How to Calculate Your Mortgage Payment — Step by Step
Understanding exactly how your mortgage payment is calculated gives you real power when buying a home. Most people accept whatever number the bank gives them — but when you understand the math, you can negotiate better, choose the right loan term, and make smarter decisions about extra payments.
This guide walks you through the complete mortgage payment formula, explains every variable, shows worked examples with real numbers, and explains how amortization works over the life of your loan.
The Core Mortgage Payment Formula
The standard formula for calculating a fixed-rate monthly mortgage payment is derived from the present value of an annuity. It looks intimidating at first, but every variable has a clear, practical meaning:
Where:
M = Monthly payment
P = Principal loan amount
r = Monthly interest rate (annual rate ÷ 12)
n = Total number of payments (loan term in years × 12)
What Each Variable Means
Principal (P)
The principal is the amount you borrow — the home purchase price minus your down payment. If you buy a $350,000 home and put down $70,000 (20%), your principal is $280,000. A larger down payment reduces your principal, which reduces your monthly payment and total interest paid over the life of the loan.
Monthly Interest Rate (r)
Lenders quote interest rates annually, but mortgage payments are calculated monthly. To get the monthly rate, divide the annual rate by 12. A 6.5% annual rate becomes 6.5% ÷ 12 = 0.5417% per month, or 0.005417 as a decimal. This seems small, but compounded over 360 payments, it adds up significantly.
Number of Payments (n)
A 30-year mortgage has 30 × 12 = 360 monthly payments. A 15-year mortgage has 15 × 12 = 180 payments. Choosing a shorter term increases your monthly payment but dramatically reduces total interest paid — often saving tens of thousands of dollars.
Worked Example — $280,000 Mortgage at 6.5%
The Numbers
Home price: $350,000
Down payment: $70,000 (20%)
Principal (P): $280,000
Annual interest rate: 6.5%
Monthly rate (r): 6.5% ÷ 12 = 0.5417% = 0.005417
Loan term: 30 years
Number of payments (n): 360
Calculation:
M = 280,000 × [0.005417 × 6.8226] / [6.8226 - 1]
M = 280,000 × [0.036954] / [5.8226]
M = 280,000 × 0.006321
M = $1,769.89 per month
Total paid over 30 years: $1,769.89 × 360 = $637,160
Total interest paid: $637,160 - $280,000 = $357,160
That $357,160 in interest is more than the original loan amount. This is why understanding your mortgage is so important — and why making even small extra payments can save you enormous amounts of money.
30-Year vs 15-Year Mortgage Comparison
Using the same $280,000 principal at 6.0% (15-year rates are typically lower):
| Factor | 30-Year Mortgage | 15-Year Mortgage |
|---|---|---|
| Interest rate | 6.5% | 6.0% |
| Monthly payment | $1,769.89 | $2,364.03 |
| Total paid | $637,160 | $425,525 |
| Total interest | $357,160 | $145,525 |
| Interest savings | — | $211,635 |
Choosing a 15-year mortgage costs $594 more per month but saves $211,635 in interest. Whether that trade-off makes sense depends on your income stability, other financial goals, and how long you plan to stay in the home.
How Amortization Works
Every mortgage payment is split between interest and principal repayment. In the early years of a 30-year mortgage, the vast majority of each payment goes toward interest. Over time, as the outstanding balance decreases, more of each payment goes toward principal.
| Payment | Payment Amount | Interest Portion | Principal Portion | Remaining Balance |
|---|---|---|---|---|
| 1 | $1,769.89 | $1,516.67 | $253.22 | $279,746.78 |
| 12 | $1,769.89 | $1,510.16 | $259.73 | $278,487.52 |
| 60 | $1,769.89 | $1,479.83 | $290.06 | $272,878.65 |
| 180 | $1,769.89 | $1,351.18 | $418.71 | $248,800.22 |
| 360 | $1,769.89 | $9.56 | $1,760.33 | $0.00 |
The Power of Extra Payments
Making extra payments toward your principal is one of the most effective ways to save money on a mortgage. Even $100 extra per month on a $280,000 30-year mortgage at 6.5% would save approximately $58,000 in interest and pay off the loan 4 years early.
Extra payments are most effective early in the loan term when the balance is high and interest charges are greatest. Some lenders charge prepayment penalties — always check your loan agreement before making extra payments.
Other Costs in Your Monthly Payment
The formula above calculates principal and interest only. Your actual monthly housing cost typically includes:
Property taxes — typically 1-2% of home value annually, divided into 12 monthly escrow payments.
Homeowner's insurance — typically $100-$200 per month depending on location and coverage.
PMI (Private Mortgage Insurance) — required if your down payment is less than 20%, typically 0.5-1.5% of the loan amount annually.
HOA fees — if applicable, can range from $50 to $500+ per month.
Our mortgage calculator includes fields for property tax and insurance so you can see your complete estimated monthly payment, not just the principal and interest portion.
Key Takeaways
The mortgage payment formula M = P × [r(1+r)^n] / [(1+r)^n - 1] gives you the exact monthly principal and interest payment for any fixed-rate loan. The three key levers you control are your down payment (which sets the principal), the interest rate you negotiate, and your choice of loan term. Understanding amortization shows why extra payments early in the loan term have such a dramatic impact on total interest paid. Use our free mortgage calculator to run your own numbers instantly without sign-up or data entry.