interest

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.

'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.

Not all places to keep cash are equal. A checking account pays almost nothing, a big-bank savings account barely more, a high-yield savings account (HYSA) can pay roughly ten times as much for the same instant access and the same FDIC insurance, and a CD pays a little more still — but only if you lock the money up and don't touch it. The simulator grows one balance in all four at once so you can see the idle-cash cost of the wrong account, then drag a slider to break the CD early and watch the withdrawal penalty erase its edge. The durable lesson: match the account to how soon you'll need the money — liquidity has value, and leaving cash in checking is a quiet, recurring tax.

The return on your money has two parts: the headline (nominal) rate the statement shows, and the real rate left after inflation eats its share. They can disagree completely — a savings account paying 1% while prices rise 3% has a positive nominal return and a negative real one, so the balance grows on paper while it buys less every year. This lesson separates the two. The simulator grows the same amount in cash, bonds, and stocks but draws every line in real, inflation-adjusted dollars, with a break-even line marking where purchasing power holds steady. Drag inflation up and watch the slow earners cross below it. The durable lesson: judge any return by what's left after inflation, because the nominal number alone can't tell you whether you're winning.

Once you've decided to buy, the next decision is the loan itself — and the most misunderstood lever on it is discount points. A point is cash paid at closing, conventionally 1% of the loan, that buys your interest rate down a notch. A lower rate means a smaller monthly payment and less interest over the life of the loan, so points look like a pure win. They aren't free, though: you hand over the money today, while the savings dribble back a little every month. That makes buying down your rate the same shape of decision as renting versus buying — a break-even that hinges on how long you stay. The break-even is simply the up-front cost divided by the monthly saving: pay $8,000 in points to cut your payment by about $130 a month and you start ahead only after roughly five years. Keep the loan past that point and the points were a bargain; sell the house, refinance, or pay the loan off early before then and you'd have been better off keeping the cash and taking the higher rate. The simulator plots the running net position of paying for points: it starts underwater by the cost of the points, climbs as the lower payment saves money each month, and crosses into the black at the break-even. The exact same arithmetic governs refinancing — closing costs paid now against a lower payment later — so the mental model you build here transfers directly. The durable lesson: a lower rate is worth paying for only if you'll keep the loan long enough to collect the savings.

A fixed-rate mortgage locks one payment for 30 years; an adjustable-rate mortgage (ARM) starts lower and then changes. The number that names it — 5/1, 7/1 — tells you the deal: the first figure is how many years the low 'teaser' rate is fixed, the second how often it adjusts after that. When the rate adjusts, the loan re-amortizes — the remaining balance is spread over the remaining term at the new rate — so a higher rate means a higher payment, often a sharply higher one. Where does the new rate come from? A published index (a market rate) plus a fixed margin the lender sets, together the 'fully-indexed rate.' Caps limit the damage: a typical 2/2/5 structure means the first adjustment can move the rate at most 2 points, each later one at most 2 points, and it can never rise more than 5 points above where it started. The appeal is real — for the intro years you pay less than a comparable fixed loan, sometimes much less. The risk is just as real: when the teaser ends, the payment can leap, and the early savings get eaten if you keep the loan long enough. That makes an ARM a bet — that you'll sell or refinance before the reset catches up, or that rates will fall instead of rise. The simulator races an ARM's monthly payment against the fixed loan you could take instead: a flat line for fixed, a teal staircase for the ARM that sits low through the teaser, then steps up at each reset. The crossover — where the ARM's running cost overtakes the fixed loan's — is the hidden break-even. The durable lesson: the rate on the billboard is the teaser, not the loan; an ARM only wins if you collect the discount and get out before the reset bites.

Rates dropped, so you should refinance, right? Not necessarily. Refinancing swaps your loan for a new one at a lower rate, but you pay closing costs up front and recoup them slowly through a smaller monthly payment — so the decision turns on how long you'll keep the loan. There's a clean break-even month (closing costs divided by your monthly saving): keep the loan past it and the refinance paid for itself; sell, move, or refinance again before it and the closing costs were money down the drain. And there's a subtler trap. Refinancing almost always resets the term to a fresh 30 years, and a big chunk of your 'saving' is really just the same balance stretched over more years — which can pile on more total interest even at a lower rate. This lesson makes both visible: a chart that races the cost of keeping your loan against the cost of refinancing it, crossing at the break-even, plus the lifetime-interest reality check. The takeaway: a lower payment is not always less money, and the right move is often to take the lower rate but keep paying like you never refinanced.

A HELOC (home equity line of credit) is one of the cheapest ways to borrow money, because your house backs it. But 'cheap' hides a structure no other common loan has: a draw period of interest-only payments where the balance never falls, followed by a repayment period that suddenly amortizes the whole thing — principal and interest — over whatever years are left. The payment can jump sharply the day the draw period ends, and because the line is secured by your home, missing it risks the house, not just whatever the money bought. This lesson races the HELOC's real monthly payment against what the same balance would cost if it amortized from day one, showing that the interest-only period isn't free — it costs real lifetime interest — before turning to the question that actually decides whether tapping your equity was smart: was what you bought with it worth more than the interest?