Body Mass Index
Calculate BMI from height and weight.
About the Body Mass Index
BMI is a rough screening number, not a diagnosis. It compares weight to height and says nothing about muscle, bone or where fat sits.
How to use it
- Enter your height.
- Enter your weight.
- Read the category alongside the number.
The formula
Body mass index divides weight by the square of height. Two versions exist depending on your units, and they give identical answers.
Metric: BMI = kg ÷ m²
Imperial: BMI = 703 × lb ÷ in²
The 703 is only a unit conversion, not a health constant. Height must be in inches throughout — 5 feet 9 inches is 69 inches, and using 5.9 produces a wildly wrong figure.
The squaring is the interesting part, and the source of most of the criticism. People do not scale as squares; taller people are larger in three dimensions, so BMI tends to read slightly high for tall individuals and slightly low for short ones. Adolphe Quetelet devised the ratio in the 1830s to describe populations, and it was never intended to assess an individual.
Worked examples
| Height | Weight | BMI | Standard category |
|---|---|---|---|
| 5'6" | 120 lb | 19.4 | healthy weight |
| 5'4" | 140 lb | 24.0 | healthy weight |
| 5'9" | 170 lb | 25.1 | just into overweight |
| 6'0" | 200 lb | 27.1 | overweight |
The third row shows how arbitrary a threshold can feel. At 5'9" the boundary between 'healthy' and 'overweight' falls at about 169 pounds, so a single pound moves someone across a category line that sounds far more decisive than it is. The bands are population statistics with hard edges drawn on a continuous distribution, and nothing physiological changes at 25.0.
Common mistakes
- Entering height as 5.9 instead of 69 inches. The imperial formula needs total inches. 5 feet 9 inches is (5 × 12) + 9 = 69. Using 5.9 gives a BMI in the thousands.
- Reading BMI as a measure of body fat. It measures neither fat nor its distribution. A muscular athlete and a sedentary person of the same height and weight get the same score and are in entirely different health situations.
- Applying adult categories to children. Children and teenagers are assessed on age- and sex-specific percentile charts, not the adult bands. The adult categories are meaningless before about 20.
- Treating a category as a diagnosis. BMI is a screening tool. Waist circumference, blood pressure, blood glucose, lipids and fitness all say more about health, and a doctor looks at those rather than this ratio alone.
Terms explained
- Body mass index
- Weight divided by height squared. A population-level screening ratio, not a measure of body composition.
- Underweight
- Conventionally a BMI below 18.5 in adults.
- Healthy weight
- Conventionally 18.5 to 24.9. The bands are statistical conventions rather than physiological thresholds.
- Overweight
- Conventionally 25.0 to 29.9.
- Obesity
- Conventionally 30.0 and above, often subdivided into classes.
- Waist-to-height ratio
- Waist divided by height, with roughly 0.5 as a common guideline. It captures where fat sits, which BMI cannot see at all.
Common questions
- What is a healthy BMI?
- Roughly 18.5 to 24.9 for adults. These are population averages used for screening, not personal targets, and they say nothing about an individual's health on their own.
- Why is BMI criticised?
- It cannot distinguish muscle from fat, ignores where fat is distributed, and treats height as scaling with the square when bodies are three-dimensional. Athletes routinely score overweight while being extremely healthy.
- Does BMI work for children?
- Not with these categories. Children are assessed against age- and sex-specific percentile charts, because normal body composition changes substantially during growth.
- What is the 703 for?
- It converts pounds and inches into the metric ratio. It has no medical meaning — the metric formula, kilograms over metres squared, needs no constant at all.
- Is waist measurement more useful?
- For many purposes, yes. Abdominal fat is more strongly associated with metabolic risk than total weight, and a waist-to-height ratio under about 0.5 is a common rule of thumb.
- Does BMI differ by ethnicity?
- Health authorities in several countries apply lower thresholds for people of South Asian and East Asian descent, because metabolic risk appears at a lower BMI in those populations.
- Should I try to change my BMI?
- That is a conversation for a doctor, who can look at the measurements BMI cannot see. A number from a formula is a prompt to ask a question, not an instruction to act.
- Why does one pound change my category?
- Because the bands have hard edges applied to a continuous range. Nothing physiological happens at exactly 25.0 — the boundary is a convention for reporting, not a cliff.