Body Mass Index (BMI) is a number calculated from height and weight using the formula BMI = weight(kg) / height(m)². It was developed by Adolphe Quetelet in the 19th century as a population-level statistical tool — not as a clinical diagnostic for individuals.

What are the WHO BMI categories?

The World Health Organization defines four primary ranges: underweight (BMI < 18.5), normal weight (18.5–24.9), overweight (25–29.9), and obese (≥30). Obesity is further divided into Class I (30–34.9), Class II (35–39.9), and Class III (≥40).

BMI is a population-level screening tool, not a precise measure of health for any individual. It does not distinguish between fat and muscle mass, does not account for fat distribution, and has known limitations for athletes, elderly populations, and certain ethnic groups.

What is the imperial BMI formula?

The imperial formula is BMI = 703 × weight(lbs) / height(in)². The factor 703 converts the result to the same scale as the metric formula.

How do I calculate BMI in metric units?

Divide your weight in kilograms by your height in meters squared. For example, a person who is 70 kg and 1.75 m tall: BMI = 70 / (1.75)² = 70 / 3.0625 ≈ 22.9.

Why does BMI underestimate body fat in some people?

BMI uses only height and weight, so it cannot distinguish dense muscle mass from fat. Athletes with high muscle mass may have an elevated BMI that incorrectly suggests overweight status. Conversely, someone with low muscle mass and high body fat may have a BMI in the normal range.

BMI Calculator — Body Mass Index

Q: Is BMI accurate?

BMI is a population-level screening tool, not a precise measure of health for any individual. It does not distinguish between fat and muscle mass, does not account for fat distribution, and has known limitations for athletes, elderly populations, and certain ethnic groups.

Q: What is the imperial BMI formula?

The imperial formula is BMI = 703 × weight(lbs) / height(in)². The factor 703 converts the result to the same scale as the metric formula.

Q: How do I calculate BMI in metric units?

Divide your weight in kilograms by your height in meters squared. For example, a person who is 70 kg and 1.75 m tall: BMI = 70 / (1.75)² = 70 / 3.0625 ≈ 22.9.

Q: Why does BMI underestimate body fat in some people?

BMI uses only height and weight, so it cannot distinguish dense muscle mass from fat. Athletes with high muscle mass may have an elevated BMI that incorrectly suggests overweight status. Conversely, someone with low muscle mass and high body fat may have a BMI in the normal range.

Body Mass Index (BMI) is a number derived from height and weight using a formula developed by the Belgian statistician Adolphe Quetelet in the 1830s. BMI = weight (kg) ÷ height (m)². It is a population-level screening metric — not a diagnostic tool for individual health. The World Health Organization publishes standard classification ranges used globally.

The formula

BMI (metric)   = weight (kg) ÷ height (m)²
BMI (imperial) = 703 × weight (lbs) ÷ height (in)²

The factor 703 converts the imperial result to the same scale as the metric formula. Both inputs and outputs are identical — only the unit system differs.

WHO classification table

The World Health Organization (Technical Report Series 894, 2000) defines these ranges for adults 18 and older:

Category	BMI range
Underweight	Below 18.5
Normal weight	18.5 – 24.9
Overweight	25.0 – 29.9
Obese — Class I	30.0 – 34.9
Obese — Class II	35.0 – 39.9
Obese — Class III	40.0 and above

These thresholds were established for populations of European descent. The WHO also notes that some Asian populations may have higher health risks at lower BMI values.

Practical examples

Example 1 — Metric: Weight 75 kg, height 1.78 m. BMI = 75 ÷ (1.78)² = 75 ÷ 3.1684 = 23.7 — Normal weight.

Example 2 — Imperial: Weight 165 lbs, height 5 ft 9 in (69 inches total). BMI = 703 × 165 ÷ (69)² = 116,000 ÷ 4,761 = 24.4 — Normal weight.

Example 3 — Athlete with high muscle mass: A 95 kg (209 lb) bodybuilder at 1.80 m (5 ft 11 in) has a BMI of 29.3 — classified as Overweight — even though body fat percentage may be in the athletic range. BMI does not account for lean mass.

Common mistakes

Using height in centimeters without converting. The metric formula requires height in meters. A height of 175 cm must be entered as 1.75 m. Entering 175 gives a BMI of 0.0024 — not a useful result.

Using imperial inputs with the metric formula. Entering pounds and feet into BMI = kg/m² gives a meaningless number. Use the imperial formula (703 × lbs/in²) or convert to metric first.

Treating BMI as a health score. BMI is a ratio of weight to height squared. It does not measure body fat, muscle mass, bone density, or fat distribution — all of which affect health more precisely than mass alone.

What BMI does not measure

BMI's known limitations are well-documented in clinical literature:

Limitation	Who is affected
Cannot distinguish fat from muscle	Athletes, bodybuilders, strength-trained individuals
Does not account for fat distribution	People with centrally distributed fat (higher metabolic risk)
Age-related changes not reflected	Older adults who lose muscle mass while maintaining weight
Ethnic thresholds differ	South Asian, East Asian, and other populations
Does not apply to children and teens	Under 18 — requires age- and sex-specific percentile charts

International and regional variations

System / Region	Notes
WHO global standard	Overweight ≥25, Obese ≥30. Published in TRS 894 (2000).
WHO Asia-Pacific (2004)	Overweight ≥23, Obese ≥27.5 — lower thresholds for Asian populations.
US CDC	Uses same thresholds as WHO global; provides separate percentile charts for ages 2–19.
UK NHS	Uses WHO global thresholds; also notes lower thresholds for South Asian, Chinese, and Black African/Caribbean groups.

For a companion reference covering body fat percentage ranges by sex (ACSM), measurement method accuracy, and the Navy formula, see the Body Fat Percentage Reference Chart →.

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