How Mortgage Amortization Works

If you’ve ever looked at a mortgage statement after a year of payments and wondered why the balance has barely moved, you’ve met amortization. It isn’t a trick, and it isn’t the bank front-loading its profit. It’s arithmetic — and once you see how it works, a few big decisions (extra payments, refinancing, 15 vs 30 years) become much easier.

One payment, two jobs

Every monthly mortgage payment does two things: it pays the interest the lender is owed for that month, and whatever is left over reduces the principal — the amount you still owe.

Interest for the month is simple to compute: it’s the outstanding balance times the monthly rate.

monthly interest = balance × (annual rate ÷ 12)

On a $320,000 loan at 6.5%, the first month’s interest is 320,000 × 0.065 ÷ 12 = $1,733. The scheduled payment on a 30-year loan is $2,023, so only $290 goes to principal in month one. Next month the balance is $319,710, the interest is $1,731, and $292 goes to principal. Each month the interest slice shrinks a little and the principal slice grows a little.

Why the payment is fixed

The lender sets the payment so that this process reaches exactly zero at the end of the term. The formula is:

Payment = P × r ÷ (1 − (1 + r)^−n)

where P is principal, r is the monthly rate, and n is the number of payments. You don’t need to memorise it — the mortgage calculator does it — but it’s worth knowing that the payment is derived from the term. Stretch the same loan from 15 to 30 years and the payment drops, but because the balance falls more slowly, you pay interest on a larger balance for longer. Total interest roughly doubles.

$320,000 at 6.5%	Monthly payment	Total interest
15 years	$2,788	$181,700
30 years	$2,023	$408,100

The crossover point

On a 30-year loan at today’s rates, the month where principal finally exceeds interest arrives around year 19–20. On a 15-year loan it’s around year 4–5. That’s why people who sell or refinance after 5–7 years — which is most people — have built surprisingly little equity from payments alone; most of their equity came from the down payment and price appreciation.

What this means for extra payments

Because interest is charged on the balance, every extra dollar you pay reduces the balance immediately and stops accruing interest for the remaining life of the loan. An extra payment in year 1 saves far more than the same payment in year 25.

Run the numbers: adding $200 a month to the $320,000 loan above pays it off in about 23½ years instead of 30 and saves roughly $105,000 in interest. One extra full payment per year — the classic “bi-weekly” trick — has a similar effect. The calculator’s extra-payment field shows the exact figure for your loan.

Before you do this, check two things: that your lender applies extra money to principal (some apply it to next month’s payment unless told otherwise), and that there’s no prepayment penalty.

What this means for refinancing

Refinancing replaces your loan with a new one — and a new amortization schedule that starts at the interest-heavy end again. If you’re 10 years into a 30-year loan and refinance into a fresh 30-year loan, you’ll pay 40 years total, and the first years of the new loan will again be mostly interest.

That doesn’t make refinancing bad; a lower rate can still save a lot. But compare on total remaining interest, not just the monthly payment, and consider refinancing into a shorter term (20 or 15 years) so you don’t reset the clock.

Reading an amortization schedule

The schedule the calculator produces lists, for each year, how much went to principal, how much to interest, and the closing balance. Three things worth looking at:

1. The balance at the year you expect to sell. That’s what you’ll need to pay off from the sale proceeds.
2. Cumulative interest at that point. That’s the true cost of the loan to you.
3. The crossover year. If it’s beyond your planning horizon, extra payments will have the largest effect.

Amortization rewards patience and punishes short holds — the longer you keep a loan, the better the deal you got up front looks. Use the mortgage calculator to see your own schedule, and the loan calculator for the same view of any other fixed-rate loan.