A mortgage is likely the largest debt most people ever take on — and mortgage interest is how lenders make money on that debt. Over the life of a typical 30-year mortgage, you may pay more in interest than the original amount you borrowed. Understanding exactly how that interest is calculated, how amortization works, and how your choices around loan term, rate, and extra payments affect the total cost can save you tens of thousands of dollars.
How Lenders Make Money on Mortgages
When a bank lends you $400,000 to buy a home, it takes on risk (the possibility you won't repay) and the opportunity cost of that capital. The interest rate is the price you pay for the use of that money. Lenders profit on the spread between what they pay depositors and the rate they charge mortgage borrowers. They also sell many mortgages to secondary market investors (like Fannie Mae and Freddie Mac), recouping capital to make new loans.
Fixed vs Adjustable Rate Mortgages
A fixed-rate mortgage maintains the same interest rate for the entire loan term — your monthly payment never changes, making budgeting simple and predictable. An adjustable-rate mortgage (ARM) starts with a fixed period (often 5, 7, or 10 years) and then adjusts annually based on a benchmark index like the Secured Overnight Financing Rate (SOFR). A 5/1 ARM has a fixed rate for 5 years, then adjusts every year thereafter. ARMs typically offer a lower initial rate than 30-year fixed mortgages but carry the risk of payment increases if rates rise. ARMs make sense if you plan to sell or refinance before the adjustment period begins.
How Monthly Interest Is Calculated
The formula is simple: Monthly interest = (Annual interest rate ÷ 12) × Outstanding loan balance. On a $400,000 mortgage at 6% annual interest: (0.06 ÷ 12) × $400,000 = $2,000 in interest for the first month. If the total monthly payment is $2,398, then only $398 reduces the principal balance in month one. As the balance decreases over time, the interest portion of each payment shrinks — which is the essence of amortization.
Amortization: Why Early Payments Are Mostly Interest
Mortgage payments are structured so the total monthly payment remains constant over the loan term — this is called amortization. In month one of a $400,000, 6%, 30-year mortgage, $2,000 of the $2,398 payment goes to interest (83.4%) and only $398 reduces principal. By year 10, the interest-to-principal split is roughly 50/50. By year 25, most of each payment is going to principal. This front-loading of interest benefits the lender and means that most of the interest cost of a mortgage is locked in early — which is why refinancing or selling early in a loan has less equity-building impact than many homeowners expect.
30-Year vs 15-Year: A Real Comparison
The choice between a 30-year and 15-year mortgage is one of the most consequential financial decisions homeowners make. On a $400,000 loan at current market rates (approximately 6% for 30-year, 5.5% for 15-year):
- 30-year fixed at 6%: Monthly payment $2,398 | Total interest paid ≈ $463,000
- 15-year fixed at 5.5%: Monthly payment $3,268 | Total interest paid ≈ $188,000
- Interest savings from 15-year: approximately $275,000
- Monthly payment difference: $870 more per month on the 15-year
- Break-even thinking: that $870/month could be invested — compare the mortgage interest savings vs investment returns
APR vs Interest Rate: What's the Difference
The interest rate is the annual cost of borrowing, expressed as a percentage. The Annual Percentage Rate (APR) includes the interest rate plus other costs: origination fees, discount points, mortgage broker fees, and certain closing costs. The APR is always higher than the stated interest rate (unless no fees are charged) and provides a more complete picture of the true cost of the loan. When comparing mortgage offers from different lenders, compare APRs — not just interest rates — to get an apples-to-apples comparison.
Points: Buying Down Your Rate
Mortgage points (also called discount points) are upfront fees paid to the lender in exchange for a lower interest rate. One point equals 1% of the loan amount. On a $400,000 loan, one point costs $4,000. Paying one point might lower your rate from 6.5% to 6.25%. Break-even analysis: if that 0.25% rate reduction saves $60/month, you break even in 67 months (5.6 years). If you plan to stay in the home for at least 6 years, paying points makes financial sense. If you might sell or refinance sooner, skip the points and keep the cash.
Credit Score Impact on Your Rate
Your credit score is one of the most powerful factors lenders use to price your mortgage. On a $400,000, 30-year mortgage, a 760+ credit score might qualify for a 6.0% rate while a 620 credit score might receive 7.5% — a 1.5% difference that translates to over $130,000 in additional interest over the loan life. Key credit score thresholds for mortgage pricing: 760+ (best rates), 740–759, 720–739, 700–719, 680–699, 660–679, 640–659, and 620–639 (minimum for conventional loans). Improving your credit score before applying for a mortgage is one of the highest-return financial moves available.
Private Mortgage Insurance (PMI)
If your down payment is less than 20% of the purchase price on a conventional loan, lenders require Private Mortgage Insurance (PMI). PMI protects the lender — not you — if you default. It typically costs 0.5%–1.5% of the loan amount per year, added to your monthly payment. On a $400,000 loan, PMI might add $167–$500 per month. PMI can be canceled once you reach 20% equity in your home (by paying down the principal or through appreciation). FHA loans have their own version of mortgage insurance (MIP) that can last the full loan term regardless of equity.
Escrow Accounts
Most mortgage lenders require an escrow account — a separate account managed by the servicer — to collect monthly contributions for property taxes and homeowner's insurance. Your total monthly mortgage payment (often called PITI) includes: Principal, Interest, Taxes (property), and Insurance. The escrow portion is not interest, but it is a required part of your payment. Each year, the servicer recalculates the required escrow amount based on actual tax and insurance costs. Increases in property taxes or insurance premiums will raise your total monthly payment even if your interest rate is fixed.
The Mortgage Interest Deduction
The mortgage interest deduction allows homeowners to deduct interest paid on mortgage debt from their federal taxable income. Since 2018, the deduction is limited to interest on up to $750,000 of mortgage debt (down from $1 million). To benefit, your total itemized deductions must exceed the standard deduction ($14,600 for singles, $29,200 for married filing jointly in 2024). With high mortgage interest in the early years of a loan and potentially high property taxes and charitable contributions, many homeowners — especially those with recent large mortgages in high-tax states — can still benefit from itemizing.
Strategies to Pay Off Early and Save Interest
- Make one extra principal payment per year — can reduce a 30-year loan by 5–6 years
- Add $100–$200 to every monthly payment and specify it goes to principal reduction
- Switch to biweekly payments — 26 half-payments equal 13 full payments per year (one extra)
- Apply windfalls (tax refunds, bonuses) directly to principal
- Refinance to a shorter term when rates drop and you can afford the higher payment
When Refinancing Makes Sense
Refinancing replaces your existing mortgage with a new one, typically to get a lower rate or change the loan term. The traditional rule of thumb — refinance when rates drop 1% or more — is oversimplified. The real question is the break-even point: divide your total closing costs by your monthly savings. If closing costs are $6,000 and you save $200/month, break-even is 30 months (2.5 years). If you plan to stay longer, refinancing makes sense. Pitfalls: resetting to a new 30-year term extends the loan and may cost more in total interest even at a lower rate; closing costs add up; and cash-out refinancing can dangerously erode home equity.
Use an amortization calculator to see exactly how extra payments reduce your total interest and shorten your loan term. Even $100/month in extra principal payments on a $400,000, 6%, 30-year mortgage saves approximately $47,000 in interest and eliminates about 5 years from the loan.
