loans

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.

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'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.

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?

There are two ways to pay a college bill, and they cost very different amounts. Save ahead in a 529 — a tax-advantaged college-savings account where money grows and comes out tax-free for education — and tax-free compounding quietly pays for a chunk of the tab, so you put in less than the sticker price. Borrow the same bill as a student loan and you pay the full sticker price plus years of interest, so you pay more. The simulator races the two as cumulative-cost lines over time: the saver pays a little, steadily, in the years before college, while the borrower pays nothing until the bill is due and then a lot, for a decade after. At the defaults — a $120,000 bill, $400 a month for 14 years — the 529 grows to about $105,000 (you contributed $67,200; tax-free growth added the other $38,000), covering all but ~$15,000 of the bill, which you borrow. Saving ahead costs about $88,000 all in; borrowing the whole thing costs about $164,000 — the sticker price plus $44,000 of interest. That's roughly $76,000 less for the family who planned ahead, and it splits almost evenly into two forces: tax-free growth working for you, and loan interest you never pay. The deeper lesson is the same one behind compound interest and opportunity cost: time is the lever. A dollar saved years early is multiplied by tax-free growth; a dollar borrowed is multiplied by interest. Start early and small beats start late and large. The honest caveats: this ignores financial aid, scholarships, and grants (which can shrink the bill for either family), 529 rules on leftover money, and the fact that not all of college has to be paid by you — but the core trade-off, save-ahead-cheap vs borrow-later-expensive, holds.

Most people with a federal student loan never realize they chose a repayment plan — they took the default. But the choice between the standard 10-year plan and an income-driven plan (IDR) can swing the total cost by tens of thousands of dollars, in either direction. The standard plan is a fixed amortizing payment that clears the loan in 10 years: the highest monthly bill, the least interest, and debt-free fastest. An income-driven plan instead caps your payment at a share (often 10%) of your discretionary income — the part above roughly 150% of the poverty line — and forgives whatever balance is left after 20 or 25 years (10 years for public-service workers under PSLF). That lower payment is real relief when money is tight, but it hides a trap: when the payment is smaller than the month's interest, the unpaid interest is added to the balance and the loan GROWS — 'negative amortization.' So for a moderate earner, IDR can mean paying more in total, over twice as long, even after some forgiveness — the lower payment was just a longer, costlier loan. For a low earner whose income genuinely can't support the standard payment, the same plan is a lifeline: a tiny or zero payment, and a large balance wiped clean. The simulator races both balances over time so you can see the standard plan dive to zero while the income-driven balance climbs above what you borrowed before forgiveness erases the rest. The durable lessons: judge a loan on total cost and time, not the monthly payment; income-driven repayment is a safety net for unaffordable payments, not a default to reach for; forgiveness can be taxed; and refinancing a federal loan to a private one trades these protections away for good.

After a house, a car is the biggest check most people write — and the one they think about least clearly. The trap is the price tag: people compare monthly payments and sticker prices and miss the two things that actually decide what a car costs them. A car's true cost of ownership is how much value it LOSES while you own it (depreciation) plus what you pay to BORROW (financing interest). Everything else — the monthly payment, the down payment — is just how you split those two costs across time. Seen that way, the famous advice 'buy a lightly-used car' stops being folksy wisdom and becomes arithmetic: a new car sheds roughly a fifth of its value in the first year and close to half in five, so buying the same model a few years old lets the first owner absorb that 'depreciation cliff' for you. Leasing is a different shape entirely — a low monthly payment that buys you a perpetually-new car but never any equity, so you pay forever and own nothing. This lesson races the all-in cost of all three paths over the years you keep the car. The headline: the cheapest way to put miles on a car is almost always to buy it a few years used and drive it for a long time; leasing's low monthly is the most expensive option in disguise; and the depreciation cliff, not the interest rate, is the number that dominates the decision.