Line of Credit Interest Rate: Calculate the Cost

A line of credit interest rate is the annual rate a lender charges on the amount you borrow, not necessarily on your entire credit limit. This calculator estimates interest for one month from your outstanding balance and annual APR; enter those figures to see the estimated monthly interest and equivalent monthly rate.

Formula

The estimate is the outstanding balance multiplied by the annual percentage rate divided by 12 months:

Monthly interest = balance × (APR ÷ 100 ÷ 12)

• Balance is the amount currently borrowed, in dollars.
• APR is the annual percentage rate entered as a percentage, such as 8 for 8%.
• Monthly rate is APR ÷ 12, expressed as a percentage. In the interest calculation, APR ÷ 1,200 converts the percentage to a monthly decimal rate.

The calculation assumes a constant balance for a full month. Actual statements may use daily balances, a daily periodic rate, transaction dates, and the lender’s specific interest calculation method, so the result is an estimate rather than a statement prediction.

Worked Example

Using the calculator’s defaults—an outstanding balance of $10,000 and an APR of 8%:

1. Convert the annual percentage rate to a monthly decimal rate: 8 ÷ 1,200 = 0.0066667.
2. Multiply the balance by that rate: $10,000 × 0.0066667 = $66.6667.
3. Round to cents: estimated interest for one month is $66.67.

The equivalent monthly rate shown is 8 ÷ 12 = 0.6667%. This is a nominal monthly conversion, not a promise that every statement will charge exactly $66.67.

Assumptions

• The entered APR remains fixed for the month being estimated.
• The balance is unchanged throughout that month, with no draws, payments, or other balance adjustments.
• Interest is approximated monthly at APR ÷ 12; the calculator does not model daily compounding or a daily-balance method.
• Fees, taxes, insurance, promotional pricing, and other account charges are not included.

How a Line of Credit Rate Works

A line of credit gives a borrower access to funds up to a stated limit. Interest generally relates to money actually borrowed and still outstanding, rather than the unused portion of the limit. For example, if a person has a $20,000 limit but has drawn $5,000, the interest calculation generally starts with the outstanding borrowed balance—not the full $20,000. The lender’s agreement and statement determine the balance and method used.

The rate may be variable or fixed. A variable rate can change under the terms of the account, often in relation to a reference rate plus or minus a stated margin. A fixed rate stays unchanged for the period specified by the agreement, although a product can have different terms for different draws. Check the account disclosure for how changes are determined, when they take effect, and whether a promotional rate expires.

APR is useful for expressing a yearly borrowing rate, but it does not by itself tell you the exact interest on a particular statement. The outstanding balance can change day by day as money is borrowed and repaid. Some accounts calculate interest using daily balances and a daily periodic rate; others may apply different contract-specific procedures. Statement timing and rounding can also affect the amount.

What Changes the Interest Cost

The two most direct drivers in this estimate are the balance and APR. Holding the rate constant, doubling the balance doubles the estimated monthly interest. Holding the balance constant, a higher rate raises the estimate proportionally. The period you carry a balance matters too: paying down principal reduces the balance on which later interest may accrue, subject to the account’s calculation method.

• Rate changes: If the APR is variable, a new rate can change interest even if the balance stays the same.
• Draws and repayments: New advances raise the balance; payments may reduce it, but a payment can also include interest or other charges.
• Fees: Annual, transaction, or other fees may add to borrowing costs, but they are not part of this monthly-interest estimate.
• Time outstanding: Carrying the balance for more days can change statement interest when a daily-balance method is used.

A common misconception is that the stated APR multiplied by the credit limit gives the interest due. It does not: the limit is the maximum available credit, while interest is generally based on borrowed funds and the account’s rules. Another important distinction is APR versus the periodic rate. For this estimate, an 8% APR becomes 8% ÷ 12, or about 0.6667% per month—not 8% each month.

How to Use and Interpret the Estimate

Use the current amount owed, not the total credit limit, and enter the APR as a percentage number: enter 8 for 8%, not 0.08. If the rate varies, use the rate that applies to the balance for the period you want to approximate. For an estimate over a different balance or rate, change the inputs and treat the result as a simple one-month illustration.

Compare the calculation with the account’s latest rate disclosure and statement. Look for the balance used, the periodic rate, whether rates differ by transaction type, and how payments are applied. Cash advances, purchases, and promotional balances can have distinct rates or terms on some accounts. This calculator has one balance and one APR, so it cannot separate those categories or model changing balances.

Do not read the monthly interest figure as a required payment or as the total cost of keeping a line open. A payment may include principal, interest, and fees, and account terms may set minimum-payment requirements. The calculator does not estimate a payoff date, future variable rates, or the effect of additional borrowing. Obtain the applicable rate and account-specific calculation details from the lender’s disclosure or statement.

Frequently Asked Questions

Is line of credit interest charged on the full credit limit?
Usually, interest is based on the amount borrowed and outstanding, rather than the unused limit. The account agreement describes how the balance is determined.

How do I convert a line of credit APR to a monthly rate?
For a nominal monthly conversion, divide the APR percentage by 12. For example, 8% ÷ 12 is about 0.6667% per month. A statement’s actual periodic rate may be calculated differently.

Can a line of credit interest rate change?
It depends on the account terms. A variable-rate line can change as specified in its agreement, while a fixed rate applies for the period set out in the terms. Check the disclosure for the rate-change mechanism and timing.

Why is my statement interest different from this estimate?
The calculator assumes one unchanged balance for a full month. A statement may reflect daily balances, transactions on different dates, rate changes, payment timing, rounding, or fees that the estimate does not include.

Does paying more reduce line of credit interest?
A payment that reduces the outstanding principal can reduce the balance used in later interest calculations. The amount and timing of the effect depend on the account’s balance method and payment processing terms.

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

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in bal | Outstanding balance | $ | 10000 | 0 | 100000000
in apr | Interest rate (APR) | % | 8 | 0.01 | 100
out int | Estimated interest for one month | $ | bal * (apr / 1200) | 2
out mr | Monthly rate | % | apr / 12 | 4