Health calculators
The standard formulas, cited — with their limits stated.
Every health calculator here follows the same rules: results update live as you type, the formula is shown with a worked example, and the math runs entirely in your browser — nothing you enter is uploaded anywhere. Each one is unit-tested against published reference values before it ships.
What these numbers can and cannot tell you
Every calculator in this section is a screening formula built from population data. That is a genuinely useful thing and a genuinely limited one. A screening formula is designed to sort a large group into rough bands cheaply — it is not designed to describe you, and applying a population average to an individual is where most of the harm in health numbers comes from.
BMI is the clearest example. It was devised to study populations, uses only height and weight, and cannot distinguish muscle from fat or tell where fat is carried. A muscular athlete and a sedentary person of identical height and weight get identical BMIs and very different health outlooks. Waist measurement and body-fat estimates add the information BMI structurally cannot hold, which is why those calculators sit alongside it rather than beneath it. Treat any single number here as one reading on one instrument, not a verdict.
Calories: why the estimate moves so much
The calorie calculators work in two steps. First they estimate basal metabolic rate — the energy your body uses at complete rest — using the Mifflin–St Jeor equation, which is the modern standard and typically lands within about 10% of measured values for most adults. Then they multiply by an activity factor to reach daily maintenance energy. That second step is where the uncertainty concentrates: the gap between "lightly active" and "moderately active" is a matter of judgement, and it can move the answer by several hundred calories a day.
The right way to use the result is as a starting hypothesis. Eat at the estimate for two or three weeks, weigh yourself under consistent conditions, and adjust based on what actually happens. Your body is the measurement instrument; the equation just saves you from starting at a random number. Anyone whose weight is medically supervised should be working from measured values, not an equation.
Due dates are estimates, and most births miss them
The due-date calculator applies Naegele's rule: last menstrual period plus 280 days, assuming a 28-day cycle with ovulation on day 14. Both assumptions are averages that many people do not match, which is why an early ultrasound — which measures the pregnancy directly rather than inferring it from a date — supersedes this calculation whenever the two disagree. Fewer than one in twenty births happen on the estimated date; the great majority arrive within a fortnight either side. The date is a planning anchor for appointments and leave, not a deadline.
Ideal weight, and why the calculators disagree
There is no single accepted formula for ideal body weight. Devine, Robinson, Miller and Hamwi were each developed for different purposes — several originated in drug-dosing rather than health assessment — and they can differ by several kilograms for the same person. Our page shows the competing results side by side instead of hiding the disagreement behind one confident number, because the spread between them is honest information: it tells you the concept itself is fuzzy, and that a target anywhere inside that range is defensible.
Privacy matters more here than anywhere else on the site
Body measurements, pregnancy dates and weight goals are among the most sensitive things a person types into a browser. Nothing entered on these pages is transmitted anywhere: the arithmetic runs on your device, there is no server receiving it, and no account associates a result with a person. This is a property of how the site is built rather than a policy we ask you to trust — you can load a page, disconnect from the internet, and the calculator still works.