Bank Loan Interest Payment Calculator and Formula

A bank loan interest payment is the part of a scheduled payment that covers the cost of borrowing; the rest usually reduces the amount owed. Use this calculator to estimate a fixed-rate loan’s monthly payment and total interest by entering the loan amount, annual interest rate, and term.

Formula

For a fixed-rate loan with equal monthly payments, the payment is calculated using the standard amortization formula:

Payment = P × r ÷ [1 − (1 + r)⁻ⁿ]

• P is the amount borrowed, in dollars.
• r is the monthly interest rate as a decimal: annual APR ÷ 12 ÷ 100.
• n is the number of monthly payments: loan term in years × 12.

Total interest is estimated as monthly payment × n − P. The calculator uses the unrounded payment formula internally for its displayed monthly payment, rounded to cents, and calculates total interest from that displayed payment. Because actual payment schedules can round each installment and adjust the final one, a lender’s figures may differ slightly.

Worked Example

Using the calculator’s default inputs—$20,000 borrowed, 8% APR, and a five-year term:

1. Monthly rate: 8 ÷ 12 ÷ 100 = 0.0066667.
2. Number of payments: 5 × 12 = 60.
3. Monthly payment: $20,000 × 0.0066667 ÷ [1 − (1.0066667)⁻⁶⁰] = approximately $405.53.
4. Total of displayed payments: $405.53 × 60 = $24,331.80.
5. Estimated total interest: $24,331.80 − $20,000 = $4,331.80.

These results assume the displayed monthly payment is made for all 60 months. A lender’s schedule can vary because of payment-date conventions, rounding, or other charges.

Assumptions

• The interest rate stays fixed for the full term.
• Payments are equal monthly installments, paid at the end of each monthly period.
• Interest accrues monthly at the annual APR divided by 12.
• Fees, taxes, insurance, late charges, and other loan costs are not included.

How Loan Interest Payments Work

Interest is generally calculated on the outstanding principal, not on the original loan amount forever. In an amortizing loan, each payment covers the interest accrued since the previous payment and then pays down principal. As the principal balance falls, the interest portion usually shrinks and the principal portion grows, while the scheduled payment remains level when the rate and terms are fixed.

For example, at the start of the calculator’s sample loan, monthly interest is about $133.33 ($20,000 × 0.08 ÷ 12). Of the first $405.53 payment, roughly $272.20 therefore reduces principal. Later, interest is calculated on a smaller balance. This explains why multiplying the original loan amount by the annual rate gives an interest estimate for one year at the beginning, but not the total interest over an amortizing loan’s full term.

What the Results Tell You

The monthly payment is the recurring principal-and-interest amount in this simplified estimate. Total interest is the estimated amount paid above the principal over the full schedule. It is not necessarily the total amount a borrower pays: charges such as origination fees, insurance, or taxes may be separate, and some costs may be added to the balance.

APR is an annualized rate used here as the input for a monthly calculation. A quoted APR may reflect some borrowing costs depending on the product and disclosure, but this calculator does not separately model fees or lender-specific methods. Enter the rate and loan amount from the lender’s disclosure for a closer arithmetic estimate. If the rate can change, a single fixed-rate calculation cannot represent future payments accurately.

Common Mistakes and Limits

• Entering the wrong rate units: Enter an annual percentage such as 8, not 0.08. The formula converts the percentage to a monthly decimal.
• Confusing interest with payment: A monthly payment includes principal as well as interest. Interest alone is not the full installment.
• Ignoring the term: A longer repayment period generally spreads principal over more payments and changes total interest. The calculator assumes the full stated term is paid as scheduled.
• Assuming all loans amortize this way: Interest-only, variable-rate, balloon-payment, and irregular-payment loans need different calculations.
• Expecting an exact payoff quote: Daily interest accrual, payment dates, fees, extra payments, and rounding can change the amount due. Use the lender’s payoff statement for an exact figure.

The tool is useful for understanding the mathematical relationship between principal, rate, and term, not for determining eligibility or deciding whether to borrow. Get the actual rate, payment schedule, and any included charges from the lender.

Frequently Asked Questions

How do I calculate interest on a bank loan?
For a fixed-rate amortizing loan, calculate the level monthly payment with the principal, monthly rate, and number of monthly payments. The interest over the schedule is the total of payments minus the amount borrowed, subject to the lender’s rounding and payment rules.

Is the loan payment the same as the interest payment?
No. A standard installment usually contains both interest and principal. Interest pays the borrowing cost for that period; principal repayment reduces the outstanding balance.

Why does the interest portion decrease over time?
On a typical amortizing fixed-rate loan, interest is based on the remaining principal. As payments reduce that balance, less interest accrues for each subsequent period, leaving more of a level payment to reduce principal.

Does a longer loan term mean more total interest?
With the same principal and fixed rate under this formula, extending the term generally lowers the required monthly payment but increases the number of periods over which interest accrues. The calculator’s total-interest result reflects those assumptions.

Can I use this calculator for a variable-rate loan?
It can show a hypothetical payment if you enter one rate, but it cannot model future rate changes. A changing rate can alter both the payment and total interest; consult the lender’s schedule for product-specific calculations.

Why is my lender’s payment different?
The lender may use a different payment date, day-count convention, rounding method, fee treatment, or loan structure. Compare the calculator’s assumptions with the disclosure and ask the lender to explain any difference. The result is an estimate, and the lender’s disclosure gives the exact figures.

Related

• First Payment Gateway Benefits Explained
• First Loan Payment Date Calculator & Timing
• First Payment Default: Meaning and Loan Impact

in p | Loan amount | $ | 20000 | 1 | 100000000
in apr | Interest rate (APR) | % | 8 | 0.01 | 100
in yrs | Loan term | years | 5 | 0.0833 | 50
out pmt | Monthly payment | $ | p * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)) | 2
out int | Total interest | $ | pmt * yrs * 12 - p | 2