The Quick Answer
BMI (Body Mass Index) is a number calculated from your height and weight. The formula is:
BMI = weight (kg) ÷ [height (m)]²
A BMI between 18.5 and 24.9 is classified as "normal weight" by the World Health Organization. Below 18.5 is underweight, 25–29.9 is overweight, and 30+ is obese.
Example: A person who is 175 cm tall and weighs 70 kg has a BMI of 70 ÷ (1.75 × 1.75) = 22.9 (normal weight).
BMI is a screening tool, not a diagnosis. It does not measure body fat directly.
How to Calculate BMI
There are two versions of the formula — one for metric units, one for imperial. Both produce the same result.
Metric formula
BMI = weight (kg) ÷ [height (m)]²
Steps:
- Convert height from centimeters to meters (divide by 100)
- Square the height (multiply it by itself)
- Divide weight by the squared height
Worked example: 82 kg, 180 cm
- Height in meters: 1.80
- Height squared: 1.80 × 1.80 = 3.24
- BMI = 82 ÷ 3.24 = 25.3 (overweight category)
Imperial formula
BMI = [weight (lbs) × 703] ÷ [height (inches)]²
The constant 703 converts the imperial result to the standard kg/m² scale.
Worked example: 154 lbs, 5 ft 7 in (67 inches)
- Weight × 703 = 154 × 703 = 108,262
- Height squared = 67 × 67 = 4,489
- BMI = 108,262 ÷ 4,489 = 24.1 (normal weight)
Where does 703 come from?
It's a unit conversion factor. Since 1 kg ≈ 2.205 lbs and 1 m ≈ 39.37 inches, the conversion works out to approximately 703 (2.205 × 39.37² ≈ 703). This ensures the imperial formula produces the same BMI value as the metric one.
The Four BMI Categories
The World Health Organization defines these standard thresholds for adults aged 20 and older:
| Category | BMI range | Example (175 cm / 5′9″) |
|---|---|---|
| Underweight | Below 18.5 | Below 57 kg / 125 lbs |
| Normal weight | 18.5 – 24.9 | 57–76 kg / 125–169 lbs |
| Overweight | 25.0 – 29.9 | 77–91 kg / 169–202 lbs |
| Obese | 30.0 and above | 92+ kg / 203+ lbs |
Some clinical guidelines subdivide the obese category further:
- Class I obesity: 30.0 – 34.9
- Class II obesity: 35.0 – 39.9
- Class III obesity: 40.0 and above
These thresholds are statistical boundaries based on population-level data. They are not individual health diagnoses.
What BMI Gets Right
BMI became widely used for a reason. As a simple, inexpensive screening tool:
- It requires no equipment — only a scale and a tape measure (or their digital equivalents)
- It's easy to calculate — one formula, two inputs, instant result
- It works well at population level — for large groups, average BMI correlates with health outcome trends
- It's consistent — unlike subjective assessments, the same inputs always give the same result
- It's useful for tracking change over time — since the formula doesn't change, your BMI over months or years shows a clear trend
For these reasons, BMI remains a standard first-pass screening metric in healthcare, research, and public health policy.
What BMI Gets Wrong
BMI has well-documented limitations. Because it uses only height and weight, it misses several factors that matter:
1. Muscle vs. fat
BMI cannot tell the difference between muscle mass and fat mass. A kilogram of muscle weighs the same as a kilogram of fat, but they have very different health implications. Muscular athletes routinely score BMI values of 25–30+ while carrying low body fat.
2. Body fat distribution
Where fat is stored matters. Visceral fat (around internal organs) carries different risks than subcutaneous fat (under the skin). Two people with identical BMIs can have very different fat distribution patterns. BMI provides no information about this.
3. Age
Body composition changes with age. Older adults tend to lose muscle mass and gain fat even if their weight stays constant. This means BMI may underestimate body fatness in elderly populations.
4. Sex
On average, women carry more body fat than men at the same BMI. The fixed thresholds do not adjust for this difference.
5. Ethnicity
Research has shown that BMI thresholds may not apply equally across all ethnic groups. Some populations face elevated health risks at lower BMI values, while others may not. Some countries (particularly in Asia) use adjusted thresholds for this reason.
6. Frame size and bone density
People with naturally larger or denser bone structures will weigh more at the same height, potentially pushing their BMI higher without any difference in fat.
