A home equity line of credit (HELOC) splits into two distinct phases: the draw period (typically 10 years) when you can borrow and make interest-only payments, and the repayment period (typically 10 to 20 years) when the line closes and you pay back principal plus interest. Many homeowners underestimate the total interest cost because the monthly payment jumps sharply when the repayment period starts, and the final bill depends on how much you actually drew, when you drew it, and how aggressively you paid down the balance during the draw period.

The Two-Phase Structure and What It Costs

During the draw period, you pay interest only on the outstanding balance. Most HELOCs carry a variable rate tied to the prime rate, so your payment fluctuates as rates change. According to the Consumer Financial Protection Bureau, borrowers should understand that low initial payments during the draw period do not reduce the principal (CFPB, 2026). You are servicing the debt, not retiring it.

When the draw period ends, the HELOC converts to a fully amortizing loan. Your payment now covers both principal and interest, calculated to retire the remaining balance over the repayment period. The Federal Reserve tracks interest rate movements that drive HELOC rate adjustments (Federal Reserve, 2026), and even a modest rate increase during the repayment phase significantly raises the monthly cost on a large balance.

The formula for total interest paid combines the interest-only draw period cost and the amortizing repayment period cost. As covered in Principles of Finance (OpenStax, 2022), the present value and amortization formulas underpin installment loan calculations. For a HELOC, you sum the interest paid each month during the draw period (which varies with your balance and the rate at a given time), then add the interest component of each payment during the repayment period (calculated via standard amortization).

A Worked Example with Realistic US Numbers

Assume you open a $50,000 HELOC at 7.5% APR (as of August 2026; rates change daily, verify current terms with a licensed lender before deciding). The draw period is 10 years, the repayment period is 15 years, and you draw the full $50,000 immediately and make only the required interest-only payments for the first 10 years.

Draw period (years 1 to 10):
Monthly interest = $50,000 × 0.075 / 12 = $312.50
Total interest paid over 10 years = $312.50 × 120 months = $37,500

At the end of the draw period, you still owe the full $50,000 principal because you made no principal payments.

Repayment period (years 11 to 25):
The $50,000 amortizes over 15 years at 7.5% APR.
Monthly payment (principal and interest) = approximately $463.51
Total paid over 15 years = $463.51 × 180 months = $83,431.80
Total interest during repayment period = $83,431.80 - $50,000 = $33,431.80

Read also: Home Equity Line of Credit Explained: 7 Key Points About How a HELOC Works in the US

Total interest over the life of the HELOC:
$37,500 (draw period) + $33,431.80 (repayment period) = $70,931.80

You borrowed $50,000 and paid $70,931.80 in interest alone. The total cost of the credit is $120,931.80.

If you had paid $100 extra per month toward principal during the draw period, you would enter the repayment period with a lower balance and pay substantially less interest overall. The actual savings depend on the timing of extra payments and rate fluctuations, which a calculator captures precisely.

Why Borrowers Underestimate the Cost

Many homeowners focus on the low draw-period payment and do not model the repayment-period jump or the cumulative interest. Because HELOCs carry variable rates, a rate increase of even 1 to 2 percentage points during the repayment period can add thousands of dollars to the total interest paid. The U.S. Department of Housing and Urban Development recommends that borrowers evaluate whether they can afford the fully amortizing payment before opening a HELOC (HUD, 2026).

The calculator shows you the month-by-month interest accrual, the balance at the end of the draw period, the new payment when repayment starts, and the total interest over the full term under your specified rate scenario. Run different assumptions to see how extra principal payments during the draw period, rate changes, or a shorter repayment term alter the final cost.

What to Do Next

Understanding the two-phase interest structure helps you plan realistically for a HELOC and compare it to a fixed-rate home equity loan or cash-out refinance. Use the calculator to model your specific scenario: enter your credit line amount, the draw and repayment period lengths, the current rate, how much you plan to draw and when, and any extra payments you intend to make. The result is a clear projection of your total interest cost and monthly payment over the life of the loan.

This information is educational and general in nature, not personalized financial or lending advice. HELOC rates, terms, and availability vary by lender, location, and your credit profile. Verify current terms with a licensed lender and confirm your eligibility before making a borrowing decision. Consult a HUD-approved housing counselor or financial professional for guidance tailored to your situation.