Amortization Calculator: How Every Payment Splits Between Interest and Principal
An amortization calculator shows what a fixed monthly payment really does over the life of a loan. It takes the loan amount, the annual rate, and the term, works out the fixed payment, then splits every single payment into two parts: the interest charged on the balance that month, and the principal that actually reduces what you owe. The payment never changes, but the split inside it changes every month, and that shifting split is the whole story of a loan.
Early payments are mostly interest because the balance is at its largest, so interest has the most to feed on. As the balance falls, the interest portion shrinks and the principal portion grows, slowly at first and then faster. By the final years, nearly the whole payment attacks the principal. Reading a schedule row by row makes that invisible process visible, and it explains why extra payments early in a loan are worth so much more than the same dollars paid late.
How the payment is worked out
The fixed monthly payment comes from a standard formula: payment equals principal times the monthly rate times one plus the monthly rate raised to the number of payments, divided by one plus the monthly rate raised to the number of payments minus one. The monthly rate is the annual rate divided by 12, and the number of payments is the term in months. The formula guarantees the balance reaches exactly zero on the final payment, assuming the rate never changes and every payment arrives on time.
Each month's split then follows a simple rule. Interest for the month equals the starting balance times the monthly rate. Principal for the month equals the payment minus that interest. The ending balance equals the starting balance minus the principal portion. Run that rule month after month and you'll get the full schedule: 360 rows for a 30 year loan, each one built from the row before it. No step in the chain needs anything beyond those three lines.
Every input, explained
Loan amount. The principal you borrow, or the balance you still owe if you are modeling an existing loan. Rate (%/yr). The fixed annual interest rate, from 0% to 100%. The calculator divides it by 12 for the monthly rate, so a 6.5% annual rate becomes about 0.5417% a month.
Term (years) and Term (months). The repayment period, up to 50 years plus up to 11 extra months, with at least 1 month in total. Years and months combine into the payment count, so a 30 year loan means 360 payments. Extra every month. An optional additional amount paid toward principal each month, on top of the fixed payment. Extra money goes straight to principal, which is why it shortens the loan instead of just reducing each payment.
One-time lump sum and At payment #. An optional single extra payment, plus the payment number it lands on, from 1 to the total payment count. Loan start date. The date of the first payment period, which the calculator uses to date every row of the schedule. The schedule can be viewed month by month or rolled up year by year.
A worked example: $250,000 at 6.5% for 30 years
A borrower takes $250,000 at a fixed 6.5% annual rate for 30 years, which is 360 monthly payments. The monthly rate is 0.065 divided by 12, and the formula gives a fixed payment of $1,580.17. The first payment shows the split at its most lopsided: interest of $1,354.17 and principal of only $226.00. The first year alone charges $16,167.73 in interest, which is more than ten full monthly payments, while the balance barely moves.
| After year | Balance remaining | Principal paid so far |
|---|---|---|
| 1 | $247,205.69 | $2,794.31 |
| 5 | $234,027.44 | $15,972.56 |
| 10 | $211,940.32 | $38,059.68 |
| 15 | $181,397.85 | $68,602.15 |
| 20 | $139,163.21 | $110,836.79 |
| 25 | $80,760.41 | $169,239.59 |
The table shows the slow start and the fast finish. After 5 years, only $15,972.56 of principal is gone, and after 10 years the borrower still owes $211,940.32. The crossover, where the principal portion first passes the interest portion, doesn't arrive until payment 233, well into year 20. But the last 5 years clear $80,760.41 of principal, more than the first 15 years combined. Over the full term, total interest reaches $318,861.22, so the loan costs $568,861.22 in total, which is more than double the amount borrowed.
Extra payments rewrite that story. Adding $200 a month to the same loan finishes it in 265 months instead of 360, saving 7 years and 11 months, and cuts total interest to $221,243.10, a saving of $97,618.12. A single $10,000 lump sum at payment 12 finishes the loan in 323 months and saves $49,272.98 in interest. Both work the same way: money paid toward principal early removes the interest that balance would have generated in every month after, so early extra dollars are the most productive dollars in the whole loan.
Common mistakes
- Judging a loan by its payment alone. Two loans can share a payment and carry wildly different total interest. Always compare total interest and total cost alongside the monthly figure.
- Expecting the balance to fall evenly. It doesn't. A borrower who checks after 5 years and sees $234,027.44 still owed on a $250,000 loan has not been cheated; the schedule simply pays interest first.
- Assuming extra payments reduce the monthly payment. In a standard amortizing loan, extra payments shorten the term instead. The payment stays fixed unless the lender formally recasts the loan.
- Using the schedule for a variable-rate loan. The schedule assumes a fixed rate for the whole term. If the rate resets, the payment and every later row must be recalculated from the new rate and the remaining balance.
- Forgetting fees, taxes, and insurance. This schedule covers principal and interest only. A mortgage payment that includes escrow for tax and insurance will be higher than the figure shown here.
Practical tips
Read your own schedule before deciding about extra payments, and look at the interest column for the next 12 months: that is the interest your next extra dollars can start erasing. If you can only afford one extra payment a year, make it early, since the same dollars save more interest in year 2 than in year 20. When you're comparing lenders, ask for the amortization schedule, not just the rate, because two rates that look close can hide very different fee structures. And if you refinance, remember the clock restarts: a new 30 year schedule puts you back in the interest-heavy years, so compare the remaining interest on the old loan with the total interest on the new one before signing.
Frequently asked questions
What is an amortization schedule?
It's the row-by-row plan of a loan: every payment split into interest and principal, with the remaining balance after each one. On a $250,000 loan at 6.5% for 30 years, the first row shows $1,354.17 of interest and $226.00 of principal, and the split flips in favor of principal only at payment 233.
Why is my first payment almost all interest?
Because interest is charged on the full starting balance. At 6.5% on $250,000, one month of interest is $1,354.17, which leaves only $226.00 of the $1,580.17 payment for principal. As the balance falls, the interest portion falls with it.
How much does an extra $200 a month save on a $250,000 loan?
At 6.5% for 30 years, an extra $200 a month finishes the loan in 265 months instead of 360, saving 7 years and 11 months, and cuts total interest from $318,861.22 to $221,243.10, a saving of $97,618.12.
Does an amortization calculator work for car loans and student loans?
Yes. The same fixed-payment math applies to any fixed-rate installment loan: mortgages, auto loans, personal loans, and standard student loans. Revolving credit like credit cards doesn't follow a fixed schedule, since the balance and minimum payment both move.
What happens if my interest rate changes during the loan?
The schedule no longer holds. A rate change means a new payment calculated from the remaining balance, the new rate, and the remaining term, and every later row changes with it. Re-run the calculator from the current balance when a reset happens.
Related calculators: the Mortgage Calculator for a home loan payment with taxes and insurance, the Mortgage Payoff Calculator for extra-payment strategies, and the Loan Calculator for a general loan payment and cost breakdown.
A loan feels like a single monthly number until its schedule is laid out row by row, and then the real shape appears: years of interest first, principal catching up late, and early extra payments worth far more than their size suggests. Run your own loan through the schedule, find your crossover month, and test one extra payment before you commit to it. You will find the rest of this guide series on the CalcMega Blog.
Disclaimer: this guide explains loan amortization math for education only and is not financial, tax, or legal advice. Actual loan terms, fees, rate adjustments, and payoff rules vary by lender and contract, so confirm your own schedule with your lender before making payment or refinancing decisions.
Further reading: the U.S. Consumer Financial Protection Bureau explains mortgage costs and amortization in plain language for borrowers, on its official consumer finance site.