BMI calculator, with the limits explained
Calculate Body Mass Index in metric or imperial units, with a category result.
This calculates Body Mass Index — weight in kilograms divided by height in metres squared — in your browser, with imperial input converted automatically. BMI is a population-level screening measure, not a diagnosis. It was never designed to assess an individual, it cannot distinguish muscle from fat, and it is known to misclassify athletes, older adults and several ethnic groups. Read the number with those limits in mind. The standard adult categories are under 18.5 underweight, 18.5 to 24.9 healthy, 25 to 29.9 overweight and 30 or above obese. The imperial form is (pounds ÷ inches²) × 703. The measure was devised by Adolphe Quetelet in the 1830s to describe populations, and applying it to one person is a later repurposing its author never intended. Because it squares height it also overstates for tall people and understates for short. Children require age and sex percentile charts instead.
About the BMI Calculator
Calculate your Body Mass Index (BMI) in seconds. Switch between metric (kilograms and centimeters) and imperial (pounds, feet, and inches) units. We show your BMI value along with the World Health Organization category so you can interpret the result at a glance.
How to use the BMI Calculator
- 01Choose your unit system: metric (kg + cm) or imperial (lb + ft + in).
- 02Enter your weight.
- 03Enter your height.
- 04Read your BMI value and category — Underweight, Normal, Overweight, or Obese.
Why use our BMI Calculator
- 01
Metric & imperial
Toggle freely between unit systems without losing your input.
- 02
WHO categories
Result is mapped to the standard WHO BMI bands.
- 03
Instant
BMI updates as you type.
- 04
Private
Your weight and height never leave the browser.
What is BMI actually measuring?
The formula was devised in the 1830s by Adolphe Quetelet, a Belgian mathematician and astronomer who was studying the statistical distribution of human characteristics across populations. He explicitly did not intend it as a measure of individual health, and it was not used that way until the 1970s, when the physiologist Ancel Keys evaluated several indices and found Quetelet's the best available proxy for body fat in large population samples. That is the origin of its authority: it is cheap, requires only a scale and a tape measure, and correlates reasonably with body fat percentage across a large group. The correlation across a group is not the same as accuracy for a person, and the distinction has been blurred by decades of use in clinical settings where nothing better was practical. Understanding that history is what makes the limitations below make sense — they are not flaws that were introduced later, they are properties the measure always had.
Who BMI misclassifies, and in which direction
BMI cannot distinguish between fat and lean mass, so muscular people are systematically overestimated. A trained athlete frequently registers as overweight or obese while carrying low body fat, which is the most widely known limitation. The opposite error is less discussed and more consequential: older adults lose muscle mass with age, so someone can fall in the normal range while carrying a high proportion of body fat — sometimes called normal-weight obesity — and the number offers false reassurance. Fat distribution is invisible to BMI entirely, yet visceral fat around the organs carries substantially more metabolic risk than subcutaneous fat, so two people with identical BMI can have very different risk profiles. Ethnicity matters too: cardiometabolic risk rises at lower BMI thresholds in South Asian, Chinese and other Asian populations, which is why the UK's NICE guidance recommends lower action thresholds for those groups rather than applying the standard cut-offs. BMI is also not valid during pregnancy or for children, who need age-and-sex-specific percentile charts.
Better measures to use alongside it
Waist circumference is the single most useful addition and requires only a tape measure. It captures central adiposity, which BMI misses entirely, and the commonly cited thresholds are above 94 cm for men and 80 cm for women indicating increased risk, with 102 cm and 88 cm indicating substantially increased risk. Waist-to-height ratio is arguably better still and has an unusually memorable rule: keep your waist under half your height. It applies across ethnic groups more consistently than BMI thresholds and requires no chart. Waist-to-hip ratio is another established measure with good evidence for cardiovascular risk. Beyond tape measurements, body composition assessment via DEXA, bioelectrical impedance or skinfold measurement gives an actual fat percentage, though accuracy varies considerably by method. The practical position is that BMI is a reasonable first screen precisely because it is so easy, and any concern it raises — or fails to raise — is better resolved by a conversation with a clinician than by a different formula.
WHO BMI categories for adults
| Category | BMI range | Note |
|---|---|---|
| Underweight | Below 18.5 | May indicate undernutrition |
| Normal weight | 18.5 – 24.9 | Lower thresholds apply for some Asian populations |
| Overweight | 25.0 – 29.9 | Screening threshold, not a diagnosis |
| Obesity class I | 30.0 – 34.9 | |
| Obesity class II | 35.0 – 39.9 | |
| Obesity class III | 40.0 and above |
Not valid during pregnancy or for children, who require age- and sex-specific percentile charts.
Frequently asked questions
How is BMI calculated?
Weight in kilograms divided by height in metres squared. Imperial figures are converted automatically before the calculation.
Is BMI accurate for individuals?
Not reliably. It was designed as a population-level statistical measure and cannot distinguish muscle from fat or account for fat distribution.
Why do athletes get a high BMI?
Muscle is denser than fat and BMI counts only total weight. A trained athlete can register as obese while carrying very low body fat.
Can BMI be falsely reassuring?
Yes. Older adults who have lost muscle mass can sit in the normal range while carrying a high fat percentage — sometimes called normal-weight obesity.
Do the thresholds apply to everyone?
No. Cardiometabolic risk rises at lower BMI in South Asian, Chinese and other Asian populations, and guidance such as NICE recommends lower action thresholds accordingly.
Is BMI valid in pregnancy or for children?
No. Pregnancy changes weight for reasons BMI cannot interpret, and children require age- and sex-specific percentile charts rather than adult categories.
What is a better measure?
Waist-to-height ratio is a strong and simple one — keep your waist under half your height. Waist circumference and waist-to-hip ratio also capture central fat, which BMI misses.
Should I act on my BMI result?
Treat it as a prompt for a conversation, not a conclusion. A clinician can interpret it alongside waist measurement, history and blood markers.
Is my health data uploaded?
No. The calculation runs entirely in your browser and nothing you enter is transmitted or stored.
Last updated
Categories per World Health Organization BMI classification. Ethnic-specific thresholds per NICE guideline PH46. Waist circumference thresholds per WHO and NICE guidance. Not medical advice.