See both the interest-only payment and the full repayment payment on your home equity line.
A HELOC payment calculator estimates what a home equity line of credit costs during both of its phases, the interest-only draw period and the principal-plus-interest repayment period, so you can see how the payment jumps when the line converts.
A HELOC usually starts with a draw period (often around ten years) when you can borrow against the line and typically pay interest only, keeping payments low. When that ends, the repayment period begins and you must pay down the principal too, often over ten to twenty years. That switch can raise the monthly payment substantially, sometimes doubling it or more, which is the number most borrowers underestimate.
Most HELOCs carry a variable rate tied to the prime rate, so payments can rise if rates climb, especially during the draw period when you're paying interest only. Because the home secures the line, falling behind puts the property at risk. Model a higher rate than today's to make sure the repayment-period payment would still fit your budget before you draw heavily on the line.
During the draw period, payment is usually just the interest on your balance. In repayment, it amortizes the balance over the remaining term, like a regular loan, a larger payment.
When the draw period ends you begin repaying principal as well as interest, and the balance is amortized over a shorter remaining term, so the payment rises, often sharply.
Usually variable, tied to the prime rate, so payments can change as rates move. Some lenders offer a fixed-rate conversion option on part of the balance.
See the exact formula and a worked example on our methodology page.