Debt & Credit

Every fixed-rate loan — mortgage, car, student — runs on the same engine: a constant payment whose mix flips over time from mostly interest to mostly principal. The simulator splits each payment into the slice that pays the bank and the slice that pays the debt, then lets you add an extra payment and watch years of interest disappear.

A credit card is a loan whose required payment shrinks as your balance does — so progress slows every single month, by design. The simulator races the minimum-payment path against a fixed payment you choose: the same $5,000 balance takes 26 years one way and under 5 the other. Drop the minimum a notch and it never pays off at all.

APR is the sticker rate; APY is what compounding actually does to your money over a year. The gap between them grows with the rate and the compounding frequency — a 24% APR card compounded monthly really charges 26.82%. The simulator lets you crank the frequency and watch the real rate climb away from the quote.

When you owe on several debts at once, the only real decision is which one gets your spare dollars first. Avalanche (highest rate first) is mathematically optimal; snowball (smallest balance first) hands you a paid-off debt far sooner and keeps you motivated. The simulator runs both orderings on the same debt mix and budget: on the default $25,000 mix, avalanche saves $3,792 — and snowball's first win arrives 15 months earlier.

A credit score is a single number — 300 to 850 — that lenders use to price your risk. It's built from five factors with fixed weights, and almost everyone gets the priorities backwards. Payment history (35%) and credit utilization (30%) are two-thirds of the score; credit mix and new credit are 10% each. The simulator draws the five factors as one bar where width is how much a factor matters and fill is how well you're doing — so you can see, not just be told, that paying on time and keeping balances low is most of the game.

People talk about debt as if it were a single thing — and as if the only virtuous move were to avoid all of it. But two people can take the exact same loan, with the same amount, rate, and term, and end up in completely different places. The difference has nothing to do with the loan and everything to do with what the borrowed money bought. Borrow to buy something that grows in value — a home, an education that lifts your earning power, a business — and the asset can outpace the interest, so the debt quietly pays for itself: that's good debt, leverage working in your favor. Borrow to buy something that loses value — a car, a vacation, everyday consumption on a credit card — and you lose twice: you pay interest on the loan AND watch the thing shrink, often so fast that for years you owe more than it's worth. That last bit has a name people know from car loans: being underwater, or upside down. This lesson makes the split visual. Two borrowers take the identical loan; one buys an appreciating asset, the other a depreciating one. The simulator races each borrower's net worth — the asset's value minus the loan still owed — over the life of the loan. Both start at exactly zero. The good-debt line climbs steadily into the black; the bad-debt line dives below zero into the shaded underwater zone before clawing its way back. The unifying rule is the same crossover that governs the pay-down-or-invest question: the loan's interest rate is the hurdle. An asset growing faster than the rate makes borrowing worthwhile; an asset growing slower — or shrinking — means the leverage is working against you. Good debt isn't a category of loan you can spot by its name. It's any borrowing where the thing you bought out-earns the cost of the money.

You have a credit-card balance charging real interest, and no real cushion in savings. Every spare dollar this month could go one of two places: attack the debt, or start an emergency fund. This is one of the most common early-money questions there is, and it has two right-sounding answers that pull in opposite directions — 'a guaranteed 22% return beats any savings account' versus 'what if something goes wrong before the debt is gone?' This lesson races both orderings' net worth over five years and shows that they're not actually in conflict: the same guaranteed-return logic from the pay-debt-vs-invest lesson decides who wins on paper (attacking the debt, almost always, at a real card's rate), but that verdict hides a separate, real cost the net-worth number doesn't capture — attacking the debt first means running with an EXACT $0 cushion for however long the balance survives, so any real emergency in that window becomes brand-new debt at the card's rate, no exceptions. A modest starter fund doesn't usually win the spreadsheet. It buys insurance the spreadsheet doesn't price in.

