The Basic Formula: Principal Times Rate Divided by 12

Your HELOC interest is calculated monthly on the outstanding balance you actually owe, not on your total credit limit. The lender takes your current balance, multiplies it by your annual interest rate, and divides by 12 to get that month's interest charge. If you owe $50,000 at 8% annual interest, you pay roughly $333 in interest that month ($50,000 × 0.08 ÷ 12). The next month, if you've paid down $5,000, the interest drops to about $300.

This is different from a fixed-rate loan, where the same payment covers both principal and interest every month. With a HELOC, your interest-only payment changes whenever your balance changes or your rate changes. Many HELOCs have a draw period (typically 5 to 10 years) when you can borrow and repay repeatedly, followed by a repayment period (typically 10 to 20 years) when you can no longer draw and must pay down the balance.

The actual dollar amount you pay depends on three things: how much you've borrowed, what interest rate your lender charges you, and whether you're in the draw period or repayment period. Understanding each one helps you predict what your monthly bill will be.

Key Takeaways

  • HELOC interest is calculated monthly on only the balance you owe, so your payment shrinks as you pay down the debt.
  • Most HELOCs have a variable interest rate that moves with the prime rate, so your payment can increase or decrease several times a year.
  • During the draw period, you typically pay interest only; during the repayment period, you must pay principal plus interest.
  • Your lender will send a monthly statement showing the current balance, interest rate, and minimum payment due.

Variable Rates and How They Move

Nearly all HELOCs carry a variable interest rate, which means the rate changes over time. Your rate is usually tied to the prime rate — a benchmark rate that major banks publish and that the Federal Reserve influences. When the Fed raises or lowers its benchmark rate, the prime rate follows within days, and your HELOC rate adjusts shortly after.

Your lender adds a fixed margin (often 0.5% to 2%) to the prime rate to set your actual rate. If the prime rate is 7% and your margin is 1%, your rate is 8%. When the prime rate rises to 7.5%, your rate becomes 8.5% automatically. This is why HELOC payments can jump unexpectedly — you're not locked into a single rate the way you are with a fixed-rate mortgage.

Some HELOCs include a rate cap, which sets a ceiling on how high your rate can go. A common structure is a 10% lifetime cap, meaning your rate will never exceed 10% no matter how high the prime rate climbs. Read your promissory note or disclosure documents to find out whether your HELOC has a cap and what it is.

Draw Period Versus Repayment Period Payments

During the draw period, you can borrow money whenever you want (up to your credit limit) and repay it on your own schedule. Most lenders require you to pay at least the interest that accrues each month. If you borrow $30,000 at 7% during the draw period, you owe roughly $175 per month in interest alone. You can pay more if you want to reduce the principal, but the minimum is usually interest only.

When the draw period ends and the repayment period begins, you can no longer borrow. Now you must pay down the entire remaining balance over the repayment period (often 10 to 20 years). Your payment jumps because it now includes both interest and principal. That same $30,000 balance, spread over 15 years at 7%, costs roughly $237 per month — and that's before any rate increases.

This transition is where many borrowers run into trouble. If you've only been paying interest for 10 years and haven't reduced the principal, your payment can double or triple when repayment begins. Plan ahead by paying down principal during the draw period, or by refinancing before the repayment period starts.

How Lenders Calculate Your Minimum Payment

During the draw period, your minimum payment is usually the interest accrued that month, sometimes with a small principal component added. Lenders calculate this by taking your current balance, multiplying by your current rate, and dividing by 12. Some lenders add a fixed percentage of principal (like 0.5% of the balance) to the interest charge, so you pay down the debt slowly even during the draw period.

During the repayment period, lenders use an amortization schedule, the same method used for mortgages and car loans. They calculate a fixed payment that covers both interest and principal over the remaining term, so the balance reaches zero on the final payment date. Your lender provides this schedule when the repayment period begins, and your payment stays the same each month — unless your variable rate changes, in which case the lender recalculates the payment to keep you on track to pay off the balance by the end of the term.

Your monthly statement will show the interest portion and the principal portion separately. Early in the repayment period, most of your payment goes to interest; later, more goes to principal. This is normal and expected.

What Happens When Your Rate Changes

When the prime rate moves, your HELOC rate adjusts, usually within 30 days. Your lender sends a notice of the new rate and recalculates your minimum payment. If you're in the draw period paying interest only, your payment may go up or down slightly. If you're in the repayment period, the lender recalculates your amortized payment to ensure you'll still pay off the balance by the end of the term.

A rate increase during the repayment period can raise your payment noticeably. If your rate goes from 7% to 8% with 10 years left on a $50,000 balance, your monthly payment might jump from $580 to $610. The higher rate means more of each payment goes to interest, so you pay down principal more slowly — the lender adjusts the payment upward to keep you on schedule.

If rates fall, your payment decreases, but you still owe the same balance. You can choose to keep paying the higher amount and pay off the debt faster, or pay the new lower minimum and extend the payoff date. Most borrowers pay the new minimum, which is why falling rates feel like a relief but don't actually reduce your debt any faster.

Fees That Add to Your Interest Cost

Beyond the interest rate itself, HELOCs often include fees that increase your total cost. An annual fee (typically $50 to $300) is charged once per year just to keep the account open. A draw fee (typically $25 to $50) is charged each time you access the credit line. Some lenders charge an inactivity fee if you don't borrow for a certain period, or a prepayment penalty if you pay off the balance early.

These fees don't change your interest calculation, but they do add to your total cost of borrowing. When comparing HELOCs from different lenders, ask about all fees upfront. A HELOC with a slightly higher interest rate but no annual fee may cost less than one with a lower rate and a $200 annual fee.

Frequently Asked Questions

Does my HELOC interest accrue daily or monthly?

Interest accrues daily on your outstanding balance, but it's usually charged and added to your account monthly. Your lender calculates the daily interest and totals it at the end of each month. This is why paying down your balance mid-month can reduce the interest you owe that month — you've lowered the average daily balance.

What's the difference between my interest rate and my APR?

Your interest rate is the percentage charged on your balance. Your APR (annual percentage rate) includes the interest rate plus certain fees, expressed as an annual rate. For a HELOC, the APR is usually very close to the interest rate because most fees are charged separately, not rolled into the rate itself. Your disclosure documents will show both.

Can I lock in a fixed rate on my HELOC?

Some lenders offer the option to convert part or all of your HELOC balance to a fixed-rate loan, usually for a fee. This locks in your rate for the remaining term, so your payment won't change if the prime rate rises. Ask your lender whether this option is available and what it costs.

What happens to my interest if I only make the minimum payment?

If you only pay interest during the draw period, your balance never shrinks, so you owe interest indefinitely. When the repayment period begins, you'll owe a much larger payment because you must now pay down the full balance. Paying more than the minimum during the draw period reduces the shock when repayment begins.

How do I know what my next payment will be?

Your lender sends a monthly statement showing your current balance, interest rate, and minimum payment due. If your rate changed that month, the statement will show the new rate and the new payment. You can also call your lender or log into your online account to see the current figures anytime.