The short answer
BMI is your weight in kilograms divided by your height in metres squared. The WHO healthy range for adults is 18.5 to 24.9. It is a useful screening number, but it cannot tell muscle from fat, so it describes populations far better than it describes any individual person.
What is BMI and what is it actually for?
Body mass index was devised in the 1830s by Adolphe Quetelet, a statistician, to describe populations. It was never designed as a personal diagnostic, and that origin explains both its usefulness and its limits. As a cheap way to screen large groups for weight-related risk, it works well. As a verdict on one individual, it is crude.
The formula is deliberately simple:
- Metric: BMI = weight (kg) ÷ height (m)²
- Imperial: BMI = weight (lb) ÷ height (in)² × 703
A person at 175 cm and 70 kg has a BMI of 22.9. At 85 kg the same person is at 27.8.
How do you read your BMI? The WHO categories
| BMI | Category | What it signals |
|---|---|---|
| Below 18.5 | Underweight | Possible undernutrition, higher risk of bone loss and low immunity |
| 18.5 to 24.9 | Healthy weight | Lowest average risk across populations |
| 25.0 to 29.9 | Overweight | Modestly raised risk of metabolic conditions |
| 30.0 to 34.9 | Obesity class I | Clearly raised cardiometabolic risk |
| 35.0 to 39.9 | Obesity class II | High risk |
| 40.0 and above | Obesity class III | Very high risk |
These are the international adult cut-offs published by the World Health Organization. They apply to adults only: children and teenagers are assessed with age and sex specific percentile charts, not these fixed numbers.
Where BMI gets it wrong
This is the part most calculators leave out, and it matters more than the number itself.
It cannot tell muscle from fat. Muscle is denser than fat, so a rugby player and a sedentary person of the same height and weight get an identical BMI despite completely different bodies. This is not a rare edge case: it affects most people who lift seriously.
It ignores where fat sits. Visceral fat around the organs carries much more metabolic risk than fat on the hips and thighs. Two people at BMI 27 can have meaningfully different risk profiles. Waist circumference captures this, and BMI does not.
The thresholds are not universal. The WHO notes that health risk appears at lower BMI values in Asian populations and suggests additional action points near 23 and 27.5. A single global cut-off flattens real differences between groups.
It says nothing about trend. A BMI of 26 that has been stable for a decade is a different situation from a BMI of 26 that was 22 two years ago. The direction of travel usually matters more than the snapshot.
So is BMI worth calculating at all?
Yes, as a starting point. If your BMI sits comfortably in the healthy range and your weight has been stable, that is genuine reassurance. If it sits well outside, it is a reasonable prompt to look at the measures that carry more information: waist circumference, body fat percentage, blood pressure, and your weight trend over years rather than weeks.
What BMI should never be is a target you chase. Optimising a ratio of two numbers is not the same as improving health, and people who pursue a BMI figure through aggressive restriction usually lose muscle alongside fat, which makes the underlying picture worse while the number improves.
What actually moves the number
Body weight responds to the gap between the energy you eat and the energy you spend, sustained over months. Nothing else changes it durably. The practical difficulty was never understanding that; it is knowing what you actually ate, day after day, without the process becoming a second job.
That is where most tracking attempts collapse. Manual logging takes long enough that people skip meals, and a food log with gaps in it cannot tell you anything useful. CalAlly removes that friction: you say what you ate, the AI breaks it into ingredients, calories and macros, and one tap logs it. About 3 seconds. The AI coach then reads your real data and tells you whether the week is actually going where you think it is.