How to Calculate BMI

Body Mass Index (BMI) is a simple number derived from your weight and height. It is used by clinicians and public health organisations worldwide as a first-pass screening tool for weight-related health risks. This guide explains the formula, shows worked examples, and covers what the result means.

The BMI Formula

BMI is defined as body weight in kilograms divided by the square of height in metres. The result is a unit-free number.

BMI = \frac{weight (kg)}{height (m) ^{2}}

When working with imperial measurements (pounds and inches), a conversion factor of 703 is applied:

BMI = \frac{703 \times weight (lbs)}{height (inches) ^{2}}

Step-by-Step Calculation (Metric)

Measure your weight in kilograms.
Measure your height in metres (e.g. 175 cm = 1.75 m).
Square your height: 1.75 × 1.75 = 3.0625
Divide your weight by that value: 70 ÷ 3.0625 ≈ 22.86
Look up your result in the WHO category table.

Worked Example — Metric

Person: 70 kg, 175 cm tall.

BMI = \frac{70}{( 1.75 ) ^{2}} = \frac{70}{3.0625} \approx 22.9

ℹResult: BMI 22.9 — falls in the Normal weight range (18.5–24.9).

Worked Example — Imperial

Person: 154 lbs, 5 ft 9 in tall. First convert height: 5 × 12 + 9 = 69 inches.

BMI = \frac{703 \times 154}{6 9 ^{2}} = \frac{108262}{4761} \approx 22.7

ℹResult: BMI 22.7 — also Normal weight. Both unit systems produce equivalent results.

BMI Weight Categories (WHO)

The World Health Organization defines six adult BMI categories:

Category	BMI Range	Health Risk
Underweight	< 18.5	Increased risk (nutritional deficiency, osteoporosis)
Normal weight	18.5 – 24.9	Lowest risk
Overweight	25.0 – 29.9	Moderately increased risk
Obese — Class I	30.0 – 34.9	High risk
Obese — Class II	35.0 – 39.9	Very high risk
Obese — Class III	≥ 40.0	Extremely high risk

Limitations of BMI

⚠BMI is a population-level screening tool. It does not directly measure body fat and should never be used alone to diagnose health conditions.

Muscle vs fat — a heavyweight athlete and a sedentary person can share the same BMI despite very different body compositions.
Age — older adults tend to carry more fat at the same BMI; children require age-specific percentile charts.
Sex — women typically carry more fat than men at the same BMI.
Ethnicity — some populations face higher metabolic risk at lower BMI values; adjusted thresholds exist for South and East Asian populations.
Fat distribution — BMI ignores where fat is stored; abdominal fat carries higher cardiovascular risk than fat stored elsewhere.

BMI Calculator — Calculate your BMI instantly — metric and imperial
BMI Formula — Formula, variable definitions, and derivation

Frequently Asked Questions

What is considered a healthy BMI?

The WHO defines 18.5–24.9 as the normal weight range for adults. Values below 18.5 indicate underweight; 25–29.9 is overweight; 30 and above falls within the obese range.

Is BMI accurate for athletes and bodybuilders?

No. Athletes with high muscle mass often register as overweight or obese by BMI despite having very low body fat percentages. For muscular individuals, methods such as skinfold measurements or DEXA scans are more informative.

Can I use BMI to track weight loss progress?

BMI can give a rough sense of direction, but it changes slowly. Tracking actual weight, waist circumference, or body fat percentage typically gives a clearer picture of progress.

What is the BMI formula for children?

The same mathematical formula applies to children, but the result is interpreted using age- and sex-specific BMI-for-age percentile charts rather than the adult category thresholds.

Why does the imperial formula use 703?

The factor 703 converts pounds per square inch to the metric equivalent (kg/m²). It comes from the unit conversion: 1 kg = 2.2046 lbs and 1 m = 39.3701 in, giving 1 / (2.2046 / 39.3701²) ≈ 703.07.