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How BMI Is Calculated — Formula, Categories & Limitations
BMI is a simple screening tool. Understanding the formula and its limits helps you interpret your number correctly.
The BMI Formula
Worked Example: 70 kg, 170 cm tall
BMI Categories (WHO Standard)
| Category | BMI Range |
|---|---|
| Underweight | Below 18.5 |
| Healthy 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 and above |
Why BMI Has Limits
- Muscle vs fat: Athletes can have high BMI with low body fat
- Age: Older adults may have normal BMI but high body fat due to muscle loss
- Ethnicity: Asian populations face higher health risks at lower BMI values
- Sex: Women carry ~10% more body fat than men at the same BMI
- Fat distribution: BMI cannot detect visceral (abdominal) fat
Healthy BMI Range by Age — Adults
The standard healthy range is 18.5–24.9 for all adults, but risk interpretation shifts with age. Guidance for adults 65+ suggests a slightly higher BMI may be protective.
| Age Group | Healthy BMI Range | Notes |
|---|---|---|
| 18–24 | 18.5 – 24.9 | Standard WHO range applies |
| 25–34 | 18.5 – 24.9 | Standard range; monitor waist circumference |
| 35–44 | 18.5 – 24.9 | Standard range; metabolic risk rises toward 25+ |
| 45–54 | 18.5 – 24.9 | Waist circumference becomes an important co-indicator |
| 55–64 | 18.5 – 27.0 | Slightly higher BMI may be acceptable; discuss with doctor |
| 65+ | 22.0 – 27.0 | Higher BMI may be protective against osteoporosis and frailty |
Source: WHO Global Database on Body Mass Index; CDC Healthy Weight guidelines. Last reviewed June 2026.
Healthy BMI by Ethnicity
Different ethnic groups carry health risk at different BMI thresholds. Our calculator applies these automatically when you select your ethnicity.
| Ethnicity | Overweight threshold | Obese threshold | WHO recommendation |
|---|---|---|---|
| General / White / Black / Hispanic | BMI 25.0 | BMI 30.0 | Standard cutoffs |
| East & South-East Asian | BMI 23.0 | BMI 27.5 | Revised WHO 2004 cutoffs |
| South Asian (Indian subcontinent) | BMI 23.0 | BMI 27.5 | Revised WHO 2004 cutoffs |
BMI Calculator — Frequently Asked Questions
Answers to the most common questions about BMI, healthy weight ranges, and how to interpret your result.