A HELOC has two phases. During the draw period, commonly ten years, you can borrow and repay repeatedly, and many lenders require interest only. During the repayment period, the balance amortizes and the payment steps up.
Interest-only gives you flexibility, but it retires none of the balance. After ten years of interest-only payments on $75,000 you still owe $75,000, and now it has to be repaid over twenty years instead of thirty. That step-up regularly doubles the payment. On the chart above, the flat stretch on the HELOC line is the draw period.
The second issue is rate risk. HELOC rates typically float with the prime rate, so neither your payment nor your total interest is knowable in advance. This calculator holds the rate constant because it cannot know where prime will go, so treat the result as a snapshot rather than a forecast. The CFPB’s explanation of HELOCs covers the disclosures to look for.
A fixed home equity loan removes both problems with a fixed rate, payment and payoff date, at the cost of the flexibility to draw only what you need, when you need it.