Amortization: How Loan Payments Really Work

A fixed-rate loan has one of the most useful properties in personal finance: it is completely predictable. Every payment, every split between interest and principal, every dollar of total interest can be calculated on the day you sign.

Almost nobody does the calculation, which is why the first mortgage statement is such a shock.

What a payment is made of

Each month, two things happen in order:

1. Interest is charged on the balance you currently owe.
2. Whatever is left of your payment reduces the balance.

The payment is fixed. The split is not. Because the balance is largest at the start, the interest charge is largest at the start — so the principal portion begins small and grows every month.

Nothing about that is a trick or a bad deal being hidden from you. It is what borrowing a large sum for a long time costs.

The formula, for the curious

The level payment that exactly clears a loan is:

Payment = P × r ÷ (1 − (1 + r)⁻ⁿ)

where P is the amount borrowed, r is the periodic interest rate (annual rate ÷ 12 for monthly payments) and n is the number of payments.

Every mortgage, car loan and personal loan calculator is running this one line and then repeating the two-step process above to build the schedule.

What extra payments do

An extra payment is special because all of it goes to principal. It skips the interest step entirely, which permanently reduces every future interest charge.

The effect is largest early, when the balance is at its highest, and shrinks toward the end. This is why extra payments in year two are worth far more than the same amount in year twenty.

Before committing to this, two practical checks: confirm the lender applies extra amounts to principal rather than holding them as a prepaid future payment, and check whether any prepayment penalty applies.

Interest rate vs APR

• Interest rate — what you pay on the principal.
• APR — the rate plus certain lender fees, expressed annually.

Two loans can share an interest rate and differ meaningfully in APR because one carries higher fees. When comparing offers, APR is generally the fairer comparison, though it still does not capture everything (for a mortgage, closing costs and points deserve their own look).

Longer terms: lower payment, higher cost

Stretching a loan reduces the monthly payment and increases the total interest, since you owe money for longer. Both numbers are real, and they answer different questions — "can I afford this month?" and "what does this cost me in total?"

The mistake is looking at only one of them. Dealerships and lenders often quote the payment, because it is the smaller, friendlier number.

Common mistakes

1. Shopping by monthly payment instead of total cost.
2. Assuming extra payments go to principal automatically — some lenders need to be told.
3. Refinancing to a lower rate and restarting a 30-year clock, resetting the amortization to its most interest-heavy stage.
4. Ignoring fees and comparing rates only.
5. Not checking for prepayment penalties before planning to pay early.

What to do before signing anything

• Calculate the total interest, not just the payment.
• Compare APRs across offers.
• Ask explicitly how extra payments are applied.
• Test what one extra payment a year does to the total.