BMI Alternatives and Complementary Measurements
Because of BMI's limitations, other measurements can provide additional context:
| Measurement | What it captures | Typical method |
|---|---|---|
| Body fat percentage | Proportion of body mass that is fat | Calipers, bioelectrical impedance, DEXA scan |
| Waist circumference | Central adiposity (abdominal fat) | Tape measure at navel level |
| Waist-to-hip ratio | Fat distribution pattern | Tape measure (waist ÷ hip) |
| Waist-to-height ratio | Central fat relative to body size | Tape measure (waist ÷ height) |
| DEXA scan | Full body composition (fat, muscle, bone) | Specialized X-ray equipment |
No single metric tells the complete story. BMI is one data point among several.
BMI for Children and Teenagers
The BMI formula is the same for all ages, but the interpretation differs for children and teens (ages 2–19). Instead of fixed thresholds, pediatric BMI uses age- and sex-specific percentile charts:
- Below 5th percentile: Underweight
- 5th to 84th percentile: Normal weight
- 85th to 94th percentile: Overweight
- 95th percentile and above: Obese
This approach accounts for the fact that body composition changes naturally as children grow. Growth charts are maintained by organizations such as the WHO and the CDC.
A Brief History of BMI
BMI was first described by Belgian mathematician and statistician Adolphe Quetelet in the 1830s. Originally called the "Quetelet Index," it was designed to study population-level patterns — not to assess individual health.
The term "Body Mass Index" and its modern use as a health screening tool were popularized by American physiologist Ancel Keys in a 1972 paper. Keys explicitly noted that BMI was better suited for population studies than individual diagnosis — a caveat that is still relevant today.
The WHO adopted BMI as a standard classification system in the 1990s, establishing the thresholds (18.5, 25, 30) that are still used worldwide.
Common Misconceptions
"BMI measures body fat"
BMI does not measure body fat. It measures the ratio of total weight to height. Two people with the same BMI can have very different body fat percentages.
"If my BMI is normal, I'm healthy"
BMI is one indicator, not a comprehensive health assessment. A normal BMI does not rule out metabolic conditions, nutritional deficiencies, or other health concerns.
"BMI is useless because it's inaccurate for athletes"
BMI is not designed for individual diagnosis — it's a population-level screening tool. For most people (who are not competitive athletes or bodybuilders), BMI broadly correlates with body fatness. Its limitations are real but do not make it useless; they make it incomplete.
"I need a doctor to calculate my BMI"
BMI requires only height and weight. You can calculate it yourself using the formula, or use a BMI calculator for instant results.
Frequently Asked Questions
What does BMI stand for?
BMI stands for Body Mass Index. It is a numerical value expressing the ratio of weight to height.
What is a normal BMI?
The WHO defines normal BMI as 18.5 to 24.9 for adults. This range is based on population-level statistical associations, not individual health guarantees.
How is BMI calculated?
Metric: BMI = weight (kg) ÷ height (m)². Imperial: BMI = weight (lbs) × 703 ÷ height (inches)².
Is BMI the same for men and women?
The formula is the same. The standard category thresholds (18.5, 25, 30) are also the same. However, at the same BMI, women typically have a higher body fat percentage than men.
Can BMI be too low?
A BMI below 18.5 is classified as underweight. Being significantly underweight can be associated with health concerns, but BMI alone does not determine whether someone's weight is medically problematic.
Does BMI change with age?
The formula does not change, but body composition does. Older adults tend to have less muscle and more fat at the same weight, meaning BMI may understate body fatness in older populations.
Why do some countries use different BMI thresholds?
Research has shown that health risk associations with BMI vary across ethnic groups. Several Asian countries, for example, use lower thresholds (overweight at 23, obese at 25) based on population-specific data.
Is there a better alternative to BMI?
No single metric replaces BMI entirely. Body fat percentage, waist circumference, and waist-to-height ratio each add useful information. The most comprehensive approach combines multiple measurements.
How often should I check my BMI?
BMI is most useful for tracking trends over time rather than obsessing over a single reading. Checking every few months when you weigh yourself is sufficient for most people.
Does muscle really make that big a difference?
Yes, for very muscular individuals. A 180 cm person at 95 kg with 12% body fat (a lean, muscular build) would have a BMI of 29.3 — classified as overweight despite low body fat. This is BMI's most commonly cited limitation.
What is BMI prime?
BMI prime is BMI divided by 25 (the upper limit of "normal"). A BMI prime of 1.0 means a BMI of exactly 25. Values below 1.0 are normal weight; above 1.0 are overweight. It's an alternative way to express the same data.
Can I calculate BMI for children?
The formula is identical, but results are interpreted using age- and sex-specific percentile charts rather than the fixed adult thresholds.
Calculate Your BMI
Use the BMI calculator to calculate your Body Mass Index instantly. Enter height and weight in metric or imperial units, and see your result with a visual gauge showing which category it falls into.
For a more detailed view of body composition, also try the body fat calculator or the ideal weight calculator which uses multiple estimation formulas.