A HELOC rate is really two numbers: an index plus a margin. Only one of them moves. Know which is which and the payment will not surprise you.
The index is usually the bank prime loan rate, which the Federal Reserve publishes daily in its H.15 release and defines as the rate posted by a majority of the top 25 insured US-chartered commercial banks by domestic assets. It has stood at 6.75 percent through 2026, alongside a federal funds target range of 3.50 to 3.75 percent.
The margin is the part you negotiate
Prime is the same number for everybody. The margin the lender adds on top is set by your credit profile, your combined loan-to-value and the lender’s own appetite, and it is fixed for the life of the plan in most agreements. So the only part of the price you can actually influence is the margin, and it is the number to ask about by name.
The CFPB’s shopping worksheet reflects this. It asks you to record, for each of three offers, the index used and its current value, the amount of the margin, the frequency of rate adjustments, and the rate cap and floor. Two offers quoting the same starting rate can carry different margins if one is applying a temporary discount, and that difference persists for years after the discount ends.
Ask for the margin and the fully indexed rate, meaning index plus margin with no promotional discount applied. That is the number the plan actually runs at.
What a rate move does to the payment
Round illustrative numbers chosen to show the mechanism, not market terms. On a $60,000 drawn balance during an interest-only draw period, each percentage point of rate is $600 a year, or $50 a month. Prime moves in quarter-point steps, so one step is roughly $12.50 a month on that balance and a full point is four steps.
That arithmetic scales linearly, which makes it easy to stress test. Take the balance you actually expect to carry, multiply by two or three percentage points, divide by twelve, and ask whether that monthly figure is survivable alongside everything else. If it is not, the product is wrong regardless of today’s rate.
There is a floor under the uncertainty. The CFPB notes that by law all adjustable-rate plans carry a lifetime cap limiting the rate increase over the life of the loan, and many also carry periodic adjustment caps limiting the increase from one adjustment period to the next. Ask for both numbers, and run the stress test at the lifetime cap, not a number you made up.
The payment jump that has nothing to do with rates
Rate movement is the risk people worry about. The larger one is the end of the draw period, and it arrives on a known date.
The CFPB describes plans that set a draw period, commonly ten years, after which the plan may allow renewal, may call for payment in full of any outstanding balance, or may allow repayment over a fixed period. It also cautions that a minimum payment during the draw may put too little against principal to clear the balance by the time the term runs out.
Put those together. A borrower can pay every month for ten years and arrive at the end owing what they started with, at which point the payment either balloons or converts to full amortization over a shorter remaining term. On the $60,000 balance above, an interest-only payment at 8 percent is $400 a month; amortizing the same balance over ten years at the same rate is closer to $730. The rate did not move. The structure changed.
Find out which of the three endings your plan has before you sign, not in year nine.
Who a variable rate actually suits
A line does suit some people: someone who draws modestly, repays inside a short window, and could clear the balance from savings if the rate ran against them is well matched to a line. The rate risk is bounded by the short exposure, and interest accrues only on what has been drawn.
It suits poorly the borrower carrying a large balance for the full draw period with no plan to retire it, because that person holds maximum exposure to both the rate and the structural reset. For them the fixed-rate closed-end alternative, covered in our comparison of the two products, is usually the better shape even at a higher opening rate.
Worth noticing why variable products advertise so well: the opening number is the lowest figure anywhere in the document. That is a property of the marketing, not a verdict on the pricing.
HELOC rate questions
What happens to my payment when the Fed moves?
Prime tracks the federal funds target closely, so a change in the target generally shows up in prime and then in your rate at the next adjustment date. Each percentage point costs about $10 a month per $12,000 of drawn balance. Work it on the balance you expect to carry, then check that against the plan’s lifetime cap.
Is my margin fixed or can the lender change it?
In most plans the margin is fixed for the life of the line and only the index moves. Confirm it in the agreement, and ask specifically whether the opening rate includes a temporary discount, because a discounted start with a high margin costs more over time than a plain start with a low one.
Is there a limit on how high the rate can go?
Yes. The CFPB notes that by law every adjustable-rate plan carries a lifetime cap on the rate increase over the life of the loan, and many carry periodic caps limiting each individual adjustment. Get both figures and treat the lifetime cap as the number your budget has to survive.
Can I lock part of the balance at a fixed rate?
Some plans offer a conversion feature letting you fix a portion of the drawn balance, sometimes for a fee and sometimes at a higher rate than the variable. Ask about it if you plan a large draw. Treat it as a term to compare across offers, not a standard feature.
Sources
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Every figure in this article traces to a government record or to a named independent, non-commercial research body. We do not cite affiliate comparison sites for data.
- CFPB Home equity lines of credit (HELOC)
- CFPB Closing costs and fees
- Federal Reserve Selected Interest Rates (H.15)
- FTC Mortgage shopping tips
- FDIC Consumer Resource Center