Everyday Calculations
Every formula here is one line long, and every bug is an edge case: completed years versus nominal age, 29 February, retirement age advancing month by month by birth date, BMI failing for muscular or elderly people.
The point of these guides is not the formula but when it stops applying. Knowing the boundary is more useful than memorising the arithmetic.
- Age Calculation: Completed Years, Leap Days and Legal AgeHow many years old someone is depends on which convention you use. Completed years versus the Korean age system, what happens to a 29 February birthday, and why two calculators disagree by one.
- BMI: When It Stops Being UsefulBMI is body mass divided by height squared, and it knows nothing about what that mass is. Why athletes read as obese, why the waist-to-height ratio predicts risk better, and the age groups where the thresholds do not apply.