A mortgage payment can look like a number a lender simply hands over. In fact, the monthly principal and interest payment on a standard fixed-rate loan comes from a single formula, and understanding it makes it much easier to see why rates, loan terms and loan amounts change the payment the way they do.
The formula
For a fully amortizing fixed-rate loan:
M = monthly payment (principal + interest)
P = loan amount (principal)
r = monthly interest rate (annual rate ÷ 12)
n = number of monthly payments (years × 12)
The formula finds the single, level payment that pays off the loan exactly over n months while paying interest each month on whatever balance remains.
Working through it
A $300,000 loan at 6.5% for 30 years
- P = $300,000
- r = 6.5% ÷ 12 = 0.005417 per month
- n = 30 × 12 = 360 payments
Plugging those in gives a monthly principal and interest payment of $1,896.20.
| Over the life of the loan | Amount |
|---|---|
| Monthly payment | $1,896.20 |
| Total of all 360 payments | $682,633 |
| Total interest paid | $382,633 |
Hypothetical numbers for illustration. Excludes taxes, insurance and mortgage insurance.
Over 30 years, the borrower pays $382,633 in interest on top of the $300,000 borrowed.
Where each payment goes
The payment stays the same every month, but its makeup changes. Each month, interest is charged on the remaining balance, and whatever is left of the payment reduces the principal.
| Payment | Interest | Principal | Balance after |
|---|---|---|---|
| Month 1 | $1,625.00 | $271.20 | $299,729 |
| Month 2 | $1,623.53 | $272.67 | $299,456 |
In the first month, $1,625.00 of the $1,896.20 payment goes to interest and only $271.20 to principal. As the balance shrinks, the interest portion shrinks too, and more of each payment goes to principal. This schedule is called amortization.
The effect over time is significant:
| After | Remaining balance | Principal repaid |
|---|---|---|
| 5 years | $280,833 | $19,167 |
| 10 years | $254,328 | $45,672 |
| 20 years | $166,996 | $133,004 |
After five years of payments, only about 6% of the original loan has been repaid. The balance falls slowly at first and much faster toward the end.
What changes the payment
The interest rate. On the same $300,000, 30-year loan:
| Rate | Monthly P&I |
|---|---|
| 5.5% | $1,703.37 |
| 6.5% | $1,896.20 |
| 7.5% | $2,097.64 |
A one-point change in rate moves the payment by roughly $201 a month in this example.
The loan term. A 15-year loan at the same 6.5% rate has a payment of $2,613.32 — higher each month, because the principal is repaid in half the time — but total interest drops to $170,398, compared to $382,633 over 30 years.
The loan amount. The payment scales directly with principal. A loan 10% larger has a payment 10% larger, at the same rate and term.
P&I vs. the full housing payment
The formula above covers principal and interest only. The payment a borrower actually sends each month often includes more:
- Property taxes and homeowners or landlord insurance, frequently collected monthly into an escrow account.
- Mortgage insurance, on some loans with smaller down payments.
- HOA dues, if paid alongside the mortgage.
Together, principal, interest, taxes and insurance are often abbreviated PITI. For rental analysis, it’s important to know whether a quoted “payment” is P&I or PITI — counting taxes and insurance both inside the payment and again as operating expenses is a common double-count in cash flow calculations.
Why it matters for rental analysis
The mortgage payment is usually the single largest outflow for a financed rental. It’s what separates net operating income from cash flow, and it’s the denominator of DSCR. Small changes in rate or loan size ripple through every financed metric — cash flow, cash-on-cash return and coverage ratios alike.
The principal portion of each payment also matters for total return. It isn’t counted in monthly cash flow, but it does build equity — which is why broader measures like ROI include principal paydown as its own component.
Adjustable and interest-only loans
The formula above assumes a fixed rate and a fully amortizing schedule. Other structures behave differently:
- Adjustable-rate loans recalculate the payment when the rate resets, using the remaining balance and remaining term.
- Interest-only loans charge just the interest (P × r) during the interest-only period, so the balance doesn’t fall. When amortization begins, the payment rises.
Each of those is worth modeling explicitly rather than assuming a single fixed payment for the life of the loan.
A quick way to sanity-check a payment
Rearranging the formula isn’t necessary to check a quote. Two shortcuts help:
- Interest in month one is simply the loan amount times the monthly rate: $300,000 × 0.005417 = $1,625.00. The total payment must be larger than that, or the balance would never fall.
- A payment per $1,000 borrowed makes comparisons easy. At 6.5% for 30 years, the payment is about $6.32 per $1,000. Multiply by the loan size in thousands to estimate any payment at that rate and term.
If a quoted payment is far off from these quick checks, the quote may include taxes, insurance or mortgage insurance, or it may assume a different term or rate.
The takeaway
A fixed-rate mortgage payment comes from one formula that balances the loan amount, monthly rate and number of payments. Early payments are mostly interest; later payments are mostly principal. Rate, term and loan size each move the payment in predictable ways — and because the payment drives cash flow, coverage and return, it’s worth seeing those effects side by side before settling on any one set of loan terms.