BMI Calculator
Calculate your Body Mass Index using the WHO standard formula.
What is BMI?
Body Mass Index (BMI) is a numerical value derived from a person's height and weight. It was developed by Belgian mathematician Adolphe Quetelet in the 1830s and is now the most widely used screening tool by doctors to identify weight-related health risks in adults.
A single BMI number places you into one of four WHO-defined categories: Underweight, Normal weight, Overweight, or Obese. While it is not a diagnostic tool, it is a useful first indicator for conditions such as cardiovascular disease, type 2 diabetes, and hypertension.
BMI Formula
Metric: BMI = Weight (kg) ÷ Height (m)²
Convert height to metres first — e.g. 170 cm = 1.70 m
Imperial: BMI = [Weight (lbs) ÷ Height (in)²] × 703
Convert height to total inches first — e.g. 5 ft 7 in = 67 in
The factor 703 converts the imperial result to the same scale as the metric formula. Both equations produce an identical BMI value for the same person.
BMI Prime
BMI Prime is BMI divided by 25 (the upper limit of the normal range). It expresses your BMI as a ratio to the normal upper boundary, making it easier to see how far above or below normal you are:
- Less than 0.74 — Underweight
- 0.74 to 1.00 — Normal weight
- 1.00 to 1.20 — Overweight
- Greater than 1.20 — Obese
Ponderal Index
The Ponderal Index (PI), also known as Rohrer's Index, is similar to BMI but uses height cubed instead of squared, making it less biased for very tall or very short individuals:
A normal Ponderal Index falls between 11 and 15 kg/m³. Values below 11 suggest underweight and above 17 suggest obesity, though thresholds can vary by population.
Limitations of BMI
- Muscle vs fat: Athletes and bodybuilders often have high BMI despite low body fat because muscle is denser than fat.
- Age: Older adults naturally lose muscle mass, so a "normal" BMI can mask excess body fat.
- Gender: Women generally carry more body fat than men at the same BMI.
- Ethnicity: South Asians tend to have higher body fat at the same BMI as Europeans. WHO recommends a lower overweight threshold of 23 for Asian populations.
- Children: BMI in under-18s must be interpreted using age- and sex-specific growth charts, not adult categories.
Frequently Asked Questions
What is a healthy BMI for adults?
According to WHO, a BMI between 18.5 and 24.9 is considered normal. Below 18.5 is underweight, 25–29.9 is overweight, and 30 or above is obese.
Is BMI accurate for athletes?
No. Muscle is denser than fat, so athletes may have a high BMI despite low body fat. In such cases, waist-to-height ratio or body fat percentage is a better indicator.
What is a healthy BMI for Indians?
Research suggests Indians tend to have higher body fat at lower BMI compared to Western populations. Many health organisations recommend using 23 as the overweight threshold for South Asians.
BMI Categories (WHO)
- Underweight — < 18.5
- Normal — 18.5 to 24.9
- Overweight — 25 to 29.9
- Obese — ≥ 30