How loan amortization works — and why early payments feel useless
Every amortized loan — mortgages, car loans, most personal loans — works on one rule: each month you pay interest on whatever you still owe, and only the leftover goes to the debt itself. Early on you owe the most, so interest eats most of the payment. That's the whole mystery.
The formula
The fixed monthly payment M on a principal P, monthly rate r (annual rate ÷ 12), over n months is:
M = P × r / (1 − (1 + r)^−n)
It's the one payment that exactly zeroes the balance on the last month after interest accrues every month in between.
A real example
Take a $250,000 loan at 6.5% for 30 years. The monthly rate is 0.065 ÷ 12 = 0.005417, and n = 360 payments.
- Payment:
M = 250,000 × 0.005417 / (1 − 1.005417^−360) ≈ $1,580 - First month's interest: 250,000 × 0.005417 = $1,354. Of your $1,580 payment, only about $226 reduces the loan.
- Fifteen years in, you still owe about $181,000 — so about $983 of that same payment is still interest. Even halfway through the calendar, you're not halfway through the debt.
- Over the full 30 years you pay about $568,900 total — roughly $318,900 of it interest.
The move that actually works
Anything extra you pay goes 100% to principal — and every dollar of principal you remove stops charging you interest every month for the rest of the loan. On the same loan, an extra $200 a month pays it off in about 22 years instead of 30 and saves roughly $97,000 in interest. The earlier the extra dollars arrive, the more months of interest they cancel.
Two honest caveats: check that your loan has no prepayment penalty, and tell the lender extra payments are for principal (some default them to "next month's payment"). And extra principal competes with other uses of money — a 6.5% loan is a guaranteed 6.5% return, which may or may not beat your alternatives.
See your own loan's schedule
The free Loan Amortization calculator builds the full month-by-month table for your numbers — payment, interest/principal split, and remaining balance — right on your device. Nothing you type leaves it.