How to Calculate Your HELOC Payment Across Draw and Repayment Phases

What a HELOC Actually Is, and Why the Two-Phase Structure Matters

A Home Equity Line of Credit (HELOC) is a revolving credit line secured by your home equity. It operates in two distinct phases with different payment mechanics. The draw period, typically 10 years, allows you to borrow up to your credit limit and make interest-only minimum payments. The repayment period, typically 20 years, closes the line and converts the outstanding balance into a fully amortizing loan.

Most lenders price HELOCs at a variable rate tied to the Wall Street Journal Prime Rate plus a margin. As of mid-2025, Prime stands at 7.50%, and lender margins range from 0.50% to 2.00%, producing initial rates between 8.00% and 9.50% for well-qualified borrowers. That variability means your draw-phase payment can change monthly.

Treating the two phases as one product is the source of most HELOC payment surprises. They require separate calculations and separate cash-flow planning.

How to Calculate Your Draw-Phase Payment

During the draw period, the minimum payment on a HELOC is interest only. The formula is:

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

That is it. No principal reduction occurs unless you pay above the minimum.

Worked Example: $50,000 Balance at 8.50%

Outstanding balance: $50,000 Annual interest rate: 8.50% Monthly rate: 8.50% / 12 = 0.7083%

Monthly payment = $50,000 x 0.007083 = $354.17

If the Fed raises rates and Prime increases by 0.75%, your new rate becomes 9.25%. The same $50,000 balance now costs $385.42 per month. That $31.25/month increase is modest in isolation, but it grows across every dollar you draw.

Draw more, and the payment scales proportionally. A $150,000 balance at 9.25% costs $1,156.25 per month at the interest-only minimum. No principal has moved.

How to Calculate Your Repayment-Phase Payment

At the end of the draw period, your outstanding HELOC balance converts to a fully amortizing term loan. The lender uses the standard amortization formula to set a fixed monthly payment that retires the balance over the repayment period, typically 20 years (240 months).

The formula for the monthly amortizing payment is:

M = P x (r(1+r)^n) / ((1+r)^n - 1)

Where:

• P = principal balance at end of draw period
• r = monthly interest rate (annual rate / 12)
• n = number of repayment months (typically 240)

Worked Example: $80,000 Balance at 8.75% Over 20 Years

P = $80,000 Annual rate = 8.75%, so r = 8.75% / 12 = 0.72917% n = 240

Step 1. Calculate (1 + r)^n: (1.0072917)^240 = 5.7122 (rounded)

Step 2. Calculate the numerator: 0.0072917 x 5.7122 = 0.041659

Step 3. Calculate the denominator: 5.7122 - 1 = 4.7122

Step 4. Calculate the payment factor: 0.041659 / 4.7122 = 0.008840

Step 5. Calculate the monthly payment: $80,000 x 0.008840 = $707.20/month

Compare that to the draw-phase minimum on $80,000 at 8.75%: $80,000 x (0.0875 / 12) = $583.33/month

The repayment-phase payment is $123.87 higher per month. Over 240 months, that difference represents $29,728.80 in additional cash-flow commitment, before any rate adjustment.

The Rate-Change Stress Test You Must Run

HELOCs are variable-rate products. Most lenders cap lifetime rate increases at 18%, with periodic caps of 2% per adjustment. Running only a current-rate scenario leaves a significant risk unquantified.

Apply a 2-percentage-point rate shock to the repayment-phase calculation.

Stress Test: $80,000 Balance at 10.75% Over 20 Years

r = 10.75% / 12 = 0.89583% (1.0089583)^240 = 8.3683 Numerator: 0.0089583 x 8.3683 = 0.074968 Denominator: 8.3683 - 1 = 7.3683 Payment factor: 0.074968 / 7.3683 = 0.010174 Monthly payment: $80,000 x 0.010174 = $813.92/month

That is $230.59 more per month than the draw-phase interest-only minimum, and $106.72 more per month than the base-case repayment payment. Borrowers who have not modeled this scenario face genuine cash-flow disruption.

The stress test is not optional. It is the minimum due diligence before drawing funds.

How Partial Principal Payments During the Draw Period Change the Math

Paying down principal during the draw period directly reduces the repayment-phase payment because it lowers P in the amortization formula.

Consider the same $80,000 draw at 8.75%. If the borrower pays an additional $200/month toward principal during the 10-year draw period, the outstanding balance at repayment onset drops by roughly $24,000, to approximately $56,000.

Repayment-phase payment on $56,000 at 8.75% over 20 years: $56,000 x 0.008840 = $494.98/month

That is a $212.22/month reduction compared to the interest-only strategy, and it saves approximately $50,932.80 in total repayment-phase payments. The $200/month in extra draw-period payments costs $24,000 over 10 years and returns $50,932 in reduced future payments. That is a clear trade-off worth running through the numbers before the draw period ends.

What the Total Interest Cost Looks Like Over the Full HELOC Term

Total interest cost across both phases is the number most borrowers never calculate. The draw-period interest-only minimum hides how expensive variable-rate revolving debt becomes over 30 years.

For $80,000 at 8.75% with interest-only minimums during the draw period and standard amortization during repayment:

Draw-phase total interest (120 months x $583.33): $69,999.60 Repayment-phase total payments (240 months x $707.20): $169,728 Less principal ($80,000), repayment-phase interest: $89,728

Total interest paid over 30 years: $159,727.60

That is nearly twice the original balance, paid in interest alone. This figure is what separates HELOC borrowers who model the full term from those who anchor on the draw-phase minimum and feel blindsided at year 10.

Use the CalcMoney HELOC Calculator to Model Both Phases in Minutes

The CalcMoney HELOC Calculator handles all five inputs simultaneously: draw amount, annual rate, draw period length, repayment period length, and an optional rate-shock scenario. It outputs the draw-phase monthly payment, the repayment-phase monthly payment, total interest across both phases, and the payment differential you need to plan around.

Enter your actual credit line terms, not hypothetical ones. Pull your lender's current margin from your HELOC agreement, add it to the current Prime Rate, and run the base case. Then increase the rate by 2 percentage points and run it again. The difference between those two outputs defines the range of outcomes you are accepting when you draw on a variable-rate instrument secured by your home.

The math is straightforward. Skipping it costs money.

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