The calculators below split into three groups that don't actually share much mathematics with each other, which is why they live in three subsections rather than one long list. Body covers metabolic and clinical measurements — how many calories you burn at rest, how much of your weight is fat, how well your kidneys are filtering. Pregnancy covers cycle and gestational dates, working forwards and backwards from a last period, a conception date or an ultrasound measurement. Sport covers training zones and performance numbers — heart rate ranges, pace, estimated aerobic capacity — the figures a training plan is built around rather than a medical chart.
Calculators in this section
Named formulas, not house guesses
Where a calculator has a standard published formula behind it, it uses that formula rather than an approximation invented for this site. Resting metabolic rate uses the Mifflin-St Jeor equation, which replaced the older Harris-Benedict equation in most clinical use after a 1990 study found it tracked measured metabolic rate more closely across a modern population. Kidney function uses Cockcroft-Gault, still the equation most commonly used to estimate creatinine clearance for drug dosing. Body fat percentage uses the U.S. Navy method, a set of circumference measurements the Navy adopted because it doesn't require any equipment beyond a tape measure. Heart rate training zones use the Karvonen formula, which factors in resting heart rate rather than treating everyone at a given age as having the same starting point. None of these are the only formula that exists for the job — several have competing versions — but each is the one most widely cited for that specific calculation, so that's what's implemented.
A screening number is not a diagnosis
BMI is the clearest example of a figure that gets over-interpreted. It was developed as a population-level screening tool, cheap to calculate from height and weight alone, and it correlates reasonably well with body fat across large groups. Applied to one specific person, it says nothing about muscle mass, bone density or where fat is actually distributed, which is why a heavily muscled athlete and a sedentary person of the same height and weight can return an identical BMI while being in very different metabolic shape. The same caveat applies, in a smaller way, to most of the other numbers in this section: eGFR, QRISK and body fat percentage all estimate something a lab test or a clinician would measure directly, and all three exist to give you a reasonable figure to discuss with a doctor, not to replace seeing one.
Dates that move as new information arrives
The pregnancy calculators are all doing the same underlying arithmetic — adding a fixed or estimated number of days to a known reference point — but the reference point itself changes as a pregnancy progresses. An early due date estimated from the last menstrual period gets revised once a dating ultrasound provides a direct measurement, because ultrasound-based estimates in the first trimester are more accurate than cycle-based ones. That's normal, not a sign either calculation was wrong; each one is doing correct arithmetic on the best information available at the time it was run.