Guides

Learn the idea by doing it — every guide has an interactive tutorial built in.

Is it worth overpaying your mortgage by $100 a month?GuideIs it worth overpaying your mortgage by $100 a month?$30,600 in cancels $60,772 of interest — and your required payment never drops a cent.7 min read →Simple vs compound interest: why the gap explodes over timeGuideSimple vs compound interest: why the gap explodes over timeSame $10,000 at 5% for 30 years: simple earns $15,000 in interest, compound earns $33,219.6 min read →How much to save each month, and what the interest coversGuideHow much to save each month, and what the interest coversDouble the timeline and the monthly falls to less than half — interest picks up the rest.7 min read →Roth vs traditional 401(k): at the same tax rate, it's a tieGuideRoth vs traditional 401(k): at the same tax rate, it's a tieSame pre-tax dollars, same tax rate: both accounts land on $589,435. Exactly the same.8 min read →The car loan term trap: a lower payment, far more interestGuideThe car loan term trap: a lower payment, far more interestSame $30,000 car, same rate: pick a 7-year loan instead of 3 and you pay $4,686 more.6 min read →APR vs APY: why the same rate shows up as two different numbersGuideAPR vs APY: why the same rate shows up as two different numbersA 24.99% card APR is really 28.4% APY — banks quote whichever number looks better.6 min read →Backing out sales tax: why subtracting the rate leaves you shortGuideBacking out sales tax: why subtracting the rate leaves you shortTake 8.25% off a $54.11 receipt and you're 34 cents short — the tax on the tax.6 min read →The bulk size is cheaper until you throw this much awayGuideThe bulk size is cheaper until you throw this much awaySave 22% per ounce and you can throw out 22% of it before the big size loses.6 min read →Front-loading your 401(k) can cost you the matchGuideFront-loading your 401(k) can cost you the matchMax out too early and you forfeit the match on every paycheck left in the year.6 min read →Keystone pricing: doubling the cost keeps half, not allGuideKeystone pricing: doubling the cost keeps half, not allDouble your cost and it feels like 100% profit — but that's a 50% margin.6 min read →Debt snowball vs avalanche: what does the smart one save?GuideDebt snowball vs avalanche: what does the smart one save?Avalanche is the math-smart pick — but line the debts up right and it saves $0.6 min read →Time and a half: it's not 1.5× your whole checkGuideTime and a half: it's not 1.5× your whole checkOnly hours past 40 get the 1.5x — so a 48-hour week is $1,144, not $1,584.6 min read →Is a 3% raise good? Subtract inflation firstGuideIs a 3% raise good? Subtract inflation firstA 3% raise with 3.3% inflation isn't a raise — it's a $174 pay cut in disguise.6 min read →Coast FIRE: when you can stop saving for retirementGuideCoast FIRE: when you can stop saving for retirementAt 30, $181,000 invested and never touched grows into a $1,000,000 retirement.6 min read →Rule of 70 vs Rule of 72: which one should you use?GuideRule of 70 vs Rule of 72: which one should you use?Below 5% growth, trust 70. Above it, 72 — and the honest number behind both is 69.3.5 min read →Biweekly vs semimonthly pay: why one check looks smallerGuideBiweekly vs semimonthly pay: why one check looks smallerSame salary, cut into 26 slices instead of 24 — the biweekly check is always 7.7% smaller.5 min read →How long does paying only the minimum really take?GuideHow long does paying only the minimum really take?On a $5,000 card: 16 years, and more interest than you ever borrowed.6 min read →Why 20% off then 10% off isn't 30% offGuideWhy 20% off then 10% off isn't 30% offIt's 28% — the second discount cuts a price the first one already lowered.4 min read →Markup vs margin: what's the difference?GuideMarkup vs margin: what's the difference?A 50% markup is only a 33% margin — and mixing them up quietly costs money.5 min read →The Rule of 72, learned by doingGuideThe Rule of 72, learned by doingDrag one slider and watch how your return sets how fast money doubles.4 min read →