How this is calculated
Turn on "Buying a home?" and the loan amount fills itself in from price minus down payment; property tax, insurance and HOA get added on top as their own line — they don't shrink the loan, they're recurring costs of owning, usually collected through escrow. The breakdown chart shows the split; on a typical loan, taxes and insurance can be a quarter or more of the check you write each month. The formula underneath the principal-and-interest part is the standard fixed-rate amortization:
M = P × [ i (1 + i)ⁿ ] / [ (1 + i)ⁿ − 1 ] — where P is the loan amount, i the monthly rate (APR ÷ 12), and n the number of monthly payments.
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 enough. Add one as an event and the impact list below shows its break-even date automatically, plus a one-click way to keep paying your old amount so the whole rate cut goes to payoff instead of your pocket.
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.