How loan amortization works: reading your payment schedule

A fixed-rate loan has a strange property: the payment is the same every month, but what that payment does changes every single month. Early on, most of it is interest; near the end, almost all of it is principal. That gradual handover is amortization, and understanding the schedule is the difference between knowing what a loan costs and being surprised by it.

The payment formula

Every amortized loan payment comes from the annuity formula:

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

where L is the loan amount, r is the monthly rate (annual rate ÷ 12), and n is the number of payments. For a $30,000 car loan at 6% over 60 months: r = 0.005, n = 60, and the formula lands on $579.98 per month. The clever part of the design: the formula is solved so that the payment is identical every month while the split inside it shifts — interest first, principal later.

The first few months, in numbers

Here is the schedule for that loan, month by month at the start:

Month	Payment	Interest	Principal	Remaining balance
1	$579.98	$150.00	$429.98	$29,570.02
2	$579.98	$147.85	$432.13	$29,137.89
3	$579.98	$145.69	$434.29	$28,703.60
4	$579.98	$143.52	$436.46	$28,267.14
12	$579.98	$125.75	$454.23	$24,695.96
60	$579.98	$2.88	$577.10	$0.00

Month 1 charges 0.5% of $30,000 — $150 — as interest, and the remaining $429.98 of the payment reduces the balance. Month 2 charges 0.5% of the new balance, $29,570.02, which is $147.85. The interest portion declines a little every month because the balance declines a little every month, and by month 60 the interest is $2.88 on the last few hundred dollars.

What the loan actually costs

Sixty payments of $579.98 total $34,799 — of which $30,000 is the money you borrowed and $4,799 is interest. That ratio, roughly 16 cents of interest per dollar borrowed, is the real price of the loan, and it is the number lenders quote in the APR rather than in dollars because dollars depend on the term.

The comparison that matters happens before you sign: the rate. On the same $30,000 over 60 months:

Rate	Monthly payment	Total interest over 5 years
4%	$552.50	$3,150
6%	$579.98	$4,799
8%	$608.29	$6,497

Two points of APR cost about $1,650 over the life of the loan — the difference between a $4,799 interest bill and a $3,151 one. The monthly difference looks small ($27.46), which is exactly why lenders quote monthly figures; the total is the number to negotiate on.

The lever: extra principal, paid early

Because interest is charged on the remaining balance, every dollar of extra principal kills off future interest dollars. The effect is heavily front-loaded: on this loan, an extra $1,000 in month one saves roughly $160 in total interest and about two months of payments (the balance drops immediately, so the money that would have been paid as interest on that principal is never paid); the same $1,000 in month 55 saves only the interest on the final few payments. If you have a choice between “pay extra” and “pay extra late”, the first option wins by a wide margin — and the loan amortization calculator shows both schedules side by side, with and without the extra payment.

What amortization does not cover

The formula assumes a fixed rate and a fixed term, and it is indifferent to fees and penalties. A loan with origination fees, prepayment penalties, or a variable rate is a different contract: compare those with the APR, which folds fees into the rate, and read the prepayment terms before paying extra — a penalty can erase the interest you were trying to avoid. And the schedule is an obligation, not a plan: the numbers assume you make every payment, and the compounding that works for a savings account (explained here) works against you on the unpaid balance if a payment is missed.

The one rule

Read the interest total, not the monthly payment: the payment is what you sign, but the interest total is what you buy. Two loans with nearly identical payments can differ by thousands in total interest, and the amortization schedule shows exactly why — interest first, principal later, and the rate decides how much of each you pay.