What is BMI and what does it measure?

BMI (Body Mass Index) is a numerical value calculated from a person's weight and height. It provides a simple, widely-used screening tool to categorise whether a person is underweight, normal weight, overweight, or obese. Formula: BMI = weight (kg) / height (m)² BMI was developed by Belgian mathematician Adolphe Quetelet in the 1830s and was originally called the Quetelet Index. It was adopted by the World Health Organisation (WHO) as a practical population-level obesity indicator in the 1990s. BMI is a screening tool, not a diagnostic tool. A high BMI may indicate high body fat, but BMI does not directly measure body fat percentage. Two people with the same BMI can have very different body compositions — a muscular athlete and a sedentary individual may share the same BMI despite very different health profiles.

What is a healthy BMI range?

The WHO defines healthy BMI as 18.5 to 24.9 for adults aged 18 and over. Full WHO classification: • < 16.0 — Severely Underweight • 16.0–18.4 — Underweight • 18.5–24.9 — Normal weight • 25.0–29.9 — Overweight • 30.0–34.9 — Obese Class I (Moderate) • 35.0–39.9 — Obese Class II (Severe) • ≥ 40.0 — Obese Class III (Very Severe / Morbid) Note: Some Asian health organisations use lower thresholds — 23.0 for overweight and 27.5 for obesity — because Asian populations tend to have higher body fat percentage at lower BMIs than Caucasian populations. This calculator uses standard WHO thresholds.

What is BMI Prime and how is it different from BMI?

BMI Prime is the ratio of a person's BMI to the upper limit of the Normal range (25). BMI Prime = BMI / 25 Interpretation: • 1.00 — Overweight or obese BMI Prime is unitless and allows quick intuitive understanding — a person with BMI Prime of 1.20 is 20% above the upper normal limit, regardless of the underlying BMI scale. Example: BMI 30 → BMI Prime = 30/25 = 1.20 (20% over normal upper limit) BMI 22 → BMI Prime = 22/25 = 0.88 (within normal range)

What are the limitations of BMI?

BMI has several well-known limitations: 1. Muscle vs fat: BMI cannot distinguish between muscle mass and fat mass. A muscular athlete like a bodybuilder may have a 'overweight' BMI despite very low body fat. 2. Age: Older adults tend to have more body fat at the same BMI than younger adults, and some excess weight may be protective in older age. 3. Sex: Women typically have more body fat than men at the same BMI. Some researchers argue for sex-specific thresholds. 4. Ethnicity: Asian populations have higher health risks at lower BMIs. South Asian guidelines often use BMI 23 as the overweight threshold. 5. Muscle loss: Extremely thin individuals can have a normal BMI due to muscle wasting rather than healthy lean mass. 6. Height extremes: BMI underestimates adiposity in short people and overestimates in tall people — the Ponderal Index (kg/m³) is more accurate for these extremes. Alternatives that better reflect body composition include waist circumference, waist-to-hip ratio, body fat percentage (DEXA scan), and waist-to-height ratio.

Does BMI apply to children and teenagers?

BMI is calculated differently for children and teens aged 2–19. Instead of fixed thresholds (like 18.5 for underweight in adults), children's BMI is compared to age- and sex-specific percentile charts from the CDC or WHO. Children's BMI categories (CDC): • Below 5th percentile — Underweight • 5th to 85th percentile — Healthy weight • 85th to 95th percentile — Overweight • 95th percentile or above — Obese This is called 'BMI-for-age' and accounts for the fact that children's body fat levels change with age, and girls and boys differ in body composition at different stages of development. This calculator is designed for adults aged 18 and over. For children, use the CDC BMI Percentile Calculator at cdc.gov/healthyweight.

What is the Ponderal Index and when should I use it?

The Ponderal Index (PI) divides weight by the cube of height rather than the square: PI = weight (kg) / height (m)³ Normal range: approximately 11–15 kg/m³ The Ponderal Index was proposed as a more accurate alternative to BMI for people at height extremes: • BMI underestimates adiposity in shorter people (BMI scales too favourably) • BMI overestimates adiposity in taller people (BMI penalises height) The cube relationship corrects for this — a taller person has more volume, so dividing by m³ gives a fairer comparison. In practice, BMI is more widely used and has more research behind its health risk thresholds. Ponderal Index is most useful as a supplementary check for people who are significantly above or below average height (e.g. under 155cm or over 190cm).

