How Loan Amortization Calculator Works
The Loan Amortization Calculator generates a complete payment-by-payment schedule for any fixed-rate loan, showing exactly how each payment splits between principal and interest. It supports extra payment modeling so you can see how additional payments shorten your loan term and reduce total interest.
Enter your loan amount, annual interest rate, and loan term (in years or months). The tool instantly generates your monthly payment amount and a full amortization table showing every single payment from first to last. Each row displays the payment number, payment amount, principal portion, interest portion, and remaining balance.
The extra payment feature is where this tool truly shines. Add recurring extra payments (an additional $200/month, for example), one-time lump sums (applying a $10,000 bonus in month 24), or annual extra payments (adding one extra payment per year). The schedule recalculates dynamically, showing your new payoff date and total interest savings compared to the original term.
A visual chart plots principal vs. interest over the life of the loan, clearly showing the crossover point where more of each payment goes to principal than interest. For a standard 30-year mortgage, this crossover happens around year 15-18, which surprises many borrowers.
The summary panel displays key metrics: total interest paid over the life of the loan, total cost (principal + interest), the interest-to-principal ratio, and—with extra payments—how many months/years early you'll pay off and the dollar amount saved. Export the full schedule as CSV or PDF for record-keeping or sharing with your lender.
Key Terms Explained
- Amortization
- The process of spreading a loan into a series of fixed payments over time. Each payment covers interest on the remaining balance plus a portion of principal.
- Principal
- The original loan amount borrowed, or the remaining unpaid portion of that amount. As you make payments, principal decreases until it reaches zero at payoff.
- Interest-to-Principal Ratio
- The proportion of each payment going to interest versus principal. Early in a loan, most of the payment is interest; this ratio inverts over time as the balance shrinks.
- Prepayment
- Any payment made above the required monthly amount that goes directly toward reducing principal, effectively shortening the loan term and reducing total interest.
- Loan Term
- The total length of time over which the loan is scheduled to be repaid. Common terms are 15 or 30 years for mortgages, 3-7 years for auto loans.
Who Needs This Tool
Comparing a 15-year vs. 30-year mortgage to see how much total interest they'd save and whether the higher monthly payment fits their budget.
Has 22 years remaining on a 30-year mortgage and wants to see if refinancing to a 15-year at a lower rate saves money after closing costs.
Financing a $35,000 car and wants to compare 48-month vs. 72-month terms to understand the true cost difference beyond just the monthly payment.
Wants to see the impact of paying an extra $300/month on their mortgage—specifically how many years it cuts and how much interest they save.
Has a $50K student loan at 6.5% and wants to model paying $100 extra per month to see if it significantly accelerates payoff.
Methodology & Formulas
Monthly payment uses the standard annuity formula: PMT = P[r(1+r)^n] / [(1+r)^n - 1], where P = principal, r = monthly rate (APR/12), n = total payments. Each month: interest = balance × r, principal = payment - interest, new balance = old balance - principal. Extra payments reduce balance immediately, recalculating remaining interest on the lower balance for all subsequent periods.
Pro Tips
- Even $50-$100 extra per month on a mortgage can save tens of thousands in interest and cut years off the term—run the numbers to see your specific impact.
- Compare the 15-year vs. 30-year total cost, not just the monthly payment. A 30-year mortgage often costs 60-80% more in total interest.
- If you can't commit to extra monthly payments, apply one extra full payment per year (e.g., from a tax refund). This alone cuts a 30-year mortgage to roughly 25 years.
- Check that your lender applies extra payments to principal, not advancing your due date—some require you to specify 'apply to principal' explicitly.
- Use the amortization table to find your break-even point for refinancing: compare remaining interest on current loan vs. new loan's total interest plus closing costs.