What is BMI and How to Calculate It?
BMI is one of the most widely used health screening tools in the world — used by doctors, gyms, insurance companies, and health apps. But many people don't know exactly what it measures, how it's calculated, or — crucially — what its limitations are.
This guide covers the complete picture: the formula, what your number means, how it differs for different populations, and when BMI alone isn't enough.
What Does BMI Actually Measure?
BMI (Body Mass Index) is a simple numerical value calculated from a person's weight and height. It was developed in the 1830s by Belgian mathematician Adolphe Quetelet and was adopted by the WHO as a population-level obesity screening tool in the 1980s.
Crucially, BMI does not directly measure body fat. It estimates whether a person's weight is proportionate to their height. A high BMI can indicate excess body fat, but it can also reflect high muscle mass — which is why athletes often have "overweight" BMIs despite being very healthy.
The BMI Formula
Metric Formula (kg and metres)
Imperial Formula (pounds and inches)
BMI Categories (WHO Standard)
BMI Reference Table by Height and Weight
| Height | Underweight (<18.5) | Normal (18.5–24.9) | Overweight (25–29.9) | Obese (30+) |
|---|---|---|---|---|
| 5'0" / 152 cm | Below 43 kg | 43–57 kg | 58–68 kg | 69 kg+ |
| 5'3" / 160 cm | Below 47 kg | 47–63 kg | 64–76 kg | 77 kg+ |
| 5'6" / 168 cm | Below 52 kg | 52–70 kg | 71–84 kg | 85 kg+ |
| 5'9" / 175 cm | Below 57 kg | 57–76 kg | 77–91 kg | 92 kg+ |
| 6'0" / 183 cm | Below 62 kg | 62–83 kg | 84–100 kg | 101 kg+ |
| 6'2" / 188 cm | Below 65 kg | 65–88 kg | 89–106 kg | 107 kg+ |
BMI for Different Populations: Important Adjustments
🌏 Asian Population (Including Indian)
Studies show that people of Asian descent have higher body fat percentage at the same BMI compared to Caucasian populations, and face higher health risks at lower BMI values. The WHO and many Asian health authorities recommend adjusted cutoffs:
| Category | Standard WHO BMI | Asian-Adjusted BMI |
|---|---|---|
| Normal | 18.5 – 24.9 | 18.5 – 22.9 |
| Overweight | 25.0 – 29.9 | 23.0 – 27.4 |
| Obese | 30.0+ | 27.5+ |
If you're of South Asian descent (Indian, Pakistani, Bangladeshi, Sri Lankan), it's worth using these adjusted thresholds when interpreting your BMI result.
👶 Children and Teenagers
For children and teens (ages 2–19), BMI is calculated using the same formula but interpreted differently — using age- and sex-specific percentile charts rather than fixed cutoffs. A child's BMI result is expressed as a percentile compared to peers of the same age and sex.
👴 Older Adults (65+)
For older adults, a slightly higher BMI (22–27) may actually be associated with better health outcomes. The relationship between BMI and health risk shifts with age, and being slightly "overweight" by standard BMI can offer protective benefits for elderly people.
Limitations of BMI — What It Doesn't Tell You
- Doesn't distinguish muscle from fat — a bodybuilder with 10% body fat may have a BMI of 30+ (classified "obese")
- Ignores fat distribution — belly fat (visceral fat) is far more dangerous than fat elsewhere; BMI doesn't capture this
- Doesn't account for age — body composition changes significantly with age at the same BMI
- Not accurate for all ethnicities — different populations have different BMI-to-health-risk relationships
- Doesn't reflect fitness level — a fit person with BMI 27 may be healthier than an unfit person with BMI 22
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Frequently Asked Questions
By the standard WHO classification, a BMI of 25.0 marks the beginning of the "overweight" range (25.0–29.9). However, this doesn't necessarily mean you have a health problem — context matters. For Asian populations, the overweight threshold starts at 23.0. Always discuss your individual results with a doctor.
For Indian adults, health experts often recommend using the Asian-adjusted thresholds: normal BMI is 18.5–22.9, overweight is 23.0–27.4, and obese is 27.5+. This is because South Asian populations tend to carry more abdominal fat at lower BMI values compared to Western populations, increasing metabolic risk.
Yes, significantly. Athletes and highly muscular individuals often show BMI in the "overweight" or even "obese" range because muscle is denser than fat. A professional athlete with a BMI of 28 and 8% body fat is far healthier than someone with a BMI of 22 and 30% body fat. For athletes, body fat percentage testing is a much more meaningful metric.
Reducing BMI essentially means reducing body weight relative to height (since height doesn't change in adults). This is achieved through a calorie deficit — eating fewer calories than you burn — combined with regular physical activity. A sustainable goal is 0.5–1 kg of weight loss per week. Consult a nutritionist or doctor before starting any weight loss programme.
Conclusion
BMI is a quick, free, and widely accepted starting point for assessing whether your weight is in a healthy range. It's not perfect — it ignores muscle mass, fat distribution, age, and ethnicity — but it gives you a useful baseline number in seconds.
For Indian adults especially, using the Asian-adjusted thresholds (normal: 18.5–22.9) gives a more accurate health picture than the standard WHO ranges.