What amortization means
An amortizing loan is paid off in equal payments over a set term. Each payment covers that month's interest first. Whatever's left goes to principal, the amount you actually borrowed. By the last payment, the balance is zero.
Mortgages, car loans and most personal loans work this way. Credit cards don't. A card has no fixed term, and your payment changes with the balance.
Follow the first three payments
Take a $20,000 loan at 6% for 5 years. The monthly rate is 6% / 12 = 0.5%, and the formula gives a payment of $386.66 for 60 months.
Month 1: interest is $20,000 x 0.005 = $100.00. The other $286.66 goes to principal, leaving $19,713.34. Month 2: interest on $19,713.34 is $98.57, so principal gets $288.09 and the balance drops to $19,425.25. Month 3: interest is $97.13, principal is $289.53, and you owe $19,135.72.
The payment never changes, but the split does. Interest shrinks a little each month because the balance shrinks. By payment 60, interest is just $1.92 and principal is $384.73. Over the whole loan you pay $3,199.36 in interest.
Formula notes
Payment = P x r / (1 - (1 + r)^(-n))Interest = balance x rPrincipal = payment - interest
Why long loans feel stuck at the start
On the 5-year loan above, principal is bigger than interest from the very first payment. A 30-year mortgage is different. On $250,000 at 6.5%, the first payment of $1,580.17 sends $1,354.17 to interest and only $226.00 to principal. Principal doesn't overtake interest until payment 233, nearly 20 years in.
Nothing unfair is going on. The rate is applied to a big balance for a long time. The longer the term and the higher the rate, the longer the slow stretch lasts. That's also why your home equity grows slowly in the early years unless the home's value rises.
How extra payments rewrite the schedule
Any amount you pay above the regular payment goes straight to principal, as long as your lender applies it that way. That lowers the balance, which lowers every future interest charge, which sends more of each later payment to principal. The effect snowballs.
On the $20,000 loan, paying $100 more each month finishes in 3 years and 11 months instead of 5 years. Interest drops from $3,199.36 to $2,444.38, a saving of about $755. Our amortization calculator lets you try monthly, yearly or one-time extras and shows the new payoff date.
Mistakes people make reading a schedule
A schedule is a model of your loan, so it's only as good as what goes into it.
- Using the APR instead of the note rate, which inflates the payment.
- Including taxes and insurance in the loan payment. Escrow isn't amortized.
- Assuming the schedule balance is your payoff amount. A payoff quote adds interest up to the payoff date and any fees.
- Forgetting to tell the lender an extra payment is for principal, so it gets held or applied to the next due date instead.
- Expecting a daily simple interest loan to match a monthly schedule exactly. Payment timing shifts the split.
When the schedule changes
A fixed-rate schedule stays put unless you change it. Extra payments shorten it. Refinancing replaces it with a new loan and a new schedule. Some lenders offer a recast, where you pay a lump sum and they recalculate a lower payment over the remaining term.
Adjustable-rate loans reset the payment when the rate changes, so their future schedule is only an estimate. Interest-only periods and balloon loans don't amortize in the usual way at all, and they need their own math.
Frequently asked questions
What does it mean when a loan is fully amortized?
The regular payments pay off both interest and the full principal by the end of the term, with no balloon payment left over.
What is negative amortization?
It happens when payments don't cover the interest due, so unpaid interest is added to the balance and you owe more over time.
Is amortization the same for a 15-year and a 30-year loan?
The math is the same, but a 15-year loan has bigger payments, so more goes to principal from the start and total interest is much lower.
Does making a payment early in the month save interest?
It can on loans that charge simple interest daily, because the balance is lower for more days. Check your loan agreement to see how your interest is calculated.
What is a mortgage recast?
You pay a lump sum toward principal and the lender recalculates a lower monthly payment over the remaining term. Not every lender offers it, so ask your servicer.
References
These sources support the method or guidance used for Amortization Calculator. Verify time-sensitive rules at the source.
Try the calculator
Open Amortization Calculator, enter your scenario, and compare its supporting rows with this guide's method and checks.