BMI calculator
Two measurements, one number — and a straight answer about what that number is worth for a person rather than a population.
How this is calculated
BMI = weight(kg) / height(m)²
That is the whole formula. Adolphe Quetelet proposed it in the 1830s as a way to describe the average build of a population, and the WHO cut-offs layered on top — 18.5, 25, 30 — arrived more than a century later from mortality data across large groups.
What it cannot see
Muscle weighs more than fat for the same volume, so trained people are routinely scored "overweight" by a formula that has no way to look. It also cannot see where weight sits, and visceral fat around the organs carries risk that fat elsewhere does not. Two people, same score, different futures.
Use it as one coarse input among several. If you want a single cheap improvement, measure your waist: keeping it under half your height tracks metabolic risk better than BMI does, and costs one tape measure.
These are the same numbers and the same guards the Talk2Fit app applies when it builds your plan — the calculator is the app's own arithmetic, not a separate approximation.
Questions
Is BMI accurate for me personally?
It was never meant to be. Quetelet built it in the 1830s to describe populations, and it is still useful for exactly that. For an individual it cannot distinguish muscle from fat, or where fat sits — which is why a rugby player and a sedentary person of the same height and weight get the same score and very different health outcomes.
Then why is it everywhere?
Because it needs two numbers anyone can measure, and across large groups it correlates well enough with body-fat-related risk to be worth tracking. Cheap and roughly right at scale beats precise and impractical — but that logic stops working the moment you apply it to one person.
What is a better number?
Waist-to-height ratio is the cheapest upgrade: keeping your waist under half your height is a better predictor of metabolic risk than BMI, and needs one tape measure. Body-fat percentage is better still, and a Navy-method calculator for it is on the way.
Does BMI work for children or older adults?
Not as shown here. Children are scored against age-and-sex percentiles, not fixed cut-offs, and in older adults a slightly higher BMI is associated with better outcomes. These bands are for adults roughly 20–65.