How Mortgage Payments and Amortization Work

A conventional fixed-rate mortgage is an amortizing loan. “Amortizing” means the balance is paid down to zero by the end of the term through equal scheduled payments. Each payment covers two things: the interest that has accrued on the balance since the last payment, and a slice of principal that reduces the balance.

Because interest is charged on the remaining balance, the interest part is largest on day one and shrinks a little every month. The payment itself stays level, so the principal part grows by the same amount the interest part shrinks. That is why a loan feels like it barely moves in the first years.

Many real mortgage bills also include escrow items — property tax, homeowner’s insurance and sometimes mortgage insurance. Those are separate from the principal-and-interest (P&I) payment discussed here, and they can change from year to year even when the loan rate is fixed.

With loan amount P, annual rate converted to a monthly rate r (annual rate ÷ 12), and n monthly payments, the level payment is:

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

The same formula is used for car loans, personal loans and the EMI products common in many countries. Only the labels and the term differ. If the rate is zero the formula collapses to P ÷ n.

Here r = 0.065 ÷ 12 = 0.0054167 and n = 360. The formula gives a payment of about $1,896.20.

• Month 1 interest: $300,000 × 0.0054167 = $1,625.00. Principal: $1,896.20 − $1,625.00 = $271.20.
• Total paid over 360 months: about $682,633, so total interest is about $382,633 — more than the amount borrowed.
• Balance after 5 years (60 payments): about $280,833. You will have paid roughly $113,772 in total but reduced the balance by only about $19,167.
• Balance after 10 years: about $254,328.

This is not a trick of the lender. It is what interest on a declining balance looks like when the payment is flat. The same loan at a lower rate or shorter term changes the shape considerably.

Compare the same $300,000 at 6.5%:

The 15-year payment is about $717 higher each month, but total interest is less than half. Whether that trade is right depends on cash-flow safety, other debts and what else the money could do. A shorter term is a guaranteed saving at the loan’s rate; investing the difference carries risk and uncertain returns.

Extra principal goes straight to the balance, so every later month’s interest is calculated on a smaller number. Adding $200 a month to the 30-year loan above shortens the payoff from 360 months to about 277 months (about 23 years) and cuts total interest from about $382,633 to about $279,185 — roughly $103,000 saved.

Before paying extra, check the loan terms for prepayment penalties and make sure the lender applies the extra to principal, not to next month’s payment. Biweekly plans work the same way: 26 half-payments a year equal 13 full payments, so one extra payment is made annually.

A one-percentage-point difference matters. On the same $300,000 and 30 years, a 5.5% rate gives a payment of about $1,703.37, compared with $1,896.20 at 6.5% — roughly $193 a month. Over 30 years that gap is more than $69,000 in payments. Even a quarter-point is worth modelling before you lock a rate.

An amortization table lists each payment with its interest, principal and ending balance. Three things are worth looking for. First, the crossover point where principal exceeds interest in a payment; on a 30-year loan at a high rate that can be many years in. Second, the balance at the date you expect to sell or refinance, because that is what you will owe. Third, the cumulative interest, which is the true price of borrowing.

• Using the APR instead of the note rate in the payment formula. APR includes fees and is a comparison tool, not the rate that accrues on the balance.
• Forgetting escrow, insurance, HOA dues and maintenance when judging affordability.
• Entering the term in years where a calculator expects months.
• Assuming an adjustable-rate loan will keep its starting payment after the fixed period ends.
• Treating the schedule as a promise: rounding, payment dates and day-count conventions differ slightly by lender.

Start with the Mortgage Calculator to get the payment, then open the Amortization Schedule for the month-by-month table. Try two terms and two rates, one change at a time, so you can see which lever matters. Results are educational estimates; your lender’s Loan Estimate and closing documents are the authority for your actual loan.