Body Mass Index (BMI) is the most widely used screening tool for weight status. Despite its limitations, it remains the standard starting point in clinical and public health settings worldwide. Here is how it is calculated, what the numbers mean, and where it falls short.
The BMI Formula
Metric:
BMI = weight (kg) / height (m)²
Imperial:
BMI = (weight (lbs) × 703) / height (inches)²
Example (metric): Weight 78 kg, height 1.76 m
BMI = 78 / (1.76)² = 78 / 3.0976 = 25.2
Example (imperial): Weight 172 lbs, height 5'9" (69 inches)
BMI = (172 × 703) / (69)² = 120,916 / 4,761 = 25.4
BMI Categories (WHO Standard)
| BMI Range | Category | |-----------|----------| | Below 18.5 | Underweight | | 18.5 – 24.9 | Normal weight | | 25.0 – 29.9 | Overweight | | 30.0 – 34.9 | Obese (Class I) | | 35.0 – 39.9 | Obese (Class II) | | 40.0 and above | Severely obese (Class III) |
These thresholds apply to adults aged 18 and over. Different thresholds apply for children and teenagers.
Adjusted Thresholds for South Asian Populations
Large-scale studies show that people of South Asian descent have higher cardiometabolic risk at lower BMI values. Organisations including the WHO and NHS use adjusted action points:
| Category | General population | South Asian | |----------|-------------------|-------------| | Increased risk | BMI ≥ 25 | BMI ≥ 23 | | High risk | BMI ≥ 30 | BMI ≥ 27.5 |
BMI for Children and Teenagers
Children's BMI is age- and sex-specific because body composition changes significantly with development. BMI-for-age is plotted on growth charts and expressed as a percentile:
| Percentile | Category | |-----------|----------| | Below 5th | Underweight | | 5th – 84th | Healthy weight | | 85th – 94th | Overweight | | 95th and above | Obese |
A child with the same BMI as an adult may fall in a completely different category — you cannot apply adult cut-offs to children.
The Limitations of BMI
BMI is a useful population-level screening tool but a poor individual diagnostic.
It cannot distinguish muscle from fat. A heavily muscled athlete and a sedentary person of the same height and weight will have identical BMIs but very different body compositions. Many elite athletes score "overweight" or "obese" by BMI.
It ignores where fat is stored. Abdominal fat (visceral fat, measured by waist circumference) carries much higher cardiovascular risk than fat stored in the hips and thighs. Two people can have the same BMI with very different risk profiles.
It varies by ethnicity. As noted above, the standard cut-offs underestimate risk in South Asian populations and may overestimate risk in some Black populations.
It says nothing about fitness. A metabolically healthy person with high BMI (due to muscle) may have lower cardiovascular risk than a thin, sedentary person.
More Informative Alternatives
Waist-to-height ratio: Divide waist circumference by height. Values above 0.5 indicate increased health risk. Less affected by muscle mass than BMI.
Waist circumference alone: NICE guidelines suggest increased risk above 94 cm (men) and 80 cm (women); high risk above 102 cm (men) and 88 cm (women).
Body fat percentage: Measured via DEXA scan, hydrostatic weighing, or less accurately by skinfold callipers or bioelectrical impedance. Provides direct measurement of fat versus lean mass.
Waist-to-hip ratio: Divides waist circumference by hip circumference. Captures body shape and fat distribution.
Healthy BMI and Weight Range
To find the healthy weight range for a given height, rearrange the formula:
Healthy weight range = BMI range × height (m)²
For a healthy BMI of 18.5–24.9 at height 1.75 m:
Lower: 18.5 × (1.75)² = 18.5 × 3.0625 = 56.7 kg
Upper: 24.9 × (1.75)² = 24.9 × 3.0625 = 76.2 kg
Healthy weight range: 56.7 – 76.2 kg (8.9 – 12.0 stone)
How to Use BMI Appropriately
BMI is best used as a:
- First-line screening tool in population surveys
- Starting point for a clinical conversation, not a diagnosis
- Longitudinal tracker of one person's weight trend over time
It is not appropriate as the sole determinant of health, fitness, or obesity-related risk. Clinical assessment should include waist circumference, blood pressure, lipid profile, and glucose levels.
Use our BMI Calculator to find your BMI instantly, and our Ideal Weight Calculator to see the healthy weight range for your height.