rocket Mortgage

Factoring in taxes, insurance, and PMI

Your monthly housing payment is more than just principal and interest. Property taxes and homeowners insurance are often bundled into your monthly mortgage payment, and if your down payment is less than 20%, you'll likely have to pay private mortgage insurance (PMI) as well. These costs can significantly increase your monthly outlay.

A basic mortgage calculator typically shows only principal and interest, which can understate the true cost of homeownership. Always add estimated property taxes, insurance, and PMI to get a realistic picture of your total housing expense. Flood insurance may also be required if your property is in a FEMA-designated Special Flood Hazard Area.

The amounts vary widely by location, property value, and borrower profile, so use local estimates to calculate a more accurate monthly payment.

Sources: Consumer Financial Protection Bureau

Using an affordability calculation example

Let's walk through a step-by-step example using the 28/36 rule.

From that amount, set aside portions for property taxes and homeowners insurance.

With a 30-year fixed mortgage at an interest rate of 6.85%, you can use a mortgage payment formula to estimate the loan amount. The formula for the monthly principal and interest payment is P = (r * L) / (1 - (1 + r)^(-n)), where r is the monthly interest rate (annual rate divided by 12), L is the loan amount, and n is the number of monthly payments (360 for a 30-year loan). Rearranging to solve for L, we get L = P * (1 - (1 + r)^(-n)) / r. Then (1 + r)^(-360) is approximately 0.1267, so (1 - 0.1267) = 0.8733. Dividing by r gives 153.0.

If you have a 20% down payment, the maximum home price would be the loan amount divided by 0.80.

Remember, this is an estimate. Your actual affordability depends on the interest rate you qualify for, the exact tax and insurance costs, and your down payment amount. The 28/36 rule is a starting point, and lenders will assess your complete financial picture.

Sources: NerdWallet, Consumer Financial Protection Bureau, LendingTree