A mortgage payment can look like a number a lender simply hands you, but the core of it comes from one standard formula. Once you know what goes into that formula, you can check a quote yourself, compare loan terms, and see why a small change in the interest rate can change your payment noticeably.
What goes into a mortgage payment
For a typical fixed-rate mortgage, the monthly payment you owe to the lender has two parts:
- Principal is the money you borrowed. Each payment reduces the remaining balance a little.
- Interest is what the lender charges for the loan. It is calculated each month on the balance you still owe.
According to the Consumer Financial Protection Bureau (CFPB), only the principal portion of a payment reduces your balance and builds equity. On a typical fixed-rate loan the principal-and-interest payment stays the same for the life of the loan, but the mix shifts over time: early payments are mostly interest, and later payments are mostly principal. That shift is called amortization, and we cover it in detail in our guide to mortgage amortization.
Many borrowers also pay property taxes and homeowners insurance through the same monthly payment. Those amounts are not part of the loan formula. We cover them in a later section.
The monthly payment formula
For a fully amortizing fixed-rate loan, the principal-and-interest payment is calculated with this standard formula:
M = P × r(1 + r)n ÷ ((1 + r)n − 1)
M is the monthly principal-and-interest payment. P is the loan amount (principal). r is the monthly interest rate, meaning the annual rate divided by 12, written as a decimal. n is the total number of monthly payments, meaning the loan term in years multiplied by 12.
The formula finds the single fixed payment that covers each month's interest and pays the balance down to exactly zero by the final payment. It assumes you make every payment on time, in full, and with no extra principal.
Why the rate is divided by 12
Mortgage rates are quoted as annual rates, but interest on a standard US mortgage is charged monthly. A 6.5% annual rate becomes 0.065 ÷ 12 ≈ 0.0054167 per month. Entering the annual rate directly is the most common mistake people make when they try the formula by hand.
A worked example, step by step
Example: $300,000 loan at a hypothetical 6.5% for 30 years
- Loan amount: P = $300,000.
- Monthly rate: r = 0.065 ÷ 12 ≈ 0.0054167.
- Number of payments: n = 30 × 12 = 360.
- Growth factor: (1 + r)360 ≈ 6.9918.
- Payment: M = 300,000 × 0.0054167 × 6.9918 ÷ (6.9918 − 1) ≈ $1,896.20 per month.
If every scheduled payment is made over 30 years, you would pay about $682,633 in total, of which about $382,633 is interest. Because of rounding to the cent, an actual lender schedule can differ by a few dollars, usually in the final payment.
The 6.5% rate here is a hypothetical number chosen to make the math easy to follow. It is not a prediction or a current market rate. To try your own numbers, use our mortgage payment calculator.
How rate and term change the payment
The formula has only three inputs: the loan amount, the rate, and the term. Changing any of them changes the payment, but in different ways.
Interest rate
The table below keeps the loan at $300,000 over 30 years and changes only the hypothetical rate. Each half-point step changes the monthly payment by roughly $95 to $102, and it changes total interest over the full term by tens of thousands of dollars.
| Hypothetical rate | Monthly payment | Total interest over 30 years |
|---|---|---|
| 5.5% | $1,703.37 | $313,212 |
| 6.0% | $1,798.65 | $347,515 |
| 6.5% | $1,896.20 | $382,633 |
| 7.0% | $1,995.91 | $418,527 |
| 7.5% | $2,097.64 | $455,152 |
Loan term
A shorter term means fewer payments, so each one has to be larger, but you pay interest for fewer years. At the same hypothetical 6.5% rate, a 15-year, $300,000 loan has a payment of about $2,613.32, roughly $717 more per month than the 30-year loan. Total interest over the 15 years, however, is about $170,398, less than half of the 30-year figure. In practice, lenders often price 15-year loans at different rates than 30-year loans, so compare actual offers rather than assuming the same rate.
Loan amount
The payment scales directly with the amount borrowed. At a fixed rate and term, borrowing 10% less lowers the principal-and-interest payment by 10%. That is why a larger down payment reduces the monthly payment, and it can also affect whether you pay mortgage insurance.
Costs beyond principal and interest
The formula gives you principal and interest only. Your actual monthly housing cost is often higher. People sometimes call the full amount "PITI": principal, interest, taxes, and insurance.
- Property taxes and homeowners insurance. The CFPB notes that many lenders require you to pay these through an escrow account. The servicer collects part of each payment and pays the bills when they are due. Because taxes and premiums can change from year to year, your escrow payment, and therefore your total payment, can go up or down even on a fixed-rate loan.
- Private mortgage insurance (PMI). The CFPB explains that you may be required to pay PMI on a conventional loan if your down payment is less than 20% of the purchase price. PMI protects the lender, not you. It is most commonly paid as a monthly premium added to your payment. Government-backed loans have their own mortgage insurance or fee rules.
- Homeowners association (HOA) dues. These are usually paid directly to the association rather than through the lender, but they still belong in your housing budget.
Example: Adding hypothetical escrow costs
Suppose the $300,000 loan above comes with hypothetical property taxes of $3,600 per year ($300 per month) and homeowners insurance of $1,800 per year ($150 per month). The estimated monthly payment becomes $1,896.20 + $300 + $150 = $2,346.20, before any PMI or HOA dues. Real tax and insurance costs vary widely by location and property.
Lenders look at your full housing payment, not just principal and interest, when they evaluate your debt-to-income ratio. Our mortgage affordability calculator lets you include these costs when estimating a comfortable price range.
Estimates vs. official loan figures
A calculator, including the formula above, gives an estimate. When you apply for a mortgage, the CFPB says the lender must give you a Loan Estimate within three business days of receiving your application. All lenders use the same standard form, which shows the projected payment, including estimated escrow and mortgage insurance, along with closing costs. Before closing, you receive a Closing Disclosure with the final terms.
The formula in this article applies to standard fixed-rate, fully amortizing loans. Adjustable-rate mortgages recalculate the payment when the rate changes, and interest-only or payment-option loans follow different rules. See our comparison of fixed-rate and adjustable-rate mortgages for how ARM payments can change.
Key takeaways
- The principal-and-interest payment comes from three inputs: loan amount, monthly rate, and number of payments.
- Always divide the annual rate by 12 and multiply the term in years by 12 before using the formula.
- At a hypothetical 6.5% rate, a $300,000, 30-year loan costs about $1,896.20 per month in principal and interest.
- Taxes, insurance, PMI, and HOA dues can add significantly to the total monthly cost.
- Use calculator results for planning, and rely on the official Loan Estimate for actual loan terms.
See how your own loan amount, rate, and term affect the payment, then view a full payment-by-payment schedule.
Open the mortgage payment calculatorWant to see how each payment splits between principal and interest? Try the mortgage amortization calculator or browse all mortgage calculators.
Sources
- How does paying down a mortgage work? — Consumer Financial Protection Bureau
- What is an escrow or impound account? — Consumer Financial Protection Bureau
- What is private mortgage insurance? — Consumer Financial Protection Bureau
- What is a Loan Estimate? — Consumer Financial Protection Bureau
- Loan Estimate explainer — Consumer Financial Protection Bureau
This article is for general educational purposes only and is not financial, legal, or tax advice. ClearPath Guides is not a lender, mortgage broker, or licensed adviser. Examples use hypothetical numbers, not current rates. Read our full disclaimer.
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