The basic formula and what each number means
Your monthly mortgage payment comes from four pieces of information: the loan amount, the interest rate, the loan term in years, and how many payments you'll make total. The formula banks use is called the amortization formula, and it produces a single number that covers principal and interest together.
The formula is: M = P × [r(1 + r)^n] / [(1 + r)^n − 1]. In this formula, M is your monthly payment, P is the principal (the amount you borrowed), r is your monthly interest rate (the annual rate divided by 12), and n is the total number of payments (years multiplied by 12). You don't need to memorize this—a calculator or spreadsheet does the work—but understanding what each variable represents helps you see why changing one number changes your payment.
The monthly interest rate is not straightforward the annual rate divided by 12. If your loan carries a 6% annual rate, your monthly rate is 0.06 ÷ 12 = 0.005, or 0.5%. That 0.5% applies to whatever balance remains each month, which is why the formula accounts for the compounding effect over time.
Key Takeaways
- Your monthly payment depends on three things: how much you borrowed, the interest rate, and how many months you have to repay it.
- The amortization formula calculates the payment that covers both principal and interest, and most of your early payments go toward interest rather than principal.
- A spreadsheet or online calculator applies the formula when ready; you only need to enter the loan amount, annual interest rate, and loan term in years.
- Changing the loan term from 30 years to 15 years raises your monthly payment but cuts the total interest you pay by tens of thousands of dollars.
- Property taxes, homeowners insurance, and mortgage insurance (PMI) are separate from the principal-and-interest calculation and add to your total housing cost.
Working through a concrete example
Say you borrow $300,000 at 6.5% annual interest over 30 years. Your monthly interest rate is 0.065 ÷ 12 = 0.00542 (rounded). Your total number of payments is 30 × 12 = 360. Plugging these into the formula gives you a monthly payment of roughly $1,896 for principal and interest alone.
In your first month, the interest portion is $300,000 × 0.00542 = $1,626. That means only $1,896 − $1,626 = $270 goes toward reducing what you owe. In month two, your balance is now $299,730, so the interest is slightly lower, and slightly more of your payment reduces the principal. This pattern continues for 360 months. By month 300, most of your payment finally goes toward principal rather than interest.
If you had instead chosen a 15-year loan at the same 6.5% rate, your monthly payment would be roughly $2,896—$1,000 more per month. But over 15 years you'd pay only about $434,000 total instead of $682,000 total. The shorter term means less time for interest to accumulate.
Using a spreadsheet to calculate the payment
Excel, Google Sheets, and most other spreadsheet programs have a built-in function called PMT that does the amortization calculation for you. The syntax is =PMT(rate, nper, pv), where rate is your monthly interest rate, nper is the number of payments, and pv is the loan amount (entered as a negative number).
For the $300,000 example above, you would enter =PMT(0.065/12, 360, -300000). The result is −$1,896.20 (the negative sign indicates money flowing out). If you want the result as a positive number, wrap the formula in ABS: =ABS(PMT(0.065/12, 360, -300000)).
You can change any of the three inputs and see the payment recalculate when ready. This makes it straightforward to compare a 30-year loan against a 20-year loan, or to see how a 0.5% rate difference affects your payment. Many people build a small spreadsheet with several scenarios side by side so they can compare options before locking in a rate with a lender.
Why your actual monthly payment is higher than the formula shows
The amortization formula gives you principal plus interest only. Your actual mortgage payment, called PITI, includes four components: Principal, Interest, Taxes, and Insurance. Property taxes and homeowners insurance are often rolled into your mortgage payment and held in an escrow account by your lender, who pays them on your behalf when they're due.
If your down payment was less than 20% of the home's purchase price, your lender will also require private mortgage insurance (PMI), which protects the lender if you default. PMI typically costs 0.5% to 1% of the loan amount per year, divided into your monthly payment. A $300,000 loan with 0.7% PMI adds roughly $175 per month.
Property taxes vary by location and are assessed on the home's value, not the loan amount. Homeowners insurance also varies by location, home age, and coverage level. Neither of these is part of the amortization formula, so you need to research your specific county's tax rate and get insurance quotes to know your true total payment.
