The interest rate on a home equity line of credit works as a simple sum: a public benchmark called the index, plus a fixed markup called the margin, adjusted automatically over the life of the line and bounded by caps and floors written into your agreement. That is how HELOC interest rates work in practice. The index moves with the broader economy, your margin stays put from the day you sign, and adding them produces the rate you actually pay.
The Index Sets the Starting Point
Every HELOC rate begins with an index. Federal regulation requires that this benchmark be outside the lender’s control and publicly available, so your lender can’t quietly manipulate the baseline of your rate.1eCFR. 12 CFR 1026.40 – Requirements for Home Equity Plans
Most HELOCs use the U.S. Prime Rate, which reflects the rate banks charge their most creditworthy commercial borrowers. The Wall Street Journal publishes this figure after surveying the 30 largest banks and updates it when at least three-quarters of them change.2Federal Reserve Board. Selected Interest Rates (Daily) – H.15 As of mid-2025, the Prime Rate stood at 6.75%, and it shifts whenever the Federal Reserve adjusts its target rate.
Some lenders use the Secured Overnight Financing Rate (SOFR) instead, which measures the cost of overnight borrowing backed by Treasury securities.3Federal Reserve Bank of New York. Secured Overnight Financing Rate Data SOFR gained popularity after LIBOR was retired in 2023. Whichever index applies, your lender must disclose which one it uses, how it feeds into your rate, and where you can track it yourself.1eCFR. 12 CFR 1026.40 – Requirements for Home Equity Plans
Your rate doesn’t move on the lender’s schedule; it moves on the index’s. And how often that translates into a new rate for you depends on your agreement. Some HELOCs adjust monthly, others quarterly. In a rising-rate environment, your minimum payment can climb from one billing cycle to the next, even during the draw period. In a falling-rate environment, it shrinks on the same schedule.
The Margin Is the Lender’s Fixed Markup
The margin is a fixed percentage the lender adds on top of the index. It represents profit and a risk premium, and unlike the index it stays the same for the entire life of the HELOC. Two borrowers at the same bank on the same day can receive different margins based on their individual profiles.
Lenders weigh several factors when setting a margin: credit score, debt-to-income ratio, and combined loan-to-value ratio, meaning how much you owe on all mortgages relative to your home’s value. A borrower with a credit score above 760 and low existing debt typically lands a margin in the range of roughly 0.5% to 1.5%. Someone borrowing up to 85% or 90% of the home’s value, or carrying a thinner credit file, can expect a higher margin.
The margin matters more than any other single number when you’re comparing offers, because it’s the only rate component you can negotiate. The index is the index for every lender using it.
How the Two Combine Into Your Rate
The math is straightforward. Your lender takes the current index value and adds your margin. The result is called the fully indexed rate, and that’s what you pay.
If the Prime Rate is 6.75% and your margin is 1%, your HELOC rate is 7.75%. If Prime climbs to 7.25%, your rate automatically becomes 8.25%. If Prime falls, the formula pulls your rate down by the same amount.
Once the fully indexed rate is set, the lender checks it against the cap and floor in your agreement. If the calculated rate exceeds a cap, the capped figure applies. If it falls below a floor, the floor applies. The resulting annual rate is then divided by 365 to produce a daily rate, which is applied to your outstanding balance each day.
Caps and Floors Bound the Rate
Federal law requires every adjustable-rate loan secured by a home to include a maximum interest rate for the life of the loan.4Office of the Law Revision Counsel. 12 USC 3806 – Adjustable Rate Mortgage Caps This lifetime cap is the absolute ceiling your rate can ever reach. Lifetime caps of 18% to 24% are common, though the exact figure varies by lender. Without this protection, a runaway index could theoretically push borrowing costs to any level the market dictated.
Some HELOCs also include periodic caps that limit how much the rate can rise during a single adjustment window, such as no more than 2 percentage points per year. Periodic caps are not universal for HELOCs. If your plan doesn’t impose them, the lender must disclose that fact.5Consumer Financial Protection Bureau. 12 CFR 1026.40 – Requirements for Home Equity Plans A plan without periodic caps can see larger rate jumps in a single adjustment, so check this detail before signing.
Floors work in the opposite direction. A floor is the minimum rate the line will ever charge regardless of how far the index falls. A lender might set a floor at 3.99% or 4.50%, meaning even if Prime dropped to historic lows, your rate would never go below that number. Floors protect the lender’s margin and limit how much you benefit in a falling-rate environment.
Introductory Rates Don’t Change the Underlying Formula
Many lenders advertise HELOCs with a low introductory rate, sometimes called a teaser rate. These promotional rates are typically fixed for the first six months to a year and often run about 2 to 3 percentage points below prevailing HELOC rates.
What matters more is the rate you’ll pay after the introductory window closes. When it expires, your rate snaps to the fully indexed rate: index plus your margin. If you chose a HELOC primarily because of an attractive intro number without checking the margin, you may end up paying more than a competing lender would have charged from day one. Compare the margin and the lifetime cap across offers, not just the introductory rate.
Fixed-Rate Conversion as a Hedge
Some lenders let you convert a portion of your variable-rate HELOC balance into a fixed rate. The feature lets you lock in a rate on money you’ve already drawn while keeping the rest of the line variable. It can be useful if you’ve taken a large draw for a renovation and want payment certainty on that chunk while rates are climbing.
Specifics vary. Some lenders allow up to three fixed-rate balances at once and charge a conversion fee for each lock. The fixed-rate portion typically must be repaid over a set term, and you can usually unlock it back to a variable rate later if conditions change. Not every HELOC offers this, so ask during the application process if rate stability matters to you.
The Rate Formula Doesn’t Guarantee Access
One boundary worth knowing: the way your rate is calculated says nothing about whether the credit line stays available. Federal law allows lenders to suspend your ability to draw or reduce your credit limit under certain circumstances. The most common trigger is a significant decline in your home’s value since the HELOC was approved.6Office of the Comptroller of the Currency. Can the Bank Freeze My HELOC Because the Value of My Home Declined Lenders can also freeze a line if they reasonably believe you can’t meet repayment obligations due to a material change in your finances, or if you default on a material term of the agreement.5Consumer Financial Protection Bureau. 12 CFR 1026.40 – Requirements for Home Equity Plans A locked-in margin and a favorable index are only worth what you can actually borrow against.