How Mortgage Amortization Works

Bottom line up front: On a $400,000 loan at 6.5% for 30 years, your monthly principal-and-interest payment is $2,528.27. In year one, 85% of that payment goes to interest. By year 30, nearly all of it goes to principal. Total interest paid over 30 years: $510,178 — more than the original loan amount. Run your own numbers with our mortgage calculator.

What amortization is

Amortization comes from the Latin root meaning “to kill off” — you are gradually killing off a debt through scheduled payments. In mortgage lending, a fully-amortizing loan is structured so that each fixed monthly payment covers the accrued interest first, then applies the remainder to principal. Because the payment is constant but the interest component shrinks as the balance falls, an ever-increasing share of each payment goes to principal as time passes.

This is distinct from an interest-only loan, where payments cover only interest and principal is never reduced until a balloon payment at the end. It is also distinct from a negative-amortization loan, where payments are insufficient to cover interest and the balance actually grows over time. Standard conventional mortgages underwritten under Fannie Mae guidelines are fully amortizing (Fannie Mae 2025-26 conforming loan limit announcement, FHFA).

The conforming loan limit for most of the United States stands at $766,550 for single-family properties as of the 2025-26 FHFA announcement. Loans at or below this threshold that meet Fannie Mae/Freddie Mac underwriting standards qualify as conforming and generally carry lower interest rates than jumbo loans. Our worked example at $400,000 falls well within the conforming limit.

The amortization formula

The monthly payment for a fixed-rate fully-amortizing mortgage is derived from the present value of an annuity formula. Given that the present value of all future payments must equal the loan principal, solving for the constant payment M yields:

M = P × [r(1+r)^n] / [(1+r)^n − 1]

M

Monthly principal-and-interest payment

P

Loan principal (purchase price minus down payment)

r

Monthly interest rate (annual rate ÷ 12)

n

Total number of monthly payments (years × 12)

This is a closed-form analytical solution — no iterative approximation needed. The formula comes directly from the time value of money mathematics used across all fixed-income instruments and is the standard formula cited in the CFPB's "How to calculate your mortgage payment" consumer explainer (consumerfinance.gov, Mortgages section).

Worked example: $400,000 loan at 6.5% for 30 years

Applying the amortization formula with P = $400,000, annual rate = 6.5% (r = 6.5% ÷ 12 = 0.5417% per month), and n = 360 payments:

Month 1 breakdown: Interest = $400,000 × 0.54167% = $2,166.67; Principal = $2,528.27 − $2,166.67 = $361.60; Remaining balance = $400,000 − $361.60 = $399,638.40. The first payment is 85.7% interest, 14.3% principal.

First 5 years amortization schedule

The table below shows annual totals for the first five years of the $400,000 / 6.5% / 30-year example. All values are computed from the closed-form formula — actual results will vary by a few dollars due to rounding conventions (per the CFPB's mortgage servicing rule, 12 C.F.R. §1026.36, lender rounding practices vary by payment date).

After five years and $151,695 paid, the loan balance has only decreased from $400,000 to $374,444 — a reduction of just $25,556, or 6.4% of the original principal. The remaining $126,140 paid in that period was pure interest.

Why early payments are mostly interest

The mechanics are straightforward: each month's interest charge equals the outstanding balance multiplied by the monthly rate. At the start of the loan, the outstanding balance is the full $400,000. At 6.5% annual rate, the monthly rate is 0.5417%, so the first month's interest is $400,000 × 0.5417% = $2,167. Since the monthly payment is $2,528, only $361 goes to principal reduction.

As principal is gradually whittled down, the interest charge each month decreases by a small amount, and the corresponding principal portion increases by the same amount. The shift is imperceptible in early years because the balance shrinks so slowly. It accelerates dramatically in later years — by year 25, more than half of each payment is principal.

This is why making extra principal payments early in a loan's life produces disproportionate savings. An extra $200 payment in month 1 reduces the balance on which every future month's interest is calculated — the savings compound over the remaining life of the loan. Making the same $200 extra payment in year 25 produces far less total interest savings because there are fewer remaining payment periods.

Total cost across 30 years vs. 15 years

The table below compares the total cost of a $400,000 loan at 6.5% across a 30-year and a 15-year term. The 15-year term typically carries a lower rate, but this comparison uses the same 6.5% rate to isolate the pure effect of term length.

The 15-year borrower pays $956 more per month but saves $282,981 in total interest and owns the home outright 15 years sooner. In practice, 15-year mortgages also typically carry rates 0.5–0.75% lower than 30-year rates, making the savings even larger. The right choice depends on cash-flow requirements and the opportunity cost of the extra monthly payment.

PMI and how it interacts with amortization

Private Mortgage Insurance (PMI) is required by most conventional lenders when the down payment is less than 20%, resulting in a loan-to-value (LTV) ratio above 80%. PMI premiums typically range from 0.5% to 1.5% of the loan amount annually (Freddie Mac PMI guidelines), paid as a monthly addition to the principal-and-interest payment. For a $400,000 loan, that could add $167–$500 per month in PMI costs until the LTV threshold is reached.

Under the federal Homeowners Protection Act, lenders must automatically cancel PMI when the scheduled principal payments bring the loan-to-value ratio to 78% of the original purchase price. You can request cancellation at 80% LTV — either through scheduled amortization or through principal prepayments. Freddie Mac guidelines also permit cancellation based on current appraised value if the property has appreciated, subject to additional requirements.

On our example loan with no down payment (100% LTV at origination), reaching 80% LTV means paying down the balance to $320,000. Using only scheduled payments at the $2,528/month rate, this would take approximately 10.5 years — meaning PMI could be a substantial added cost for the first decade. Making extra principal payments accelerates PMI cancellation and also reduces the interest cost simultaneously.

Refinancing and amortization reset

Refinancing replaces your existing mortgage with a new loan, typically to capture a lower interest rate, change the loan term, or access home equity. The critical implication for amortization: when you refinance, you reset the amortization schedule. If you are 8 years into a 30-year mortgage (22 years remaining) and you refinance into a new 30-year loan, you extend your repayment horizon back to 30 years and restart the interest-heavy early payment pattern on the new balance.

This reset is not inherently bad — if the new interest rate is meaningfully lower, the total interest savings can outweigh the extended timeline. The break-even calculation compares the monthly savings from the lower rate against the closing costs (typically 2–5% of the loan amount), factoring in the extra years of interest from the timeline extension.

Refinancing into a 15-year loan avoids the timeline extension problem. Even though the monthly payment is higher than on a new 30-year loan, the borrower reaches payoff sooner than with the original 30-year loan, and total interest paid over the combined period is typically less. The right refinancing decision depends on how long you plan to stay in the home and the spread between the old and new rates.

Frequently asked questions

Common questions about mortgage amortization.