How interest rate changes affect your payment
A 1% difference in interest rate changes your monthly payment by roughly 10% to 12%, depending on the loan term. On a $300,000, 30-year loan, the difference between 5.5% and 6.5% is about $190 per month. Over 30 years, that's nearly $69,000 in additional interest paid.
This is why shopping for rates across multiple lenders matters. A lender offering 6.25% instead of 6.5% saves you money every single month for the life of the loan. Some lenders charge origination fees or points to buy down the rate; the math on whether that trade-off is worth it depends on how long you plan to stay in the home.
Rate changes also affect how much of each payment goes toward interest versus principal. At a lower rate, more of your early payments reduce the principal. At a higher rate, interest consumes a larger share of each payment, and you build equity more slowly at first.
Comparing loan terms: 15-year versus 30-year
A 15-year mortgage has a higher monthly payment but costs far less in total interest. A 30-year mortgage has a lower monthly payment but you pay interest for twice as long. The choice depends on your cash flow and long-term financial goals.
| Loan Amount | Interest Rate | Term | Monthly Payment (P&I) | Total Interest Paid |
|---|---|---|---|---|
| $300,000 | 6.5% | 30 years | $1,896 | $382,000 |
| $300,000 | 6.5% | 15 years | $2,896 | $220,000 |
| $300,000 | 6.0% | 30 years | $1,799 | $347,000 |
If you can afford the higher payment, a 15-year loan saves you over $160,000 in interest on a $300,000 loan. But if your budget is tight or you want to keep monthly obligations low, a 30-year loan gives you more flexibility. Some people choose a 30-year loan but make extra principal payments when cash flow allows, which shortens the payoff without locking in the higher 15-year payment.
What happens when you make extra principal payments
Any payment above your required monthly amount goes directly toward principal, not interest. If your payment is $1,896 and you send $2,000, that extra $104 reduces your balance when ready, which means less interest accrues in the following month.
Making extra principal payments early in the loan saves the most interest, because you're reducing the balance while interest rates are still being applied to a larger amount. A single extra $100 payment in month one might save you $3,000 in total interest over the life of the loan, while the same $100 payment in month 300 saves you only a few dollars.
Before making extra payments, confirm with your lender that there is no prepayment penalty. Most conventional mortgages have no penalty, but some older loans or specialized mortgages do. If there's no penalty, extra principal payments are one of the most direct ways to reduce the total cost of your loan.
Frequently Asked Questions
Can I use an online calculator instead of doing the math myself?
Yes. Most mortgage lenders, real estate websites, and financial sites offer free calculators that do the amortization formula for you. You enter the loan amount, interest rate, and term, and the calculator shows your monthly payment when ready. These are accurate as long as you enter the right numbers, and they save you from doing the formula by hand.
Why does my actual mortgage payment differ from what the calculator shows?
The calculator shows principal and interest only. Your actual payment also includes property taxes, homeowners insurance, and possibly PMI if your down payment was under 20%. These vary by location and your specific home, so the calculator can't include them. Your lender's loan estimate will show the full PITI payment.
Does paying off the mortgage early hurt my credit score?
Paying off early does not hurt your credit score. Your score reflects your payment history and credit mix, not how quickly you pay off a loan. Paying on time every month builds your score; paying off the loan early straightforward ends the account, which has no negative effect.
What's the difference between a fixed-rate and adjustable-rate mortgage payment?
A fixed-rate mortgage has the same interest rate and payment for the entire loan term. An adjustable-rate mortgage (ARM) has a fixed rate for an initial period (often 3, 5, 7, or 10 years), then the rate adjusts periodically based on market conditions. Your payment stays the same during the fixed period, then rises or falls when the rate adjusts. The amortization formula applies to both, but with an ARM you need to know when and how the rate will change.
If I increase my down payment, how much does my monthly payment drop?
Each dollar of additional down payment reduces your loan amount by one dollar, which lowers your monthly payment proportionally. A $50,000 down payment instead of $30,000 reduces the loan from $300,000 to $280,000, which lowers your monthly payment by roughly $316 on a 30-year, 6.5% loan. A larger down payment also eliminates or reduces PMI if you reach 20% equity.