'Buy now, pay later' — store financing, BNPL apps, the credit-card swipe — all sell the same illusion: that a big purchase is really just a small monthly payment. But that monthly payment is a force, and financing points it the wrong way. When you borrow, every payment carries interest you hand to the lender; when you save up the same amount first, that money earns interest for you, and a cash buyer often gets a discount the financed buyer forfeits. This lesson races the two paths for the exact same item on the exact same monthly budget — the payment you'd owe the lender, pointed at a savings account instead. Scored as net worth, the patient saver always finishes ahead, because the interest that worked against the borrower works for the saver, and the discount lands on top. The only thing financing actually buys you is the item sooner — and the simulator puts a precise dollar price on that head start, so you can judge whether getting it now is worth what it costs. There's one honest exception, and the sim shows it too: a genuine 0% promotion with no cash discount costs almost nothing, because the only thing you give up is the small interest your money would have earned. Outside that case, the rule is simple — if the rate to borrow is higher than the rate to save (it almost always is), save up first.

You've got a couple of cards in the mid-20s% and a personal loan, and a lender offers to roll them all into one new loan at a single, lower rate. One payment instead of three, and the rate is better than any of your cards — what's not to like? The catch is the same one that trips people up on mortgage refinances: the new loan usually runs LONGER than it would have taken to pay the debts off separately, and stretching the balance over more months can add up to more total interest even at a lower rate. There's a second, quieter cost too — folding several balances into one erases the finish line on whichever debt was closest to gone, so the relief of almost being done with your worst card resets to zero. This lesson races 'keep them separate' against 'consolidate' on the same chart, so you can see exactly when the lower rate is a real win and when it's a longer, costlier version of the same debt.

You're carrying a high-rate card balance, and a new card offers 0% interest for the next 15 months if you move the debt over — for a one-time fee. It sounds like a free pause on interest, and for a while, it is. But that 0% is a countdown, not a discount: the day the promo window closes, whatever's left starts accruing at a normal — often steep — ongoing rate. This lesson races the transfer against just leaving the balance on your original card, so you can see exactly when the 0% offer is free money and when the fee plus the reverted rate quietly cost you more than doing nothing ever would have.

A payday loan's pitch is simple: borrow a few hundred dollars, pay a flat fee, pay it all back on your next payday. The fee sounds small — $15 per $100 is a common example — so it doesn't feel like 'real' interest. But that fee is for a loan lasting roughly two weeks, not a year, and annualizing it the same way every other interest rate gets annualized reveals a true APR that routinely lands north of 300%. Most borrowers can't repay the whole balance on the first due date — that's usually why they borrowed in the first place — so most lenders offer a 'rollover': pay just the fee again, and the loan resets for another two weeks. The principal never moves. Every rollover is a brand-new, full-price fee on the exact same debt. This lesson races that rollover spiral's cumulative fees against a personal loan sized to the same amount, so you can see exactly how fast a 'small' fee turns into real money — sometimes within a single missed due date.

Most 401(k) plans let you borrow against your own balance — no credit check, no application, and the interest you're charged is paid right back into your own account. That last part makes it sound like borrowing from yourself is free. It isn't, for two separate reasons: the money you removed stops compounding at the market's rate while it's gone, even though you're paying yourself interest, and that interest is paid with after-tax paycheck dollars that land inside a pre-tax account — taxed once now, taxed again on withdrawal. On top of both, if you leave your job before the loan is repaid, the outstanding balance can come due almost immediately, taxed as a withdrawal plus a penalty. This lesson races your real account value against what it would be worth had you left it alone, so you can see exactly what borrowing from yourself costs — and the one narrow case where it doesn't.

A family member or friend asks you to cosign a car loan or an apartment lease. It feels like a formality — you're not the one borrowing the money, you're just vouching for someone. That framing is wrong in a way that costs real money. A cosigner isn't a witness or a character reference; a cosigner is a co-borrower, equally liable for the entire balance from the moment they sign. Two separate costs follow, and this lesson prices both. The first is guaranteed and invisible: the loan's monthly payment counts against YOUR OWN debt-to-income ratio immediately, whether the primary borrower pays every bill early or never pays at all — a cost that can quietly shrink what you qualify for on your own next loan for years, with nothing having gone wrong. The second is a real risk, not a certainty: if the primary borrower ever misses a payment, it's the same tradeline reporting to both credit files, so the cosigner's score takes the identical hit — and if they stop paying altogether, the cosigner owes the full remaining balance, which, left unpaid, doesn't sit still: interest keeps accruing on it with nobody paying it down. The simulator charts the balance a cosigner is liable for over the loan's life two ways — if the primary borrower keeps paying, and if they stop at a month you choose — so you can watch a debt that was almost paid off reverse and grow past its original size.