BMI Calculator — Body Mass Index

The BMI Calculator computes your Body Mass Index from weight and height in both metric (kg/cm) and imperial (lbs/ft/in) units. Results include BMI value, WHO category (Severely Underweight through Obese Class III), BMI Prime (ratio to normal upper limit of 25), Ponderal Index for additional accuracy, healthy weight ran..

YOUR MEASUREMENTS

WEIGHT (kg)

HEIGHT

= 5ft 8.9in

YOUR RESULTS

BODY MASS INDEX

22.9

Normal weight

BMI PRIME

0.91

Normal = 0.74–1.00

PONDERAL INDEX

13.1

kg/m³

HEALTHY WEIGHT RANGE

56.7 – 76.3 kg

Within the healthy weight range. Maintain with balanced diet and regular exercise.

BMI GAUGE

YOUR POSITION

FULL SUMMARY

BMI22.9

CategoryNormal weight

BMI Prime0.91

Ponderal Index13.1 kg/m³

Healthy range56.7–76.3 kg

Min healthy weight56.7 kg

Max healthy weight76.3 kg

WHO BMI CATEGORIES

Severely Underweight

< 16.0

Underweight

16.0–18.4

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

Created with❤️byeaglecalculator.com

HOW TO USE

1
Select your unit system — Metric (kg and cm) or Imperial (lbs, feet and inches). Both use the same underlying formula, with a conversion factor of 703 applied in the Imperial version. Enter your weight: use the preset buttons for common values or type your exact weight. Body weight should be measured without heavy clothing, ideally in the morning.
2
Enter your height. In metric mode, enter your height in centimetres (e.g. 175 cm). In imperial mode, enter feet and inches separately (e.g. 5 ft 9 in). The calculator shows the converted equivalent so you can verify the input. Height should be measured standing straight against a wall without shoes.
3
Read the BMI result: the large number is your BMI, colour-coded by category — green for Normal, yellow for Overweight, orange/red for Obese ranges, and blue for Underweight. Below the BMI you will see your WHO category, BMI Prime (ratio to the normal upper limit of 25), and Ponderal Index for additional perspective.
4
Check your healthy weight range — the calculator shows the minimum and maximum weight for your height that falls within the Normal BMI range (18.5–24.9). If you are outside the normal range, it shows exactly how many kg or lbs you would need to gain or lose to reach a healthy weight.
5
Review the BMI gauge (half-dial with needle) and the position bar — these visuals show where you fall across all seven WHO categories from Severely Underweight to Obese Class III. The highlighted segment on the gauge and the marker on the position bar show your current BMI at a glance.

WORKED EXAMPLE

BMI calculation for two people of the same height: METRIC EXAMPLE: Person A — 70 kg, 175 cm • Height in metres: 175 / 100 = 1.75 m • Height squared: 1.75² = 3.0625 m² • BMI = 70 / 3.0625 = 22.9 • Category: Normal weight • BMI Prime: 22.9 / 25 = 0.92 • Healthy range for 175cm: 18.5 × 3.0625 to 24.9 × 3.0625 = 56.7 to 76.3 kg Person B — 100 kg, 175 cm • BMI = 100 / 3.0625 = 32.7 • Category: Obese Class I • BMI Prime: 32.7 / 25 = 1.31 • To reach healthy weight: lose 100 − 76.3 = 23.7 kg IMPERIAL EXAMPLE: Person C — 154 lbs, 5 ft 9 in (= 69 inches total) • Height squared: 69² = 4,761 in² • BMI = 703 × 154 / 4,761 = 108,262 / 4,761 = 22.7 • Category: Normal weight • Healthy range for 69": 18.5 × 4,761/703 to 24.9 × 4,761/703 = 125 to 169 lbs

REFERENCE FORMULAS

FORMULA REFERENCE TABLE

Name	Formula	Description
BMI (Metric)	BMI = weight(kg) / height(m)²	Divide weight in kg by height in metres squared
BMI (Imperial)	BMI = 703 × weight(lbs) / height(in)²	Multiply weight in pounds by 703, divide by height in inches squared
Height in metres	h(m) = h(cm) / 100	Convert centimetres to metres before squaring
BMI Prime	BMI Prime = BMI / 25	Ratio of BMI to upper limit of normal (25). Values >1.00 indicate overweight
Ponderal Index	PI = weight(kg) / height(m)³	More accurate than BMI for very tall or short individuals. Normal: 11–15 kg/m³
Healthy weight	w = BMI × height(m)²	Rearranged BMI formula: weight(kg) for any target BMI at your height

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Last updated: April 28, 2026 · Formula verified by EagleCalculator team · Source: WHO Global Database on Body Mass Index · Eagle-eyed accuracy for every calculation.