How HELOC rates work: the index, the margin, and the caps

The formula: index plus margin

The CFPB’s HELOC booklet describes a variable rate as having two parts. The index is a measure of interest rates generally that reflects trends in the overall economy; common choices include the U.S. prime rate and the Constant Maturity Treasury rate. The margin is an extra percentage the lender adds to the index. Different lenders use different indexes, so ask which one your line uses and where it is published.

The prime rate is set by individual banks, not by the Federal Reserve. The Fed reports the prime rate posted by the majority of the largest 25 banks on its H.15 statistical release, and it notes that many banks set their prime rates based partly on the federal funds target chosen by the Federal Open Market Committee. On the H.15 release, the prime rate was 7.00% from September 18 through September 24, 2026. A line priced at prime plus 0.50% would have carried a 7.50% rate that week.

Sources: CFPB: What you should know about HELOCs (PDF) · Federal Reserve: What is the prime rate? · Federal Reserve: H.15 selected interest rates

What a rate change does to your payment

During the draw period, many HELOCs allow interest-only payments. On a $50,000 balance at 7.50%, that payment is $312.50 a month. Each one-point move in the prime rate changes it by $41.67 on the same balance, up or down. The CFPB notes that with a variable rate, your payment can change even if you do not draw more money.

Once the draw period ends and the balance is repaid on a schedule, the payment is higher at every rate because principal is included. The table uses a hypothetical 0.50% margin and a 20-year repayment period. The prime rates are illustrations, not forecasts.

Sources: CFPB: What you should know about HELOCs (PDF) · CFPB: How a HELOC works

Introductory rates versus the margin

Lenders sometimes offer a temporarily discounted rate, which the CFPB describes as an introductory or teaser rate that is unusually low for a short period, such as six months. It is the easiest number to compare and, for most borrowers, the least important one, because the margin applies for the life of the line.

Consider two hypothetical offers on a $50,000 balance, with prime held at 7.00%. Offer A charges 5.99% for six months, then prime plus 1.00% (8.00%). Offer B has no introductory rate and charges prime plus 0.25% (7.25%). Offer A costs $127.50 less in the first year. Over a 10-year draw period, Offer B costs $3,247.50 less, because its margin is 0.75 points lower for the remaining nine and a half years.

Sources: CFPB: What you should know about HELOCs (PDF)

The limits written into the agreement

Federal rules limit how a HELOC rate can change. Under Regulation Z, a lender may change the rate only based on an index outside its control that is available to the public; it cannot change the rate at its own discretion or base changes on its own prime rate or cost of funds. The agreement for any variable-rate loan secured by a home must also state the maximum interest rate that can apply over its life.

Read three terms closely. The lifetime cap is the highest rate the line can reach; with a hypothetical 18% cap, the interest-only payment on a $50,000 balance would be $750.00. A periodic cap, if there is one, limits how much the rate can rise in a given period, and the disclosures must say so if there is no annual limit. A floor sets the lowest rate the line can reach, so a falling index may not lower your payment below a certain point. The CFPB’s booklet lists the rate cap and floor among the terms to compare across offers.

Sources: CFPB: Regulation Z § 1026.40, home equity plans · CFPB: Regulation Z § 1026.30, maximum interest rate · CFPB: What you should know about HELOCs (PDF)

Fixed-rate options

Some HELOCs let you convert part of the balance to a fixed rate. The CFPB notes that the fixed rate is typically higher than the variable rate, but it makes payments more predictable. That can suit a large one-time draw, such as a finished renovation, that you plan to repay over several years. Ask whether there is a fee to convert, a minimum amount, and a limit on how many fixed-rate portions you can have.

A conversion does not change the formula for the rest of the line. Any balance left at the variable rate, and any new draws, still move with the index.

Sources: CFPB: What you should know about HELOCs (PDF)

How often the rate changes

The CFPB’s comparison worksheet asks for the frequency of rate adjustments alongside the index and the margin. Ask how often your rate can change, when a change in the index shows up on your statement, and how the minimum payment is recalculated afterward. The more often a line adjusts, the faster index changes reach your payment, in both directions.

Also ask how the minimum payment is calculated during the draw period. The booklet notes that some plans set a minimum payment that covers interest plus a portion of principal, while others allow interest-only payments, and that the principal portion typically does not repay the balance by the end of the term. A payment that includes some principal will change with the rate too, but the balance will be lower when repayment begins.

Sources: CFPB: What you should know about HELOCs (PDF)

Variable line or fixed loan?

The CFPB’s booklet contrasts a HELOC, which usually has a variable rate and lets you borrow repeatedly, with a home equity loan, which usually has a fixed rate and equal payments that repay the whole loan. A fixed rate takes the index out of the picture, but you borrow the full amount at once and pay interest on all of it from the start.

The choice often depends on how the money will be used. For a single known expense paid on one invoice, a fixed-rate loan gives you a payment that does not change. For costs that arrive in stages or are uncertain, a line lets you borrow only what you need, when you need it, and accept rate risk on that balance. If you choose a line for one large draw, a fixed-rate conversion option can give you some of both.

Sources: CFPB: What you should know about HELOCs (PDF) · CFPB: Home equity loans and HELOCs

The bigger jump: when repayment begins

On many HELOCs, the largest payment change is not a rate move but the end of the draw period. The CFPB notes that monthly payments are often significantly higher once repayment begins, and in some cases the whole balance may be due at once. In the table above, moving from interest-only to a 20-year repayment schedule at 7.50% raises the payment from $312.50 to $402.80. If prime were two points higher by then, the repayment payment would be $466.07, or $153.57 more than today’s interest-only payment.

Put both numbers in your budget before you draw: the payment if rates rise, and the payment when repayment starts. If the two together would be a strain, plan to pay down principal during the draw period or borrow less.

Sources: CFPB: How a HELOC works

Your next-step checklist

• Find the index your line uses and where it is published.
• Compare margins across offers, not only introductory rates.
• Note the lifetime cap, any periodic cap, and any floor.
• Calculate the payment at a rate two points higher than today.
• Calculate the payment when the repayment period begins.
• Ask whether you can convert part of the balance to a fixed rate, and at what cost.