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 sounds like flexibility, and it is, 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. The chart above marks it: 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 that is the only honest thing a calculator can do — but a constant-rate model is a snapshot, not a forecast. The CFPB’s explanation of HELOCs covers the disclosures to look for.
A fixed home equity loan removes both problems — fixed rate, fixed payment, fixed payoff date — at the cost of the flexibility to draw only what you need, when you need it.