Amortization Calculator
An amortized loan is a debt with a planned route to zero. The payment stays level, but the hidden split changes every month: interest is taken first, and the rest reduces principal. This calculator focuses on that schedule. It estimates the scheduled monthly payment, the total interest, the total principal plus interest, the payoff time, and a first-year snapshot of how much of the early money actually lowers the balance.
Use it for a mortgage principal-and-interest check, an auto loan, a personal loan, or any fixed-rate installment debt where the payment is meant to retire the balance over a stated term. It is intentionally narrower than a full house-payment worksheet. If you need taxes, insurance, or PMI, compare the result with the mortgage calculator. If you only need a simpler payment without the schedule detail, the loan calculator is faster. If you want to see how fees change a borrowing comparison, use the APR calculator beside this page.
How the calculator follows the schedule
The form asks for loan amount, annual interest rate, loan term, and optional extra monthly principal. The calculation converts the annual rate to a monthly rate by dividing by 100 and by 12, rounds years into monthly payments, and calculates the scheduled payment from the standard amortization formula. It then loops month by month. In each month, interest equals current balance times monthly rate. Principal paid equals the scheduled payment plus any extra principal, minus that interest. The last payment is capped so the balance does not go below zero.
That loop is why the tool can report more than a single payment. It tracks first-month interest, first-month principal, principal paid in year one, interest paid in year one, total interest, payoff month, and any interest saved by the extra payment. If an extra principal amount is too small to cover the monthly interest, the loan would negatively amortize, so the calculator treats the input as invalid rather than showing a misleading payoff.
Formula
The scheduled monthly payment is:
When the interest rate is zero, the calculator uses:
For each schedule row, the split is:
The balance after that row is the previous balance minus principal paid. Total interest is the sum of the monthly interest amounts.
Example: using amortization
With the default inputs, the loan amount is $250,000, the annual interest rate is 6.5%, the term is 30 years, and extra monthly principal is $0. The calculator rounds 30 years to 360 monthly payments and uses a monthly rate of 6.5% ÷ 12, or about 0.5416667%.
The scheduled principal-and-interest payment is $1,580.17. In month one, interest is $250,000 × 0.005416667, which equals $1,354.17. The remaining $226.00 of the payment reduces principal. Over the first 12 payments, the schedule pays about $2,794.31 of principal and $16,167.73 of interest. Across the full 360-payment schedule, total interest is $318,861.22, total principal plus interest is $568,861.22, and the payoff time is exactly 30 years.
Those numbers explain why a long fixed-rate loan can feel slow at the beginning. The payment is not wasted; it is satisfying the interest due under the contract. But the balance does not fall quickly until repeated principal reductions make the next month’s interest smaller.
What extra principal changes
Extra principal does not change the scheduled payment in this calculator. Instead, it increases the principal paid each month after interest is covered. That shortens the payoff path and lowers total interest because future interest is calculated on a smaller balance. The change is strongest early in the loan, when the balance is highest and each additional dollar prevents many future months of interest.
Before committing to a prepayment plan, check whether the lender has prepayment penalties, whether extra money must be designated as principal, and whether you have higher-priority debts. A credit card balance may be better analyzed with the finance charge calculator, while a savings decision can be compared with the APY calculator. The best move is not always the one that reduces loan interest fastest; liquidity and risk matter too.
APR, APY, and EAR distinctions
Amortization answers a schedule question: given a principal, stated rate, term, and payment frequency, how does the balance decline? APR answers a cost-comparison question and may include certain fees. APY and EAR answer compounding questions. APY is usually used on deposits and savings yields; EAR is the effective annual rate produced by a nominal rate and compounding frequency, often used for both lending and investment comparisons.
Do not substitute those measures blindly. A mortgage note might accrue interest at a stated note rate, disclose an APR that includes fees, and sit beside savings accounts advertising APY. This calculator needs the rate used to accrue interest on the unpaid principal. Use the APR calculator, APY calculator, and EAR calculator when you need those separate comparisons.
Tips for accurate schedules
- Match the term to the actual loan contract. A 29-year remaining mortgage is not the same as a fresh 30-year loan.
- Use the current principal balance for a remaining-loan schedule, not the original amount borrowed.
- Enter the nominal annual interest rate as a percent, such as 6.5, not as 0.065.
- Keep taxes, insurance, escrow, and optional products out of this tool unless they are truly part of principal and interest.
- If you are modeling extra payments, confirm that the servicer applies them to principal rather than future installments.
Sources
- CFPB, What is amortization? — plain-language definition of paying a loan down over time.
- CFPB, What is a loan amortization schedule? — explanation of monthly principal and interest schedule rows.
- Federal Reserve, Consumer Handbook on Adjustable-Rate Mortgages — consumer context for loan payments, rates, and mortgage terms.