How the Lab works
Everything runs through one month-by-month simulation of your loan. Each event changes the machine's state at the month you chose:
One-time payment — the amount lands entirely on principal that month. Recurring extra — added to every payment (or every 12th, for yearly bonuses) from its start date. Recast — a lump sum hits principal and the lender re-amortizes: your required payment drops, the payoff date stays; most lenders charge a small fee (~$150–500) and require $5,000–10,000 minimum. Refinance — the balance (plus rolled-in closing costs, if chosen) becomes a new loan at the new rate and term from that month.
The "what each move is worth" list is computed by re-running the whole simulation with each event removed — so the number beside each move is its true marginal effect given everything else you stacked, not a standalone estimate.
Recast vs. extra payment vs. refinance
Same lump sum, three different outcomes: an extra payment keeps your payment the same and shortens the loan (max interest savings); a recast lowers the payment and keeps the date (max monthly relief); a refinance changes the rate itself (worth it when rates fell — see the dedicated refinance calculator for break-even detail).
Prepay or invest?
Prepaying earns a guaranteed, tax-free return equal to your mortgage rate. Investing might earn more — the comparison uses your expected-return number, invests the freed-up payment after an early payoff, and compares both paths at the same end date, apples to apples. The honest caveat: the market return is hoped for; the mortgage rate is contractual.
The HELOC line
The HELOC is modeled as its own loan — interest-only for the period you set, then amortizing — stacked into the household-debt view. HELOC rates float in reality; the model holds yours constant.