🧍 Body Mass Index (BMI)

Written and maintained by the PhDino author · Last reviewed 21 September 2026 · Checked against 2 independent reference calculations · how PhDino checks its numbers

A simple screening ratio of weight to height, and why it's a starting point rather than a diagnosis.

Body Mass Index is a simple ratio designed to estimate whether someone's weight is proportionate to their height, without needing more involved measurements like body fat percentage. It was originally developed in the 19th century for population-level statistics, not as an individual clinical diagnostic tool — a distinction worth keeping in mind given how often it gets used the second way today.

The standard categories — underweight, normal, overweight, and obese — are population-level screening thresholds. BMI doesn't distinguish muscle mass from fat mass, which is the most commonly cited limitation: a heavily muscled athlete can show a "high" BMI despite low body fat, while someone with low muscle mass and higher body fat can show a "normal" BMI despite different actual health risk.

Key formula

BMI = weight (kg) ÷ height (m)²
(equivalently, BMI = 703 × weight (lb) ÷ height (in)² in US units)

How to use the BMI Calculator calculator

Use this to compute body mass index, weight divided by height squared, as a quick screening number. Enter weight in kilograms and height in centimetres; the units picker beside each field lets you enter pounds or feet and inches and converts them for you.

BMI is a screening tool for populations and a starting point for a conversation with a health professional. It is not a diagnosis, it does not say what a weight is made of, and it should never be used on its own to make decisions about health or treatment.

Weight (kg)

Body weight in kilograms. Use a recent, accurate measurement, ideally taken at the same time of day, since weight varies by a kilogram or two through the day.

Height (cm)

Standing height in centimetres, without shoes. Height enters the formula squared, so a small error in height moves the result more than the same error in weight.

Worked example: an adult of average height

An adult is 180 cm tall and weighs 95 kg. What is their BMI, and where does that fall in the standard adult categories?

Worked by hand:

1. Convert height to metres. 180 cm ÷ 100 = 1.80 m.
2. Square the height. 1.80² = 3.24 m².
3. Divide. BMI = weight ÷ height² = 95 ÷ 3.24 = 29.3 kg/m².
4. Cross-check with the imperial formula. BMI = 703 × pounds ÷ inches², which gives the same 29.3.

A BMI of 29.3 falls in the overweight range of 25 to 29.9, close to the boundary at 30. For this height, the range labelled healthy (18.5 to 24.9) corresponds to about 60 to 81 kg. Whether the number matters for the person depends on things BMI cannot see, such as muscle mass, where fat is carried, age, and other health measures.

Reading the result: what BMI can and cannot tell you

The World Health Organization's standard adult categories are: below 18.5 underweight, 18.5 to 24.9 healthy weight, 25 to 29.9 overweight, and 30 or above obesity. Those boundaries are conventions for population statistics rather than sharp biological thresholds.

• BMI does not distinguish fat from muscle. A muscular athlete can have a high BMI with little body fat, and an older person with low muscle mass can have a normal BMI and a high fat percentage.
• It ignores where fat is stored. Waist circumference and waist-to-height ratio add information about abdominal fat that BMI misses.
• Some health bodies use lower cut-offs for people of Asian descent, because health risks appear at lower BMI values in those populations.
• For children and teenagers BMI is interpreted as a percentile for age and sex, not against these adult categories, and this calculator is not for them.

Notes & limitations

• This is a population screening tool, not an individual diagnostic measure — it doesn't account for age, sex, muscle mass, bone density, or fat distribution, all of which affect how meaningfully a given BMI number relates to an individual's actual health.
• Standard adult BMI categories don't directly apply to children and teens (who use age- and sex-specific percentile charts instead) or to some other specific populations — this calculator uses the general adult formula only.

Common mistakes

• Treating BMI as a diagnosis. It is a screening measure that prompts a conversation, not an assessment of health.
• Using adult categories for a child or a teenager, who need age- and sex-specific percentiles.
• Mixing units, entering pounds as kilograms or inches as centimetres. Use the unit selectors beside each field.
• Ignoring muscle mass, pregnancy, or medical conditions that affect weight, all of which make BMI misleading.
• Fixating on the number. Habits and other measurements say more about health than a single ratio.

Frequently asked questions

Papers worth reading

Indices of relative weight and obesity Keys, A. et al. (1972), Journal of Chronic Diseases. The study that proposed weight divided by height squared as the best simple index of relative weight and gave it the name body mass index.

Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies WHO Expert Consultation (2004), The Lancet. Why the standard BMI cut-offs fit some populations poorly, and what other cut-off points the consultation considered.