Guides
Long-form explainers. The calculator gives you a number; the guide explains what the number actually means, where the formula comes from, and where it stops being trustworthy.
How compound interest actually works
Why the formula has that exponent, why monthly compounding beats yearly only slightly, and the rule of 72 in 60 seconds.
FinanceThe 28/36 rule, and how lenders decide what you can afford
Where the two ratios come from, why both bind, and how to tell which one is the tighter constraint for you specifically.
HealthBMI, honestly: what it measures, what it doesn't
The 19th-century origin story, the legitimate uses, the famous failures (athletes, kids, ethnicity), and what better metrics exist.
SociologyMost people radically underestimate their global income percentile
A $50k salary feels middling in the US. Globally it's already top 5%. Why the intuition fails, and what the data actually says.
FinanceThe 4% rule: how much do you actually need to retire?
Where the 4% number came from (the Trinity Study), why it's debated now, and a worked example for your own retirement target.
FinanceHow amortization actually works
Why the first years of a mortgage payment are mostly interest, where the formula comes from, and what extra principal payments really do.
FinanceAPR vs APY: the 2% difference that matters
APR is what banks quote on loans. APY is what they quote on deposits. The asymmetry is no accident — and the formula that converts them.
FinanceHow big should your emergency fund be?
The 3-to-6 months cliché is a starting point. Here's how to size yours based on income stability, fixed costs, and dependents.
FinanceHow tax brackets actually work
The single most common tax misconception: that a raise into a higher bracket can lower take-home. Marginal vs effective rates, with worked numbers.
HealthWhat is TDEE, and how do I calculate mine?
Total Daily Energy Expenditure broken down: BMR, activity factor, what the formulas miss, and how to actually use the number.
FinanceHow credit scores actually work
The five FICO factors with their published weights. The two that matter (payment history + utilization), and what credit-repair scams charge for.