Amortization Calculator
Every month, every dollar — broken into principal, interest, and remaining balance for the life of the loan.
Summary
Monthly payment
$0.00
Total interest
$0
Total paid
$0
Months
—
How amortization works
Every payment on a fixed-rate, fully amortizing loan is the same dollar amount from the first month to the last. What changes is the split. Interest each month is charged on whatever principal is still outstanding, and early on that's nearly the whole loan — so most of an early payment is interest, and only a sliver chips away at the balance. As the balance shrinks, the interest charge shrinks with it, and more of each fixed payment goes to principal instead.
The extra-principal field skips straight to reducing the balance early, which is the one variable you control. A dollar of extra principal in month 1 saves interest for every month that follows it; the same dollar paid in month 350 barely matters, because there's almost no term left for it to work against.
The formula
- M = monthly payment
- P = loan amount
- r = monthly interest rate (annual ÷ 12)
- n = total number of payments (years × 12)
Each month's interest is balance × r; principal is
whatever's left of the payment after interest. Subtract that
principal from the balance and repeat.
Worked example
$250,000 at 6.25% for 30 years, no extra payment: the formula above gives a monthly payment of $1,539.29. Over 360 payments that's $554,145.48 total — of which $304,145.48 is interest, more than the loan itself.
Add $200/month extra principal and the loan pays off in 266 months (22 years 2 months) instead of 360, and total interest drops to $212,317.77 — a savings of $91,827.71. That savings costs $53,200 in extra payments (266 months × $200), so the net benefit of paying extra is about $38,600, plus getting the loan off the books almost 8 years sooner.
Full schedule
Every month for the life of the loan. Use the CSV export to drop this into a spreadsheet.
| Month | Payment | Principal | Interest | Cum. interest | Balance |
|---|
Frequently asked questions
How is this different from the mortgage calculator?
Same math. The mortgage calculator asks for a home price and down payment and works out the loan amount for you; this one takes the loan amount directly. Use this version once you already know what you're borrowing, or for a loan that isn't a mortgage at all — auto, personal, or any other fixed-rate installment loan.
Why does interest dominate the first few years?
Because interest is charged on the outstanding balance, and the balance is largest at the start. On a 30-year loan, the crossover point where principal overtakes interest in the monthly split typically doesn't arrive until year 15–20 — later if the rate is higher.
What does extra principal actually save?
It depends on when you pay it — see the worked example above. In general, extra principal paid earlier saves more, because it removes that balance from every remaining month's interest calculation, not just the next one.
Does this work for a car loan or student loan?
Yes, as long as the loan is fixed-rate and fully amortizing — a fixed payment for a fixed term. That covers most auto loans and private student loans. It won't be accurate for loans with deferment, capitalized interest, or income-driven repayment, since those change the balance in ways this formula doesn't model.