How much you can borrow against your home, what a draw costs during the interest-only years, and the payment jump when repayment begins.
Most lenders cap mortgage + HELOC at 80–85% of the home's value.
HELOC rates are variable (prime + margin); this models it as fixed.
"Principal too" pays as if amortizing over the full draw + repayment span, which avoids the payment jump.
This HELOC calculator estimates your available credit line from your home value and existing mortgage, the interest-only payment while you draw on the line, and the larger fully amortizing payment that kicks in once the repayment period starts.
Lenders typically let you borrow up to a combined loan-to-value of 80–85% of your home's value, minus what you still owe on the first mortgage.
You can no longer borrow, and the loan converts to a repayment schedule of principal plus interest. Payments often jump sharply at this point.
HELOCs usually carry a variable rate applied to your outstanding balance, so interest accrues only on what you have actually drawn.