How to Calculate Your True HELOC Payment: Prime Minus Structure vs. Fixed Rate

The Prime Rate Is Your Starting Point, Not Your Rate

Your HELOC rate is not the Prime Rate. The Prime Rate is the index. Your lender adds or subtracts a margin from that index to produce your actual annual percentage rate (APR). The margin is fixed for the life of the line. The Prime Rate moves every time the Federal Reserve changes the federal funds rate.

The WSJ Prime Rate formula is: federal funds rate plus 3.00 percentage points. When the Fed set its target at 4.50% in mid-2025, the WSJ Prime Rate stood at 7.50%. Every 25-basis-point Fed move shifts your HELOC rate by 0.25%.

Your lender's offer will read as "Prime minus 0.50" or "Prime plus 1.25." The resulting rate today would be 7.00% or 8.75%, respectively. Most borrowers skip this arithmetic entirely and simply accept the number on the disclosure page.

How to Calculate the Fully-Indexed Rate

The fully-indexed rate equals the current index plus your margin. Write it out before you accept any HELOC offer.

Formula: Fully-Indexed Rate = WSJ Prime Rate + Margin

If your margin is plus 0.50: 7.50% + 0.50% = 8.00% APR

If your margin is minus 0.25: 7.50% + (-0.25%) = 7.25% APR

That 0.75-percentage-point difference on a $150,000 outstanding balance produces an annual interest difference of $1,125. Over a 10-year draw period, assuming no rate changes, that gap compounds to more than $11,000 in total interest paid.

Rate Caps Matter as Much as the Margin

Most HELOCs carry a lifetime rate cap, typically 18.00%, and a periodic cap of 2.00% per adjustment period. A lender offering Prime minus 0.50% looks attractive today. If the Prime Rate rises to 10.00%, your rate becomes 9.50%, not the 7.00% you budgeted. Model the cap scenario, not just the current rate.

Interest-Only Draw Period: The Payment Formula

During a HELOC's draw period, most lenders require interest-only minimum payments. The calculation is straightforward.

Monthly Payment = (Outstanding Balance x Annual Rate) / 12

Worked Example 1: Interest-Only on a $80,000 Draw

A borrower draws $80,000 against a HELOC priced at Prime plus 0.75%. With the WSJ Prime Rate at 7.50%, the fully-indexed rate is 8.25%.

Monthly interest = ($80,000 x 0.0825) / 12 = $550.00 per month

Now assume the Fed raises rates twice, moving Prime to 8.00%. The rate becomes 8.75%.

Monthly interest = ($80,000 x 0.0875) / 12 = $583.33 per month

That $33.33 monthly increase seems minor. Sustained over 12 months it adds $400 to annual borrowing costs. Over a 10-year draw period with continued rate pressure, the cumulative overage reaches thousands.

Repayment Period: Amortizing the Balance

When the draw period ends, usually after 10 years, the HELOC converts to a repayment period. The outstanding balance amortizes over the remaining term, commonly 15 to 20 years. Payments rise sharply because principal repayment begins.

The amortizing payment formula: Monthly Payment = P x [r(1+r)^n] / [(1+r)^n - 1]

Where P is the outstanding principal, r is the monthly interest rate (annual rate divided by 12), and n is the number of remaining monthly payments.

Worked Example 2: Repayment Shock on a $120,000 Balance

A borrower has $120,000 outstanding when the draw period closes. The HELOC rate at that point is 8.50%. The repayment term is 20 years (240 months).

Monthly rate r = 8.50% / 12 = 0.7083%

Payment = $120,000 x [0.007083 x (1.007083)^240] / [(1.007083)^240 - 1]

(1.007083)^240 = approximately 5.3133

Payment = $120,000 x [0.007083 x 5.3133] / [5.3133 - 1]

Payment = $120,000 x [0.037635] / [4.3133]

Payment = $120,000 x 0.008725 = $1,047.00 per month

During the draw period, this same borrower paid interest only at 8.25% on a similar balance, roughly $825 per month. The repayment phase adds $222 per month, a 26.9% increase, without any rate change.

Fixed-Rate HELOC vs. Variable: When the Math Favors Locking In

Some lenders, including Figure and certain credit unions, offer fixed-rate HELOCs. The rate is higher than the current variable rate but immune to Fed movements. The breakeven analysis is simple.

If the fixed rate offered is 9.00% and the current variable rate is 7.50%, the variable rate must rise by 1.50 percentage points and hold there long enough to make the fixed option cost less in total interest. On a $100,000 balance over 5 years, each 1.00-percentage-point rate increase costs approximately $2,600 in additional interest. A 1.50-point rise costs $3,900 over that period.

If you believe the Fed will raise rates by 1.50 points or more within your draw period, the fixed-rate HELOC may produce lower total interest paid. If you expect rates to fall, the variable structure wins. Neither is inherently superior. The math depends on your draw amount, timeline, and rate forecast.

Converting a Variable Draw to a Fixed-Rate Subaccount

Many lenders allow borrowers to lock a portion of a variable HELOC into a fixed-rate subaccount. This hybrid structure lets you fix the rate on a large draw (say, a $75,000 renovation disbursement) while keeping the remaining credit line variable. Calculate each segment separately using the formulas above, then sum the monthly payments.

Run Every Scenario Before You Commit

A HELOC is not a single product. It is a set of terms that interact with an index you do not control. The payment you make in year 3 may differ from the payment in year 7 by hundreds of dollars per month. Two variables drive that difference: the Prime Rate movement and the transition from interest-only to amortizing payments.

The CalcMoney HELOC Calculator lets you input your margin, your draw amount, your draw period length, and a custom rate path. It returns month-by-month payment schedules under your current rate, a 2.00% rate increase scenario, and a 4.00% rate increase scenario. That output tells you whether your cash flow can absorb the worst-case before you open the line.

Run your own numbers at the CalcMoney HELOC Calculator before accepting any lender's term sheet.

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Results are estimates for informational purposes only. Consult a licensed financial professional before making financial decisions.