For a HELOC using a daily-balance calculation, estimate each day's interest from that day's balance and applicable daily rate, then add the days together. A month-end balance alone is not enough when you draw or repay during the cycle. Use the calculation method and day-count basis in your agreement; this example assumes a 365-day year.
What to take away
- Use the balance on each day, not only the closing balance.
- Separate the interest charge from the required payment.
- Transaction posting dates can affect the result.
Find the calculation rule first
Before opening a spreadsheet, locate the balance method, periodic rate, and date interest begins in your account disclosures. CFPB rules require explanations of how finance charges are determined. Do not assume every line uses the exact same method or that repaying before the statement arrives creates an interest-free borrowing period.
For this illustration only, use a constant 8% annual rate, a 365-day divisor, no fees, and balances that change when transactions post. The daily rate is 0.08 divided by 365. Keep that full number in the calculation and round the final dollar result.
Divide the month into balance segments
Imagine a 30-day cycle. You owe $30,000 for days 1–10, draw another $10,000 for days 11–20, then reduce principal by $5,000 for days 21–30. Each segment lasts ten days. Multiply each balance by 0.08, divide by 365, and multiply by ten.
The segments generate approximately $65.75, $87.67, and $76.71. Add the unrounded amounts to get $230.14. Adding the displayed rounded segment amounts gives $230.13, illustrating why a one-cent rounding difference is possible. Your actual statement may follow a different rounding convention.
| Days | Balance | Illustrative interest |
|---|---|---|
| 1–10 | $30,000 | $65.75 |
| 11–20 | $40,000 | $87.67 |
| 21–30 | $35,000 | $76.71 |
| Whole cycle, before rounding segments | Changing balance | $230.14 |
Check the result with an average balance
The average daily balance here is $35,000: add 300,000, 400,000, and 350,000 balance-days, then divide by 30. Multiplying $35,000 by 8% and by 30/365 gives the same $230.14 under these assumptions.
The familiar monthly shortcut, $35,000 × 8% ÷ 12, gives $233.33. It is useful for a rough budget but does not reproduce this 30-day calculation. A 31-day cycle at a constant $35,000 would instead produce approximately $237.81 using the same daily method.
Use the dates that actually count
If the $5,000 principal reduction in the example posts five days later, interest rises by about $5.48: $5,000 × 8% × 5/365. That is the cost of five additional days carrying that amount. Record posting dates rather than assuming the day you initiated a transfer is necessarily the effective date.
A midcycle rate change requires another segment even when the balance stays the same. Keep a small ledger with start date, end date, balance, rate, and estimated charge. Ask the servicer to explain any difference between that ledger and the statement before concluding the calculation is wrong.
Keep the payment question separate
An interest estimate does not establish the minimum payment. Your payment may also require principal, fees, or past-due amounts. Read the statement's payment instructions and confirm how extra funds are applied to your specific account. This ledger is most useful for understanding charges and testing draw timing, while the agreement and statement determine what you must pay.
Questions that come up
Do I pay interest on my entire HELOC limit?
This calculation uses outstanding borrowed balances. Record any separate fees or required initial draw in addition to the line's unused capacity.
Why did interest rise when my rate did not?
A larger balance, a longer billing cycle, or a later principal payment can increase the charge without a rate increase.
Does paying interest reduce my principal?
No. Principal falls only when a payment is applied to principal; paying an interest charge alone leaves the borrowed balance outstanding.
Sources & context
Primary sources checked for this guide on October 14, 2026. Examples are hypothetical, not current offers. Lender terms and individual circumstances can differ.
This guide is educational and is not individualized financial, tax, or legal advice. A home equity loan or line puts your home at risk if you cannot repay.