BMI Formula: How Body Mass Index Is Calculated
The metric and imperial BMI formulas explained with worked examples, WHO categories, known limitations, and the history behind the index.
The BMI Formula
Body Mass Index (BMI) is a ratio of weight to the square of height. It produces a single dimensionless number that places you within a standardised weight category — without measuring body fat directly. There are two versions depending on unit system.
The constant 703 converts the imperial result to the same scale as the metric formula. Variable definitions:
weight — body mass in kg or lbheight — standing height in m or inches703 — unit conversion factorBMI — dimensionless result (kg/m²)Step-by-Step Calculation
Two worked examples — one metric, one imperial — show every arithmetic step.
Step 1: Square the height → 1.78 × 1.78 = 3.1684
Step 2: Divide weight by height squared → 75 ÷ 3.1684 = 23.67
BMI ≈ 23.7 — Normal weight
Step 1: Square height in inches → 69 × 69 = 4,761
Step 2: Multiply weight by 703 → 703 × 165 = 115,995
Step 3: Divide → 115,995 ÷ 4,761 = 24.36
BMI ≈ 24.4 — Normal weight
Always use consistent units — do not mix metric and imperial in a single calculation. When converting feet and inches to total inches: multiply feet by 12, then add the remaining inches (e.g. 5 ft 9 in = 60 + 9 = 69 in).
BMI Categories and What They Mean
The World Health Organization (WHO) defines six adult weight categories based on BMI. These thresholds apply to adults aged 20 and over, regardless of sex or ethnicity.
| Category | BMI range | Health risk |
|---|---|---|
| Underweight | < 18.5 | Increased (malnutrition, bone density loss) |
| Normal weight | 18.5 – 24.9 | Lowest |
| Overweight | 25.0 – 29.9 | Mildly increased |
| Obese — Class I | 30.0 – 34.9 | Moderate |
| Obese — Class II | 35.0 – 39.9 | Severe |
| Obese — Class III | ≥ 40.0 | Very severe |
Some Asian health guidelines use tighter thresholds: overweight at ≥ 23.0 and obese at ≥ 27.5. These lower cut-offs reflect research showing that metabolic risk — including type 2 diabetes and cardiovascular disease — rises at lower BMI values in East Asian, South Asian, and Pacific Islander populations.
Limitations of BMI
BMI was designed as a population-level screening tool, not a clinical diagnostic. It has several well-documented limitations at the individual level.
Muscle mass: Highly muscular athletes often score as overweight or obese despite very low body fat. A rugby player weighing 105 kg at 1.85 m has a BMI of 30.7, which the formula classifies as obese — yet their body fat percentage may be below 12%.
Fat distribution: BMI tells you nothing about where fat is stored. Visceral fat — around the abdominal organs — carries far greater cardiometabolic risk than subcutaneous fat. Two people with identical BMIs can have very different health outcomes depending on fat distribution.
Age and sex: Older adults tend to carry more body fat at any given BMI than younger adults. Women have a higher proportion of fat than men at any given BMI. The formula adjusts for neither. Alternatives that provide more complete information include body fat percentage (via the US Navy method) and waist-to-hip ratio, which captures abdominal fat distribution directly.
A Brief History of BMI
The formula was derived by Belgian mathematician and statistician Adolphe Quetelet in 1832 as part of his work on "social physics" — a statistical description of human populations. He called it the Quetelet Index and explicitly stated it was not intended to measure individual body fatness.
In 1972, American physiologist Ancel Keys re-examined several weight-for-height indices in a large multi-country study and concluded that Quetelet's formula was the best simple proxy for adiposity in population research. Keys coined the term "Body Mass Index," and the name stuck.
The WHO formally adopted BMI cut-offs in 1995, and they remain the global standard for epidemiological reporting. Despite growing recognition of its limitations, BMI persists in clinical use because it is free, instantaneous, and requires nothing beyond a scale and a measuring tape.
Use the free BMI calculator — enter your height and weight to get your BMI, weight category, and healthy weight range instantly.
Calculate Your BMI →Frequently Asked Questions
Is BMI accurate?
BMI is a useful screening tool at the population level but imprecise for individuals. It cannot distinguish between muscle and fat, and does not account for fat distribution. Muscular athletes often score overweight, while some older adults score normal weight despite excess fat and low muscle mass.
What BMI is considered healthy?
The WHO defines a healthy BMI as 18.5 to 24.9 for adults. Some Asian-population guidelines use a tighter range of 18.5–22.9, as metabolic risk rises at lower BMI values in certain ethnic groups — particularly for type 2 diabetes and cardiovascular disease.
How do I calculate BMI for children?
Children use the same formula as adults, but the result is plotted on age- and sex-specific growth charts (CDC or WHO) rather than compared to fixed thresholds. A child is considered a healthy weight if their BMI falls between the 5th and 85th percentile for their age and sex.
What is BMI prime?
BMI prime is your BMI divided by 24.9 (the top of the normal range). A BMI prime of exactly 1.0 equals a BMI of 24.9. Values below 0.74 are underweight, 0.74–1.0 normal, and above 1.0 overweight or obese. It makes cross-population comparisons easier when different thresholds apply.
Evidence & Methodology
How This Page Is Grounded
Method
Explains and demonstrates BMI as kilograms divided by metres squared, including the equivalent imperial conversion.
Important limitation: BMI is a screening measure and does not directly measure body composition or diagnose health.
Primary Sources
- Obesity and overweight World Health Organization
- Calculate your body mass index NHS
Quality Checks
Both unit formulas and category examples are checked against published adult guidance.
See how sources are selected and corrections are handled in our Editorial & Calculation Methodology, and which automated checks this page has to pass in How We Test.