Loan Calculator: Practical Guide
Estimate fixed-rate installment payments, interest cost and the effect of extra principal payments. See an annual declining-balance schedule for planning.
What is a loan calculator?
A fixed-rate installment loan is normally repaid in regular monthly installments. Each payment covers interest on the outstanding principal and reduces the loan balance with the remainder. Early in the term, the interest portion is generally larger; later, more of each installment pays principal. This model assumes one nominal annual rate, monthly payment intervals, and no fees.
How to use the Loan Calculator
Enter the principal amount borrowed, annual nominal interest rate, and repayment period in years. Optionally enter additional principal paid each month. Results show the scheduled installment, the monthly payment including extra principal, modeled total interest and a year-end loan balance table. The display currency is a label only; it does not convert real exchange-rate amounts.
Monthly payment formula
For loan principal P, monthly interest rate r = annual percentage rate / 1200, and payment count n = years × 12, the scheduled monthly payment is:
PMT = P × r / [1 − (1 + r)^(-n)]
At 0% interest, the payment is simply P ÷ n. The calculator uses monthly compounding on outstanding balances; it is not suitable for flat-rate interest quotes without adjustment.
Worked example: $100,000 at 6% for 10 years
The loan contains 120 monthly payments at an interest rate of 0.5% per month. The scheduled payment is approximately $1,110.21 per month. Without extra payments, total installments are about $133,224, of which approximately $33,224 is interest. Small differences are possible because payments are displayed rounded while calculations use greater precision.
How extra principal changes payoff
Paying an additional amount toward principal reduces the balance used to calculate future interest. The lender must apply the extra as a principal payment, rather than treating it as an early future installment. If you pay $100 extra every month, your total monthly outflow rises but the loan may finish earlier and incur less interest. Always check prepayment penalties or restrictions. The modeled final payment can be smaller than a usual monthly installment.
What this estimate excludes
Real loan agreements can include origination fees, insurance, variable interest, interest-only periods, payment holidays, daily interest, penalties or payment rounding. The rate field represents the nominal interest rate used in this payment model, not a verified APR including fees. Total repayment figures exclude taxes, closing costs and unrelated expenses. For a binding quote, consult the lender’s amortization schedule.
Frequently asked questions
The expandable FAQ section above addresses common usage and interpretation questions. Check the calculator inputs, formula and limitations before using results for important decisions.