Loan Amortization Explained

Every loan payment you make is split into two parts: interest, which is the lender's fee, and principal, which actually reduces what you owe. Amortization is simply the schedule that decides how that split changes over the life of the loan — and understanding it explains why your balance seems to barely move in the first year.

This guide explains the amortization formula in plain language, then works through a complete $20,000 loan example payment by payment.

The monthly payment formula

For a fixed-rate loan, the monthly payment comes from this formula, where P is the amount borrowed, r is the monthly interest rate (annual rate divided by 12), and n is the number of payments:

You do not need to memorize it — our loan calculator runs it for you — but seeing it once makes the rest of the guide click. The payment stays the same every month; what changes is how much of it is interest and how much is principal.

Worked example: a $20,000 loan at 6% for 5 years

Borrow $20,000 at 6% annual interest for 60 months. The monthly rate is 0.06 ÷ 12 = 0.005.

1. Monthly payment: $20,000 × 0.005 ÷ (1 − 1.005^(−60)) = $100 ÷ 0.25863 = $386.66
2. Total paid over 5 years: $386.66 × 60 = $23,199.60
3. Total interest: $23,199.60 − $20,000 = $3,199.60

Now watch the split change. The interest in any month is simply the remaining balance times the monthly rate:

1. Payment 1: interest $20,000 × 0.005 = $100.00; principal $386.66 − $100.00 = $286.66; new balance $20,000 − $286.66 = $19,713.34
2. Payment 2: interest $19,713.34 × 0.005 = $98.57; principal $386.66 − $98.57 = $288.09; new balance $19,425.25
3. Payment 30 (halfway): the balance is roughly $10,900, so interest is about $54.50 and principal about $332.16 — the principal slice has grown by about $45 since payment 1.
4. Payment 60: interest is under $2 and almost the whole payment is principal. The loan is gone.

What an extra $50 a month does

Pay $436.66 instead of $386.66 and the loan ends in about 52 months instead of 60 — roughly 8 months sooner — with total interest of about $2,770 instead of $3,200. That is roughly $430 saved for $50 a month, and the benefit is biggest when you start early, while the balance (and therefore the interest) is highest. Check with your lender first that extra payments are applied to principal and that no prepayment penalty applies.

Common mistakes to avoid

• Shopping by monthly payment alone. A longer term lowers the payment but raises total interest dramatically. Compare total interest, not just the monthly number.
• Ignoring the rate type. A variable rate can rise, which changes the payment and the whole schedule. Fixed rates keep the formula in this guide exact.
• Assuming extra payments automatically help. Some lenders hold extra money as a "future payment" instead of applying it to principal. Ask, in writing, how extras are applied.
• Forgetting fees. Origination fees and closing costs are real money on top of the interest in the schedule.

Fixed vs variable rates: how the schedule changes

Everything above assumes a fixed rate, which is why the payment never changes. With a variable rate, the payment is recalculated whenever the rate moves, and the schedule shifts with it. Take the same $20,000 loan at 6%: after 24 payments the balance is about $12,710. If the rate then rises to 7% for the remaining 36 months, the new payment works out to about $392 a month instead of $386.66 — only about $6 more a month, but the remaining interest rises from roughly $1,210 to roughly $1,415, about $205 more. The lesson is that a small rate change late in a loan costs less than the same change early, because the balance it applies to is smaller. If you are choosing between a fixed and a variable rate, compare the worst case you can afford, not just the tempting starting rate, and check whether the variable loan has a cap on how far the rate can rise.

Frequently asked questions

Educational note: this guide explains the math for learning purposes only and is not financial, tax, or legal advice. Tax rates and rules change, and your own situation may differ — check current rules for your state or talk to a qualified professional before making a money decision.