Interactive financial models
Understand where your money actually goes.
Free models for borrowing, ownership, and investing, built to show the mechanics underneath a decision rather than a rule of thumb. Every figure on this page is real output from the models.
Each engine is checked to the dollar against hand-worked cases before it ships.
$1M · 20% down · 6% mortgage · $4,500 rent · 3% appreciation · 7% return · California
Models
The full math behind each decision.
The cost of borrowing
What actually disappears in interest, points, and fees. The monthly payment is the least informative number on the page.
Open model →The true cost of a car
Purchase price is only the beginning. Ownership is what actually leaves your account.
Open model →Rent vs. buy
Two net-worth paths through time, and whether they ever cross. Charted above.
Open model →Roth vs. Traditional 401(k)
Your retirement tax rate isn't a guess. It's what withdrawals owe after Social Security fills the low brackets.
Open model →Lump sum vs. dollar-cost averaging
Easing a windfall into the market feels safer than investing it all at once. That feeling has a measurable price.
Open model →Every figure is real model output for the inputs listed with it.
Why these numbers differ from other calculators
MethodBoth paths play by the same rules
If the buyer's exit is taxed, so is the renter's invested savings. Idle cash earns what cash actually earns. A model that favors one side by leaving something out does not ship.
Checked by hand first
Every engine is tested to the dollar against worked cases before an interface is built on top of it. The interface only displays what the engine has already proven.
What actually moves the answer
Every assumption is visible and adjustable, so you can see which one flips the result instead of trusting a single confident number.