How loan payments and amortization work
When you borrow money through an installment loan, your monthly payment is split into two components: interest (the cost of borrowing the money) and principal (repaying the original borrowed balance). Over time, as your remaining balance decreases, each successive monthly payment applies more money to principal and less to interest.
The standard loan amortization formula
Where:
- P = Principal loan amount: The initial balance borrowed.
- r = Periodic monthly interest rate: Annual interest rate (APR) divided by 12 months and 100 (e.g. 6.5% APR → 0.065 ÷ 12 = 0.005417).
- n = Total number of monthly payments: Loan term in years multiplied by 12 (e.g. 5 years → 60 months).
Monthly payment comparison across loan amounts and terms
| Loan Amount | 3 Years (36 mo @ 6.5%) | 4 Years (48 mo @ 6.5%) | 5 Years (60 mo @ 6.5%) | 7 Years (84 mo @ 6.5%) |
|---|---|---|---|---|
| $10,000 | $306.49 / mo ($1,034 int) | $237.15 / mo ($1,383 int) | $195.66 / mo ($1,740 int) | $148.49 / mo ($2,473 int) |
| $20,000 | $612.98 / mo ($2,067 int) | $474.30 / mo ($2,766 int) | $391.32 / mo ($3,479 int) | $296.99 / mo ($4,947 int) |
| $35,000 | $1,072.72 / mo ($3,618 int) | $830.02 / mo ($4,841 int) | $684.82 / mo ($6,089 int) | $519.73 / mo ($8,657 int) |
| $50,000 | $1,532.45 / mo ($5,168 int) | $1,185.74 / mo ($6,916 int) | $978.31 / mo ($8,699 int) | $742.47 / mo ($12,367 int) |
APR vs. Interest Rate: What is the difference?
- Interest Rate: The direct percentage fee charged on the borrowed principal balance each year.
- Annual Percentage Rate (APR): The comprehensive annualized cost of borrowing, which combines the interest rate with mandatory lender fees, origination charges, closing costs, and administrative fees. When comparing competing loan offers, always compare APR rather than the nominal interest rate.
Fixed-rate vs. variable-rate loans
- Fixed-Rate Loans: The interest rate and monthly payment remain constant throughout the entire life of the loan. This guarantees predictable budgeting and shields you against market rate hikes.
- Variable / Adjustable-Rate Loans (ARM): The interest rate floats with benchmark market rates (such as SOFR or the Prime Rate). Payments may start lower than fixed loans but can adjust upward if interest rates increase.
Frequently asked questions about loans
What happens if I make extra principal payments?
Making extra payments directly toward your loan principal lowers your outstanding balance faster. Because monthly interest is calculated against the remaining balance, every extra dollar paid reduces future interest charges and pays off your debt months or years ahead of schedule.
Does this calculator work for auto loans and mortgages?
Yes. The amortization formula used in this tool applies to fixed-rate auto loans, personal installment loans, student loans, and fixed-rate mortgages.
What is a prepayment penalty?
Some lenders charge an exit fee if you pay off your loan early within a specified window (typically 1 to 3 years). Always check your loan agreement for prepayment penalty clauses before making large lump-sum payoffs.
Created and calculation-tested by Velour Trends · Updated 8 November 2026. Standard amortization mathematics verified. Report a calculation issue.