Bank Loan Interest Rate Calculator and Payment Formula

A bank loan interest rate is the price charged for borrowing, stated as a percentage; the rate alone does not tell you the full cost because the loan amount, term, fees, and repayment schedule also matter. Use this calculator to estimate a fixed-rate loan’s monthly payment and total interest: enter the principal, annual percentage rate (APR), and term, and it returns both amounts.

Formula

The calculator uses the standard amortizing-loan payment formula. It converts the annual APR to a monthly rate, then finds the level monthly payment that pays interest and gradually repays the principal over the chosen number of months.

Monthly payment: M = P × r ÷ [1 − (1 + r)⁻ⁿ]. Total interest: I = (M × n) − P.

• P is the loan amount in dollars.
• APR is the annual percentage rate entered as a percentage, such as 8 for 8%.
• r is the monthly rate as a decimal: APR ÷ 1,200.
• n is the number of monthly payments: years × 12.
• M is the monthly payment in dollars, rounded to cents.
• I is the estimated total interest in dollars, calculated using the rounded monthly payment.

For the calculator’s default 8% APR, r is 0.08 ÷ 12, or 0.0066667, and a five-year term has 60 payments. The calculation assumes a fixed rate and equal monthly payments. The entered APR is treated as the rate used in the payment formula; fees and other charges are not added separately.

Worked Example

With the default inputs—$20,000 principal, 8% APR, and five years—the number of payments is 5 × 12 = 60, and the monthly rate is 8 ÷ 1,200 = 0.0066667.

1. Substitute the values: M = 20,000 × 0.0066667 ÷ [1 − (1 + 0.0066667)⁻⁶⁰].
2. The unrounded payment is approximately $405.5277, which rounds to $405.53 per month.
3. Total paid using that rounded payment is $405.53 × 60 = $24,331.80.
4. Subtract the $20,000 principal: estimated total interest is $4,331.80.

Assumptions

• The rate stays fixed for the full term.
• Payments are monthly, equal in amount, and made at the end of each monthly period.
• Interest accrues monthly using APR divided by 12.
• The calculation excludes fees, taxes, insurance, PMI, escrow, and any charges not entered as loan principal.

What a Bank Loan Interest Rate Means

A stated interest rate expresses borrowing cost as a percentage of the outstanding balance over time. On a standard amortizing loan, each scheduled payment includes interest on the remaining balance plus principal repayment. Early payments generally devote a larger share to interest because the balance is higher; as the balance falls, the interest portion usually falls too.

APR is a useful figure to review because it is intended to express an annualized borrowing cost. However, what a quoted APR includes can depend on the loan and its disclosure. Ask the lender which charges are included and whether the rate is fixed or variable. A variable rate can change under the contract, so a calculator that holds APR constant cannot predict its future payments.

What Changes the Rate and the Total Cost

A lender sets its offered rate using its own pricing and eligibility process. The borrower should obtain the actual rate, fees, and repayment terms directly from the lender rather than infer them from a general example. A rate is only one part of the comparison: a longer term can reduce the scheduled monthly payment while increasing the number of months interest accrues.

For example, extending repayment while keeping principal and APR unchanged generally spreads principal repayment over more payments. The resulting payment is lower under the formula, but total interest is higher. This is a mathematical trade-off, not a statement about what term a particular borrower should choose. Upfront fees, late charges, payment timing, and variable-rate changes may also affect the actual amount paid.

How to Use the Estimate

• Enter the loan principal, not the total of all scheduled payments.
• Enter APR as a percentage number: type 8 for 8%, not 0.08.
• Enter the term in years. The formula converts it to monthly payments.
• Compare the calculated payment and interest with the lender’s written payment schedule and disclosures.
• Check whether the lender’s figures include fees, add-on products, or a different payment frequency before comparing numbers.

The result is a planning estimate for a fully amortizing loan with fixed monthly payments. Actual payment dates, rounding practices, fees added to the balance, or a first payment period that is not a full month can cause a difference. The formula also does not model extra payments, missed payments, changing rates, or a final adjusted payment.

Common Misunderstandings and Limits

A lower monthly payment does not necessarily mean a lower overall borrowing cost; the term and fees matter. Likewise, the total interest shown is not the same as total repayment: total repayment is principal plus interest, and additional costs may apply. The displayed interest uses the monthly payment rounded to cents, so it may differ slightly from a calculation that keeps fractions of a cent throughout.

This calculator cannot determine what rate a lender will offer, whether an application will be approved, or the exact amount due on a particular date. It does not account for interest calculated daily, variable rates, irregular payments, or lender-specific fee treatment. For a real offer, use the lender’s disclosed APR, payment amount, fee information, and amortization schedule.

Frequently Asked Questions

How do I calculate interest on a bank loan?
For a fixed-rate amortizing loan, calculate the monthly payment from the principal, monthly rate, and number of payments. Total scheduled interest is the rounded payment multiplied by the payment count, minus the principal, as shown above.

Is APR the same as the interest rate?
Not always. The interest rate describes the rate used to calculate interest, while APR is an annualized cost measure that may include certain charges. Check the lender’s disclosure to see precisely what its APR includes.

Does a longer loan term lower the interest rate?
Not by itself. Term and rate are separate loan terms. For the same principal and APR, a longer term generally lowers the scheduled monthly payment but increases total interest because repayment takes longer.

Why is my bank’s payment different from this estimate?
The lender may use different rounding, payment dates, fee treatment, or interest accrual conventions. The calculator assumes a fixed APR divided by 12 and equal end-of-month payments; compare its assumptions with the lender’s schedule.

Can I use this for a variable-rate loan?
Only as a snapshot using the APR entered as if it remained unchanged. It cannot forecast future rate adjustments or the payments that may follow them.

This result is an estimate, and the lender’s disclosure gives the exact figures.

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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