Equity Line of Credit Interest Calculator

An equity line of credit interest calculator estimates the monthly interest charge on a home equity line of credit (HELOC) from the amount borrowed and its annual interest rate. Enter the balance, APR, and number of months; the calculator returns estimated interest for that period and the balance plus interest, assuming the balance stays unchanged.

Formula

The estimate compounds the balance monthly at the APR divided by 12. The formula is:

• Monthly rate: r = APR / 1200, expressed as a decimal.
• Interest over n months: I = B × ((1 + r)ⁿ − 1).
• Balance plus interest: T = B + I.

B is the amount currently borrowed, in dollars; APR is the annual percentage rate, entered as a percent; n is the number of months; I is estimated interest in dollars; and T is the resulting balance before payments or other account activity. This is a compound-interest estimate, not a required payment calculation.

Worked Example

Using the calculator’s default inputs—$20,000 outstanding, 8% APR, and one month:

1. Convert APR to a monthly rate: 8 ÷ 1200 = 0.0066667, or about 0.66667% per month.
2. Apply the monthly compounding formula: $20,000 × ((1 + 0.0066667)¹ − 1) = $133.333…
3. Round interest to cents: $133.33.
4. Add the estimated interest to the starting balance: $20,000 + $133.33 = $20,133.33.

The calculator displays $133.33 in estimated interest and $20,133.33 as balance plus interest.

Assumptions

• The entered APR remains fixed throughout the selected period.
• Interest compounds monthly, using APR divided by 12.
• The borrowed balance does not change; there are no draws or payments during the period.
• Interest is calculated for whole months, with results rounded to cents.
• Fees, taxes, insurance, payment allocation, and lender-specific billing conventions are not included.

How to Use the Estimate

Use the outstanding balance—not the total credit limit—as the amount borrowed. Enter the APR shown for your account or the rate supplied for a hypothetical example, then select a period in months. A one-month estimate can help illustrate the approximate carrying cost of an unchanged balance; a longer period illustrates compounding under the same simplified assumptions.

For example, if you have drawn $20,000 from a $50,000 line, the calculation uses $20,000. The unused $30,000 does not accrue the modeled interest because it is not part of the borrowed balance. Whether an account has fees or other charges unrelated to the outstanding balance depends on its terms; check the lender’s disclosures rather than treating this calculator as a complete account-cost estimate.

How HELOC Interest Can Differ

A home equity line of credit is a revolving borrowing arrangement secured by home equity. Unlike a single disbursement loan, a line can allow additional draws and repayments over time, subject to the account agreement. Many HELOC rates can change, so the APR used in a calculator may not remain the same throughout a future period. This calculator deliberately holds the APR constant so its arithmetic is clear; it cannot forecast rate changes.

Actual interest calculations may also depend on daily balances, the dates funds are advanced or repaid, billing-cycle length, and the lender’s rounding method. Some statements calculate interest using a daily periodic rate and the balance on each day, rather than applying a monthly compound formula to one unchanged balance. Consult the account’s rate and calculation disclosures for the exact method.

Common Mistakes and Limits

• Entering the credit limit: Use the amount actually outstanding, not the maximum amount available.
• Confusing APR with a monthly rate: Enter an annual percentage such as 8, not 0.6667. The calculator converts APR to a monthly rate.
• Assuming a variable rate stays put: A result for several months is a scenario, not a rate forecast.
• Ignoring payments or new draws: The balance is assumed unchanged. Payments generally reduce the balance, while additional draws increase it.
• Reading the total as a bill: Balance plus interest is not necessarily the amount due. Required payments and due dates depend on account terms.

The formula is useful for a consistent, simplified estimate, but it is not a substitute for a statement or lender-provided payoff figure. It does not model fees, variable-rate schedules, irregular transactions, or a payment plan.

Frequently Asked Questions

How do I calculate monthly interest on a HELOC?
For this estimate, divide the annual APR by 12 to get the monthly rate, then multiply the balance by that rate. At 8% APR, the monthly rate is about 0.66667%; on $20,000, that is about $133.33 for one month before payments or other adjustments.

Do I use my HELOC limit or current balance?
Use the amount currently borrowed and unpaid. The credit limit is the maximum available line, not necessarily the balance on which interest is calculated.

Does this calculator include monthly payments?
No. It assumes no payments and no new draws during the selected period. To estimate a changing balance, you would need the timing and amount of each transaction and the account’s interest method.

Why might my statement show a different interest amount?
Your lender may use daily balances, a variable rate, different transaction dates, fees, or different rounding rules. Compare the statement’s calculation details with the calculator’s assumptions.

Can I use this for a variable-rate HELOC?
Yes, as a what-if calculation for one APR held constant over the chosen period. It does not predict future APR changes, so results over longer periods may differ from actual charges.

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

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in bal | Outstanding balance | $ | 20000 | 0 | 100000000
in apr | Interest rate (APR) | % | 8 | 0.01 | 100
in mos | Calculation period | months | 1 | 1 | 600
out int | Estimated interest | $ | bal * ((1 + apr / 1200) ^ mos - 1) | 2
out total | Balance plus interest | $ | bal + int | 2