The Methods Behind These Calculators
Clinical estimates are population equations, not measurements. The creatinine clearance tool uses the Cockcroft-Gault equation, which estimates kidney function from age, weight, sex and serum creatinine. It was derived from population data in the 1970s and remains a standard for drug dosing — but it estimates a lab value rather than measuring it. The same logic applies to corrected calcium: roughly half of blood calcium travels bound to albumin, so a low albumin makes total calcium look falsely low, and the correction adds back what the albumin figure implies is missing.
Half-life is exponential decay. The half-life calculator models first-order elimination, where each half-life removes half of whatever remains — 50%, then 25%, then 12.5%. That is why "five half-lives" is the standard rule of thumb for a substance being effectively cleared, and why the EtG estimator can only ever express a likelihood rather than a verdict: metabolite clearance varies with hydration, liver function and dose.
Pregnancy dating counts from a date you can actually know. Naegele's rule — the basis of the standard due date calculator — adds 280 days to the first day of the last menstrual period, because ovulation date is usually unknown while period dates are tracked. IVF dating inverts this: the embryo transfer date is known to the day, so the calculator counts forward from transfer and adjusts for whether a 3-day or 5-day embryo was used. That is why IVF due dates are the most precise of any method, and why the two tools exist separately rather than as one form with a dropdown.
Fitness standards are scoring tables, not opinions. The Army body fat calculator implements the US Army's official circumference (tape) method, estimating body fat from neck and waist measurements rather than requiring calipers or a DEXA scan — exactly why the military adopted it: it is repeatable in the field by anyone with a tape. The ACFT tool scores six events against the Army's published tables. FFMI normalises fat-free mass to height much as BMI normalises weight, answering "how muscular am I for my frame?" — a question BMI famously cannot answer, since it classifies many trained athletes as overweight.
Nutrition math is arithmetic over verified data. The restaurant calculators sum published per-ingredient figures across a custom order; the math is trivial and the value is the dataset. The ABV calculator uses the standard homebrewing relationship between the drop in specific gravity and alcohol produced — the sugar that disappeared during fermentation became alcohol, and the gravity change measures how much.
The pet tools translate veterinary rules of thumb. Dog-to-human age conversion is no longer "multiply by seven": modern conversions weight the first two years heavily, since a one-year-old dog is roughly a human teenager, then scale by breed size because small dogs age more slowly after maturity. Puppy weight prediction extrapolates breed-size growth curves rather than assuming linear growth.