All guides
％ Percentages in your head
Figure a 15% tip, a 30%-off price, or sales tax in a few seconds, no phone needed. The 10% trick and when tipping math is allowed to be lazy.
🏠 What a loan really costs
The monthly payment is the least interesting number. Where the interest hides, why the first years feel useless, and what one extra payment a year does.
📈 Why saving at 25 beats saving at 35
The ten-year head start that money never catches up to, the rule of 72, and what compounding looks like with real numbers.
⚖️ What your BMI does and doesn't tell you
Why a fit rugby player scores "overweight," what the number is actually good for, and the tape-measure check that's often more useful.
Why we write these
A calculator that spits out "$1,580 per month" hasn't really told you anything. Is that good? Compared to what? What happens if you pay $100 more? The tools on this site answer the arithmetic. These guides answer the "so what." Everything here is written by us, checked against the same formulas the calculators use, and kept short enough to read while your coffee is still hot. If something's wrong or unclear, the address on the about page reaches us.