How HELOC interest works

A HELOC interest rate is calculated by adding a margin set by your lender to a publicly available index rate. The index rate changes regularly — usually monthly or quarterly — so your interest rate and monthly payment move with it. This is why a HELOC is called a variable-rate product: the rate you pay is not locked in for the life of the loan.

The math is straightforward: Index Rate + Lender's Margin = Your Interest Rate. If the prime rate (the most common index) is 8.50% and your lender's margin is 1.00%, your rate is 9.50%. When the prime rate drops to 8.00%, your rate drops to 9.00%. You pay interest only on the balance you actually draw, not on the full credit limit.

Interest accrues daily on your outstanding balance. If you borrow $10,000 at 9.50% annual interest, the lender calculates one day's interest as ($10,000 × 0.095) ÷ 365, then adds that amount each day until you pay the balance down or the rate changes.

Key Takeaways

  • Your HELOC rate is the index rate plus your lender's margin, and it changes when the index rate moves.
  • The index is usually the prime rate, which is tied to the Federal Reserve's actions and published daily by major financial institutions.
  • You pay interest only on money you actually borrow, calculated daily on your outstanding balance.
  • Most HELOCs have a draw period (usually 5 to 10 years) when you can borrow and pay interest only, followed by a repayment period when you must pay down principal.
  • Your lender's margin is set at opening and stays the same, but the index rate — and therefore your total rate — changes throughout the life of the loan.

What the index rate is and why it moves

The index rate is a published benchmark that your lender uses as the starting point for your rate. The most common index for HELOCs is the prime rate, which is the rate that banks charge their most creditworthy customers for short-term loans. The prime rate is set by individual banks but moves in lockstep with the Federal Funds Rate, which the Federal Reserve adjusts to manage inflation and economic growth.

You can find the current prime rate in the Wall Street Journal, on the Federal Reserve's website, or on your lender's website. It changes whenever the Federal Reserve meets and votes to raise, lower, or hold rates steady. These meetings happen roughly every six weeks. Your HELOC rate typically adjusts within 30 days of a prime rate change, though your loan documents will specify the exact timing.

Other possible indexes include the Treasury Bill rate or the LIBOR rate (London Interbank Offered Rate), though these are less common for consumer HELOCs. Your loan documents will state which index your rate is tied to.

What the lender's margin is and how it's set

The margin is the percentage points your lender adds to the index rate. It is set when you open the HELOC and does not change for the life of the loan, even if the index rate moves. A typical margin ranges from 0.50% to 2.50%, depending on your credit score, income, the equity in your home, and how much you are borrowing relative to your home's value.

Lenders use the margin to cover their costs and profit. A borrower with excellent credit and a large equity cushion may get a margin of 0.75%, while a borrower with fair credit or a smaller equity position may pay 1.75% or higher. You can negotiate the margin when you explore — different lenders offer different margins for the same borrower, so it pays to shop around.

Once your HELOC is open, the margin is locked in. If you close the account and reopen it later, you will receive a new margin based on current market conditions and your financial situation at that time.

How interest accrues during the draw period

Most HELOCs have a draw period, usually 5 to 10 years, during which you can borrow money and pay interest only on what you owe. Interest accrues daily on your balance. If you borrow $15,000 at 9.50% and make no payments, after 30 days you will owe roughly $119 in interest (the exact amount depends on how many days are in the month and how your lender calculates daily interest).

During the draw period, you typically have the option to pay interest only, or to pay interest plus some principal. Paying only interest keeps your monthly payment low, but it means your balance does not shrink. If you pay interest plus principal, your balance decreases and you owe less interest going forward.

Your monthly payment during the draw period is calculated as: (Outstanding Balance × Annual Interest Rate) ÷ 12. If you owe $20,000 at 9.50%, your monthly interest-only payment is roughly $158. If rates rise to 10.50%, that same $20,000 balance now costs roughly $175 per month.

What happens when the draw period ends

When the draw period ends, most HELOCs move into a repayment period, usually 10 to 20 years. During repayment, you can no longer draw new money. Instead, you must pay down your balance through fixed monthly payments that cover both interest and principal.

Your lender will calculate a new payment amount based on your remaining balance, the current interest rate, and the length of the repayment period. This payment is usually much higher than your interest-only payment was, because now you are paying down principal on a schedule. If you owe $50,000 when repayment begins and have 15 years to pay it off at 9.50%, your monthly payment will be roughly $395.

Interest continues to accrue daily during repayment, but because you are paying principal each month, your balance shrinks and the interest you owe decreases over time. Some HELOCs allow you to convert to a fixed rate during repayment, which locks in your rate for the remainder of the loan term.

How rate caps protect you

Most HELOCs include rate caps that limit how high your interest rate can go. These come in three forms: a periodic cap (how much the rate can change at each adjustment, usually 1% or 2%), a lifetime cap (the highest rate you will ever pay, often 5% to 8% above your starting rate), and sometimes a floor (the lowest rate you will pay, even if the index drops below it).

For example, if your HELOC starts at 7.50% with a 2% periodic cap and a 12.50% lifetime cap, your rate can rise no more than 2% at each adjustment, and can never exceed 12.50% no matter how high the prime rate climbs. Rate caps protect you from payment shock if rates rise sharply, but they also mean your lender bears some of the risk if rates spike.

Read your loan documents carefully to understand your caps. A HELOC with a low periodic cap but a high lifetime cap will protect you in the short term but could still expose you to large increases over time.

How to estimate your monthly payment

To estimate what you will pay each month, you need three pieces of information: your outstanding balance, your current interest rate, and whether you are in the draw period or repayment period.

During the draw period with interest-only payments: Multiply your balance by your annual rate and divide by 12. A $25,000 balance at 9.50% costs $198.96 per month in interest.

During repayment with principal and interest: Use an online HELOC calculator or ask your lender for an amortization schedule. The payment depends on how many months remain in the repayment period. The same $25,000 balance at 9.50% over 15 years costs roughly $198 per month, but over 10 years costs roughly $265 per month.

Keep in mind that if your rate changes, your payment changes too (unless you have converted to a fixed rate). Set aside a cushion in your budget for the possibility that rates will rise and your payment will increase.

Frequently Asked Questions

What happens to my HELOC rate if the Federal Reserve raises rates?

Your HELOC rate will rise within 30 days of the Federal Reserve's action, because the prime rate (the index your HELOC is tied to) rises when ready. The amount of the increase depends on how much the Federal Reserve raises the Federal Funds Rate. Your lender's margin stays the same, so your new rate is straightforward the new index plus your margin.

Can I lock in a fixed rate on my HELOC?

Some lenders allow you to convert all or part of your HELOC balance to a fixed rate, usually during the repayment period. This locks in your rate for the remainder of the loan term, so your payment stays the same even if the prime rate rises. Ask your lender whether this option is available and what the fixed rate would be.

Why does my HELOC payment change if I haven't borrowed any more money?

Your payment changes because the index rate changed. Even if your balance stays the same, a rise in the prime rate means a rise in your interest rate, which means a higher monthly payment. This is the main risk of a variable-rate product like a HELOC.

Is the interest I pay on my HELOC tax-deductible?

HELOC interest may be deductible if you used the borrowed money to buy, build, or improve your home. Interest on a HELOC used for other purposes is generally not deductible. Consult a tax professional about your specific situation, as tax rules vary by individual circumstances.

How often does my HELOC rate adjust?

Most HELOCs adjust monthly or quarterly, though the exact schedule is set in your loan documents. Your lender will notify you of rate changes and how they affect your payment. Even if your rate adjusts frequently, your payment may not change every month if the index rate does not move.