What is amortization?

Amortization is the process of paying off a loan with a series of regular payments, each of which covers that period's interest and retires part of the balance. On a fixed-rate mortgage the payment stays the same every month, but the split inside it shifts: early payments are mostly interest, later ones are mostly principal. An amortization schedule is the table that shows that split, one row per payment, from the first payment to a zero balance.

How an amortization schedule is built

A fixed-rate, fully amortizing mortgage starts from three inputs: the loan amount, the annual interest rate and the number of monthly payments. From those, the scheduled principal and interest (P&I) payment is set once, at a level that pays the loan off exactly over the term if every payment is made on time.

M = P × r × (1 + r)^n ÷ ((1 + r)^n − 1)

The schedule is then filled in one row at a time, and every row follows the same three steps:

1. Interest for the month is the balance at the start of the month multiplied by the monthly rate: balance × annual rate ÷ 12, rounded to the nearest cent the way a lender statement shows it.
2. Principal for the month is the payment minus that interest.
3. The new balance is the old balance minus the principal. It becomes the starting balance for the next row.

Because step 1 uses the remaining balance, interest falls a little every month, and because step 2 is "payment minus interest", principal rises by exactly the same amount. The payment never changes; only its composition does. For the pieces that sit around P&I, such as taxes, insurance and PMI, see how mortgage payments work.

A worked example

Take a $300,000 loan at 6.5 percent over 30 years. The scheduled P&I payment is approximately $1,896 per month; the rounded figure carried through the schedule is $1,896.20. Here are the first two rows, the twelfth row and the last row.

Row 1 is easy to check by hand: $300,000 × 6.5 percent ÷ 12 is $1,625.00 of interest, so $1,896.20 − $1,625.00 = $271.20 goes to principal and the balance drops to $299,728.80. Row 2 repeats the steps on that smaller balance: interest is $1,623.53 (about $1.47 less), so principal is $272.67 (about $1.47 more). By payment 12, monthly interest is down to $1,608.40 and principal is up to $287.80.

Summed over the whole schedule, the estimated interest is roughly $382,600, more than the amount borrowed. That is what 360 months of interest on a slowly declining six-figure balance adds up to. Principal first exceeds interest on this loan at payment 233, more than 19 years in.

Why the final payment is different

The last payment in the table is $1,900.91, not $1,896.20. That is the final-payment adjustment, and it is normal.

The exact payment that clears this loan is $1,896.2041..., but no one can pay a fraction of a cent, so it is rounded once to $1,896.20. Paying a sliver less than the exact amount for 359 months leaves slightly more balance than a regular payment covers, so the last payment picks up the difference. On other loans the residue goes the other way: a $100,000 loan at 4.5 percent over 30 years has a regular payment of $506.69 and a final payment of $502.86.

The rule is simple. In the final period, principal is set to whatever balance remains, and the payment is that balance plus the month's interest. This is what stops a schedule from ending with a phantom balance of a few cents or an extra 361st row.

Why per-period rounding matters

There are two ways to total the interest on a loan. The quick way multiplies the unrounded payment by the number of payments and subtracts the loan amount. The careful way builds the schedule row by row, rounds each month's interest to the cent before applying it, and adds up the interest column.

The answers differ by a few dollars on a 30-year loan: about $382,633 by the quick method versus about $382,637 row by row for the example above. The gap is small, but it is why two calculators can disagree slightly on the same inputs, and why the row-by-row figure is the one to compare against a real statement. Lenders bill in cents, so a schedule that rounds in cents matches what you actually see.

Reading the schedule and the balance chart

A schedule has one row per payment with the columns shown in the example. A few checks help you read it with confidence:

• Interest plus principal in any row equals the payment for that row.
• Starting balance minus principal equals the ending balance.
• The principal column adds up to the original loan amount, and the last ending balance is zero.
• Interest falls every row and principal rises every row, except in the adjusted final row.

The balance chart plots the ending-balance column over time. On a fixed-rate loan it is not a straight line; it bows outward, falling slowly at first and steeply at the end. On the example loan the balance is still approximately $254,300 after 10 years and approximately $167,000 after 20 years, so more than half the loan remains after two thirds of the term. For the year-by-year split, see principal vs. interest.

How extra payments shorten the schedule

An extra payment toward principal changes one step in the row calculation: principal for the month becomes payment minus interest plus the extra amount. The scheduled payment stays the same, so the loan is not re-amortized; it simply reaches zero sooner.

Because next month's interest is charged on a smaller balance, the effect compounds. Adding $250 per month to the example loan moves the first row's principal from $271.20 to $521.20 and, based on the assumptions entered, clears the loan in 262 payments instead of 360. That is approximately 8 years 2 months sooner and saves about $120,300 of estimated interest.

The final-payment rule still applies with extras. In this scenario the last payment is $2,139.49, less than the usual $2,146.20, because the extra is capped at whatever balance is left.

What changes the outcome

• Interest rate. A higher rate raises the interest in every row and slows the early paydown. The same $300,000 loan at 7 percent instead of 6.5 percent has an estimated payment of about $1,996 and roughly $35,900 more lifetime interest.
• Term. A shorter term means a larger payment but far fewer interest-bearing months. At 6.5 percent, the 15-year version of the example loan has an estimated payment of about $2,613 and roughly $170,400 of total interest, about 55 percent less than the 30-year schedule.
• Loan amount. Interest scales with the balance, so borrowing less shrinks every interest cell proportionally.
• When extra principal starts. Early extras remove interest for the longest stretch of the schedule; the same dollar added in year 25 saves far less.

Common mistakes and misconceptions

• Expecting a straight-line payoff. Halfway through the term you have not repaid half the loan. On a 30-year schedule at 6.5 percent, roughly 73 percent of the balance remains at the 15-year mark.
• Confusing P&I with the total monthly housing payment. Taxes, insurance, PMI and HOA dues ride on top of the schedule and do not touch the interest or principal columns.
• Reading the final-payment difference as an error. A last payment a few dollars above or below the regular one is rounding residue being settled, not a fee.
• Assuming extra money reduces next month's bill. On a standard fixed-rate loan the scheduled payment does not change; the benefit is a shorter schedule and less total interest.
• Ignoring negative amortization. If a payment is smaller than the month's interest, principal is negative and the balance grows. Standard fixed-rate mortgages are structured so this cannot happen, but some loan types allow it, and the balance chart would slope upward.

Amortization

Paying off a loan through regular payments that each cover the period's interest and reduce the balance until it reaches zero.

Amortization schedule

A table with one row per payment showing starting balance, interest, principal, payment and ending balance.

Final-payment adjustment

Setting the last payment to exactly the remaining balance plus that month's interest, so rounding residue does not leave a phantom balance.

Negative amortization

A situation in which a payment is smaller than the interest due, so the unpaid interest is added to the balance and the loan grows.

Try it

The mortgage calculator builds the full schedule for the loan amount, rate and term you enter, shows the balance chart alongside it, and lets you add an additional monthly payment to see the estimated months and interest saved. If you are still weighing terms and down payments, the plain-language mortgage guide walks through those choices first.