How to Use BMI Correctly (And When Not To)

BMI (Body Mass Index) was invented in the 19th century by a Belgian mathematician, not a doctor. It is a quick screening tool, not a definitive health measure. This guide explains how to use BMI correctly, when it is misleading, the WHO Asian / South Asian cutoffs that most articles ignore, and worked examples for typical South Asian builds.

What BMI tells you (the standard WHO cutoffs)

BMI is calculated as weight (kg) divided by height (m) squared. The standard WHO categories for adults:

It is useful as a population-level screening tool and as a quick personal sanity check, but it is not a diagnosis.

Asian / South Asian BMI cutoffs (different and important)

In 2004, WHO recognised that South Asians (Pakistanis, Indians, Bangladeshis, Sri Lankans, Nepalis) and other Asian populations develop diabetes, hypertension, and cardiovascular disease at lower BMI levels than European populations. WHO published recommended Asian-specific cutoffs:

If you are Pakistani, Indian, or another South Asian ethnicity, your doctor will likely use the lower thresholds. A Pakistani with BMI 24 is "normal" by global WHO standards but already in the overweight risk zone for South Asians. This is the single most underrepresented fact in the standard BMI explainers ranking on Google.

Reference: Limits of BMI for South Asian populations, NIH.

Worked example: BMI of 70 kg at 5 feet 7 inches

5'7" in metres: 5 feet 7 inches is 67 inches; 67 × 0.0254 = 1.7018 m. Squared: 2.896.

BMI = 70 / 2.896 = 24.2

By global WHO cutoffs this is normal (just under the 25 overweight line). By Asian WHO cutoffs this is overweight (above 23). For a Pakistani man or woman at this build, modest weight loss may still be useful even though the global BMI looks fine.

Worked example: BMI of 40 kg at 150 cm

Height in metres: 1.50 m. Squared: 2.25.

BMI = 40 / 2.25 = 17.8

Underweight by both global and Asian cutoffs. This person needs more calories, micronutrients, and likely a medical workup to rule out underlying causes, not weight loss advice.

What is a BMI of 27 kg/m²?

A BMI of 27: - By global WHO cutoffs: overweight (above 25, below 30) - By Asian WHO cutoffs: overweight (above 23, below 27.5 obese line)

So a BMI of 27 sits in the overweight range under both classifications, but it is much closer to the obese threshold for South Asians. For a Pakistani or Indian adult, a BMI of 27 is a meaningful health-risk marker, not just "a bit above ideal."

What BMI does NOT measure

BMI is a single ratio of weight to height squared. It does not capture:

• Muscle vs fat composition (a bodybuilder and a sedentary person can share a BMI)
• Where fat is stored (visceral fat around organs is more dangerous than subcutaneous fat on hips and thighs)
• Bone density
• Frame size or skeletal proportions
• Age-related body composition changes (muscle loss with same weight)
• Ethnic differences in body composition (the basis for Asian-specific cutoffs)

A muscular athlete and an out-of-shape person can show the same BMI with very different health risks.

When BMI is misleading

• Athletes and bodybuilders: often categorised as overweight or obese despite very low body fat percentage. Pakistani cricketers and weightlifters routinely show BMI above 27 with single-digit body fat.
• Elderly: muscle loss with age means BMI can stay stable while body composition shifts toward more fat. A "normal" BMI 65-year-old can still be metabolically unhealthy.
• Children and teenagers: paediatric BMI uses age-and-sex percentile charts, not adult cutoffs. Do not apply adult ranges to a 12-year-old.
• Pregnancy: pregnancy weight changes are physiological, not adiposity. BMI is not used during pregnancy.
• Amputees and people with limb-length differences: skews the height-vs-weight ratio.
• Different ethnic groups: as covered above, South Asians need lower cutoffs. Some Pacific Islander and African populations may need higher.

BMI for female vs male

BMI categories are the same regardless of gender. However, women typically carry more body fat than men at the same BMI (this is biological and not unhealthy). Two practical implications:

• Women athletes can be classified as overweight while having lower body fat than the average sedentary man with a "normal" BMI.
• Men carrying excess weight as visceral / abdominal fat (apple shape) often face higher cardiometabolic risk than women carrying the same excess as gluteofemoral fat (pear shape) at the same BMI.

For a complete picture, women and men should both look at BMI alongside waist circumference and body fat percentage rather than relying on BMI alone.

Better metrics to combine with BMI

• Waist circumference: South Asian-specific thresholds are tighter than global ones, for South Asian men, waist circumference above about 90 cm signals elevated risk. For South Asian women, above about 80 cm. (Check current official guidance from your local cardiology / endocrinology society for exact published thresholds.)
• Waist-to-hip ratio: a different shape metric. Ratios above roughly 0.9 in men and 0.85 in women are commonly cited as elevated-risk thresholds in WHO guidance.
• Body fat percentage: calculate via our Body Fat Calculator for a more direct adiposity measure.
• Resting heart rate, blood pressure, fasting glucose, lipid panel: combine with BMI for a real cardiometabolic-risk picture.

Use BMI as one signal among several rather than as a verdict.

How to use BMI well

1. Calculate it once a month at most, not daily. Weight fluctuates with hydration, food, and time of day.
2. Use the cutoff appropriate for your ethnicity (Asian if South Asian).
3. Combine with waist circumference and how you feel, not just the number.
4. Talk to a doctor if you are far outside the normal range or if metrics conflict.
5. Track the trend across months, not single readings.

Use our calculator

Try our BMI Calculator for a quick estimate. Run our Body Fat Calculator alongside for a more nuanced picture, and our BMR Calculator and Calorie Calculator if you want to design a calorie plan around the result.