How Mortgage Payments Are Calculated (With Formula)
Your monthly mortgage payment is calculated from the loan amount, the monthly interest rate, and the number of monthly payments, run through a fixed amortization formula. Change any one of those and the payment changes with it, but not in equal steps, which is why a $10,000 bigger down payment saves more than $10,000 worth of interest over the life of the loan.
The formula behind every fixed-rate mortgage
Lenders use the standard amortization formula:
M = P × [r(1+r)^n] / [(1+r)^n − 1]- M = monthly principal and interest payment
- P = loan amount (home price minus down payment)
- r = monthly interest rate (annual rate divided by 12)
- n = total number of monthly payments (loan term in years times 12)
The formula spreads the loan into equal monthly payments for the full term. What varies month to month is the split inside that payment: how much goes to interest versus how much reduces your principal. Interest is charged on whatever balance remains, so as the balance shrinks, the interest portion shrinks with it and the principal portion grows, even though the total payment stays flat for the entire term.
A 30-year fixed loan at 6.5% has r = 0.065 / 12 = 0.005417 and n = 360. Plug those into the formula along with P and you get a fixed monthly number that does not move unless you refinance, pay extra, or the rate resets (on an adjustable loan).
Worked example: $400,000 home, 20% down, 6.5%, 30 years
Take a $400,000 home with a $80,000 down payment (20%), a 6.5% annual rate, and a 30-year term.
- Loan amount (P): $320,000
- Monthly principal & interest payment: $2,023
- Total interest paid over 30 years: $408,142
- Total cost of the loan (principal + interest): $728,142
Look closely at that last number. On a $320,000 loan, you end up paying $728,142 by the time it’s fully amortized, more than double the amount borrowed. That is the cost of borrowing money over three decades at 6.5%, not a fee or a markup, just compounding interest on a large balance for a long time.
Here is how the balance and cumulative interest move at different points in the term:
| Year | Remaining balance | Cumulative interest paid |
|---|---|---|
| 1 | $316,423 | $20,695 |
| 5 | $299,555 | $100,912 |
| 10 | $271,284 | $193,998 |
| 15 | $232,189 | $276,260 |
| 30 | $0 | $408,142 |
Two things stand out. First, after five full years of payments (60 payments, over $121,000 paid in), the balance has only dropped from $320,000 to $299,555, a reduction of about $20,000. Most of what you paid went to interest. Second, the pace of principal payoff accelerates later: between year 15 and year 30, the balance drops by $232,189, more than double the $87,811 reduction seen in the first 15 years combined. This is the direct result of the formula: interest is charged on the current balance, so a large early balance produces large early interest charges, and a shrinking balance produces shrinking interest charges, leaving more of each fixed payment for principal.
How much does your down payment matter
Same $400,000 home, same 6.5% rate, same 30-year term, two down payment scenarios:
| Down payment | Loan amount | Monthly P&I | Total interest over the loan |
|---|---|---|---|
| 10% ($40,000) | $360,000 | $2,275 | $459,160 |
| 20% ($80,000) | $320,000 | $2,023 | $408,142 |
Doubling the down payment from $40,000 to $80,000 (an extra $40,000 up front) cuts the monthly payment by $252 and cuts total interest by roughly $51,000 over the life of the loan. The mechanism is the same as above: a smaller starting balance means the formula charges interest on less money every single month, for 360 months. That extra $40,000 paid today effectively buys back $51,000 in future interest, on top of the smaller monthly payment. It’s also why lenders often price loans below 20% down with private mortgage insurance on top: a bigger loan against the same asset is more risk to them.
Calculate it with your own numbers
Enter your own home price, down payment, rate, and term below. The widget also lets you add annual property tax and home insurance for a fuller monthly estimate, and shows the full year-by-year amortization schedule.
| Payment | Principal | Interest | Remaining balance |
|---|
What people get wrong
Treating the P&I figure as the full payment. The formula above returns principal and interest only. Most homeowners also pay property tax and home insurance monthly, usually collected through an escrow account and added on top. If your annual property tax is $4,800 and annual insurance is $1,200, that’s another $500/month regardless of your loan terms. Leaving these out understates your real housing cost, sometimes by hundreds of dollars a month.
Assuming the principal/interest split is constant. It isn’t. Early payments on a 30-year loan are interest-heavy; late payments are principal-heavy. Someone budgeting based on “half my payment builds equity” is usually wrong in year 2 and closer to right in year 25.
Not weighing what a shorter term actually buys. A 15-year term raises the required monthly payment noticeably, since the same balance is repaid over half the time. But it cuts total interest dramatically, because the balance never gets a chance to sit at a high level for three decades collecting interest. If your budget can absorb the higher payment, the term length is one of the most direct levers you have on total cost.
Ignoring extra principal payments. Any payment above the required monthly amount, applied directly to principal, reduces the balance that future interest is calculated on. Because interest recalculates off the current balance every period, even modest extra payments made early in the loan (when the balance is largest) remove a disproportionate amount of future interest, and can shave years off the payoff date.
Frequently asked questions
What is amortization? Amortization is the process of paying off a loan through fixed periodic payments, where each payment covers that period’s interest plus a portion of the principal. The schedule that shows the balance, interest, and principal for every payment is called an amortization schedule, and it’s exactly what the calculator above generates.
Does a 15-year term really save that much interest? Yes, substantially. Because the balance is repaid twice as fast, it spends far less total time accruing interest, and the effective rate charged over the shorter term is often lower too. The tradeoff is a materially higher required monthly payment, so the right term depends on what your budget can sustain, not just on minimizing total cost.
Can I pay off my mortgage faster without refinancing? Yes. Making extra principal payments, whether a lump sum or a bit extra each month, reduces the balance that interest is calculated on going forward. Check your loan for prepayment penalties first, but most conventional mortgages allow extra principal payments with no fee, and even small, consistent extra payments compound into meaningful interest savings and an earlier payoff date.
Why is my actual monthly payment higher than the principal and interest number? Because your lender is likely collecting property tax and home insurance through escrow and folding it into one monthly bill. The formula in this article calculates principal and interest only; add your annual property tax and insurance divided by 12 to estimate your total monthly housing payment.
Does the interest rate or the loan term affect the payment more? Both move the payment, but not symmetrically. A rate change affects every payment for the life of the loan, front-loaded because balances are largest early on. A term change redistributes the same total principal over more or fewer payments. In practice, a full percentage point on the rate typically moves the payment and total interest more than shortening the term by a few years, which is why shopping for rate matters as much as negotiating price.