Mortgage Calculator (Washington State)

Your monthly mortgage payment can be calculated using the following formula: M = P [ I(1 + I)^N ] / [ (1 + I)^N - 1] 

Here's a breakdown of the variables within the formula to help you understand it better: 

• M = monthly payment: This is the value you want to find. 
• P = principal amount: It represents your loan balance, which is the amount you're aiming to pay off. 
• I = interest rate: Be sure to use the base interest rate, not the APR. To get the monthly interest rate, divide the annual rate by 12. 
• N = number of payments: This is the total count of payments throughout your loan term. For example, in a 30-year mortgage with monthly payments, there would be 360 payments in total. 

It's worth noting that mortgage calculators typically assume fixed-rate mortgages. However, if you have an adjustable-rate mortgage (ARM) with changing interest rates over time, you can set up an amortization table using spreadsheet software like Microsoft Excel. This allows you to adjust the formula when the interest rate changes, ensuring your calculations remain accurate.

To calculate the principal of your mortgage, you start by deducting your down payment from the total purchase price of your home. 

Let's illustrate this with an example: If you're purchasing a house for $450,000 and you make a 20% down payment, which amounts to $90,000, your remaining loan amount is $360,000. This $360,000 is your principal balance. 

Understanding the principal is crucial when determining your home affordability because it's the base amount you borrow, and interest begins accumulating as soon as you secure the loan. To get an estimate of your monthly mortgage payment within your budget, you can use our mortgage calculator. By entering details like the purchase price, down payment, and other relevant factors, the calculator provides you with a rough monthly payment estimate. Remember that when setting a comfortable mortgage payment, you should also consider other expenses such as maintenance, insurance, taxes and repairs associated with homeownership.

Continuing with our example, suppose you have a 30-year mortgage with an annual interest rate of 7%. Because you're making monthly payments, the 7% annual interest rate is divided by 12 to calculate the monthly interest. In this case, your initial monthly payment would consist of $2,625 in interest, which is computed as follows: ($450,000 principal × 0.07 annual interest rate) ÷ 12 months. 

To break it down further: The principal is determined as the purchase price minus the down payment. The monthly interest is computed by taking the principal and multiplying it by the interest rate, then dividing it by 12 months. The monthly principal can be found by subtracting the interest payment from the total monthly mortgage payment. In summary, the initial monthly payment on your mortgage is divided between the interest and principal components, with the interest being a portion of the outstanding loan balance, while the principal is the remainder.

On a $300,000, 30-year mortgage with an 8% APR, you can expect a monthly payment of about $2,201.29, not including taxes and interest (these vary by location and property).

On a $400,000, 30-year mortgage with an 8% APR, you can expect a monthly payment of about $2,935.06, not including taxes and interest (these vary by location and property).

 Prospective homebuyers' ability to afford a property and the size of the loan they can secure largely depend on various factors, but the main considerations include their income, debt, assets and liabilities.

• Gross income: This is your total income before taxes and other financial commitments. It encompasses your base salary, any additional income like bonuses, part-time earnings and self-employment income, as well as benefits such as Social Security, disability, alimony and child support. 
• Front-end ratio: This ratio, also known as the mortgage-to-income ratio, is determined by your gross income and represents the percentage of your annual gross income that can be allocated to your monthly mortgage payment. The monthly mortgage payment includes principal, interest, taxes and insurance (PITI). A general guideline is that the front-end ratio, based on PITI, should not exceed 28% of your gross income. However, some lenders allow borrowers to surpass 30% or even 40%. 
• Back-end ratio: The back-end ratio, or debt-to-income ratio (DTI), calculates the portion of your gross income needed to cover your debts, including credit card payments, child support and other outstanding loans (such as auto or student loans). For example, if you pay $2,000 in debt services each month and earn $4,000 monthly, your ratio is 50%—half your income goes toward debt. Most lenders recommend that your DTI does not exceed 43% of your gross income. 
• Credit score: Mortgage lenders evaluate your credit score to assess the risk of lending to you. A low credit score may result in a higher interest rate (APR) on your loan. To secure a home, monitor your credit reports and address any inaccuracies promptly.
• The 28%/36% rule: This heuristic helps calculate the appropriate amount of housing debt to assume. According to this rule, a maximum of 28% of your gross monthly income should cover housing expenses, and no more than 36% should go toward total debt service, which includes housing and other debts like car loans and credit cards. Lenders use this rule to gauge whether to extend credit to borrowers and sometimes it is adjusted to use slightly different percentages, such as 29%/41%.