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Understanding Your Mortgage Payment
A step-by-step guide to how mortgage payments are calculated, what affects your monthly payment, and how to evaluate different loan scenarios.
The standard mortgage payment formula is:
\[M = P \times \frac{r(1 + r)^n}{(1 + r)^n - 1}\]A $300,000 loan at 6% for 30 years produces an estimated monthly payment of $1,799. Over the full term, you pay approximately $347,515 in interest on top of the original $300,000. This guide explains each piece of the formula and how different choices change the outcome.
What goes into a mortgage payment
Most monthly mortgage payments include four components, often called PITI: principal (the portion that reduces your loan balance), interest (the cost of borrowing the money), taxes (property taxes, often collected by the lender and held in escrow), and insurance (homeowner’s insurance, also often escrowed).
This guide focuses on principal and interest, which are the components determined by the loan terms. Taxes and insurance vary by location and property. The mortgage math guide covers how down payments and the 28/36 affordability rule fit into the larger picture of housing costs.
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Open Mortgage CalculatorThe amortization formula
The formula calculates a fixed payment amount that, if paid every month for the full term, will pay off both the principal and all accrued interest by the end of the loan. In the formula, M is the estimated monthly payment, P is the principal (the loan amount), r is the monthly interest rate (annual rate divided by 12), and n is the total number of payments (years times 12). The math balances interest charges against principal reduction so that the final payment brings the balance to exactly zero.
The monthly rate r deserves a closer look, because small differences in this number propagate through every one of the n payments. An annual rate of 6% gives r = 0.06 / 12 = 0.005. An annual rate of 7% gives r = 0.07 / 12 = 0.005833. That difference of 0.000833 per month seems tiny, but it applies to a six-figure balance hundreds of times. The post on simple vs. compound interest explains why this compounding effect is so large.
Worked example: $350,000 at 6.75% for 30 years
With P = 350,000, r = 0.0675 / 12 = 0.005625, and n = 360:
First compute (1 + r)^n: (1.005625)^360 is approximately 7.5116. Then the numerator becomes 0.005625 x 7.5116 = 0.04225, and the denominator becomes 7.5116 - 1 = 6.5116. The fraction is 0.04225 / 6.5116 = 0.006489. Multiply by the principal: 350,000 x 0.006489 = $2,271. That is the estimated monthly payment for principal and interest.
Over 360 payments, the total paid is approximately $817,560. Subtracting the $350,000 principal leaves approximately $467,560 in interest. The interest cost alone exceeds the original loan amount, which is a common result for 30-year loans above 6%.
How interest rates affect your payment
The interest rate has a large impact on both the monthly payment and the total cost of the loan. On a $300,000 loan over 30 years:
| Interest rate | Estimated monthly payment | Total interest |
|---|---|---|
| 5% | $1,610 | $279,767 |
| 6% | $1,799 | $347,515 |
| 7% | $1,996 | $418,527 |
A single percentage point increase on a $300,000 loan adds roughly $189 to the monthly payment and approximately $67,000 to total interest paid over 30 years. The reason the impact is so large is that interest compounds over the full loan term. A higher rate means more interest accrues each month, which means less of each payment goes toward reducing the principal, which means the principal stays higher longer and generates even more interest. For a detailed look at how this principal-to-interest ratio shifts over time, see the blog post on mortgage payment breakdowns.
15-year vs. 30-year terms
Shorter loan terms mean higher monthly payments but significantly less total interest. For a $300,000 loan at 6%:
| Loan term | Estimated monthly payment | Total interest |
|---|---|---|
| 15 years | $2,532 | $155,683 |
| 30 years | $1,799 | $347,515 |
The 15-year option costs $733 more per month but saves approximately $191,832 in interest over the life of the loan. The savings come from two factors: you pay the loan off in half the time, and the faster principal reduction means less interest accumulates each month. Whether this tradeoff fits depends on your monthly budget and other financial priorities.
A 20-year term splits the difference. On the same $300,000 at 6%, the estimated monthly payment is approximately $2,149, and total interest is approximately $215,838. That is $350 more per month than the 30-year option but saves approximately $131,677 in interest. Some lenders offer 20-year terms at rates slightly below the 30-year rate, making the savings even larger.
How amortization works
In the early years of a mortgage, most of each payment goes toward interest rather than principal. This happens because the lender calculates interest on the remaining balance each month, and the balance is highest at the start.
On a 30-year, $300,000 loan at 6%, roughly $17,940 goes to interest in year 1 and only $3,649 goes to principal. By year 15, the split is roughly even. By year 30, almost all of the payment goes to principal. This gradual shift is called amortization. It explains why extra payments early in the loan have the biggest impact on total interest. An extra $100 per month in the first year reduces the balance that generates interest for the next 29 years. The same extra payment in year 25 only affects the remaining 5 years.
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Open Compound Interest CalculatorThe effect of extra payments
Adding even a modest extra principal payment each month can shorten the loan and reduce total interest significantly. On a 30-year, $300,000 loan at 6% with the standard $1,799 payment:
| Extra monthly payment | Loan paid off in | Total interest | Interest saved |
|---|---|---|---|
| $0 (standard) | 30 years | $347,515 | $0 |
| $100 | 25 years, 2 months | $280,198 | $67,317 |
| $200 | 22 years | $230,477 | $117,038 |
| $500 | 16 years, 10 months | $152,247 | $195,268 |
An extra $100 per month saves approximately $67,317 in interest and eliminates nearly 5 years of payments. The math works because every extra dollar goes directly to principal, which immediately reduces the balance on which future interest is calculated. The savings multiply as the reduced balance generates less interest for every remaining month. Before making extra payments, verify with your lender that there is no prepayment penalty and that extra payments are applied to principal, not to future scheduled payments.
Interest rate vs. APR
Lenders quote two numbers: the interest rate and the annual percentage rate (APR). The interest rate is what the formula uses to calculate your monthly payment. The APR includes the interest rate plus certain fees, such as origination fees and discount points, spread over the life of the loan. It is designed to represent the true annual cost of borrowing.
Two loan offers might have the same interest rate but different APRs because one charges higher fees. The loan with the lower APR is typically the cheaper option overall. When comparing lenders, the APR gives a more complete picture than the interest rate alone.
What the calculator does not include
The mortgage calculator provides estimates for the principal and interest components of a payment. It does not account for property taxes (typically 0.5% to 2.5% of home value annually), homeowner’s insurance, private mortgage insurance (PMI, typically required for down payments below 20%), HOA fees, closing costs, or potential rate changes for adjustable-rate mortgages.
For a complete picture of housing costs, consult a mortgage professional who can account for all these factors based on your specific situation.