BMI (Body Mass Index)
Use the BMI (Body Mass Index) Calculator → Try the BMI (Body Mass Index) Quiz →BMI (Body Mass Index) is a weight-to-height ratio used to classify adults as underweight, normal weight, overweight, or obese. It is the most widely used population-level screening tool for weight status.
- BMI = weight (kg) / height (m)²; the WHO defines four categories: underweight (<18.5), normal weight (18.5-24.9), overweight (25-29.9), and obese (≥30).
- BMI cannot distinguish fat mass from muscle mass - a heavily muscled athlete may appear overweight despite very low body fat, severely limiting individual-level interpretation.
- For adults 65 and over, the optimal BMI range shifts upward to approximately 23-27.5, associated with better survival, bone density, and resilience during illness.
- South Asian and East Asian populations have higher cardiometabolic risk at lower BMI values; some guidelines recommend an overweight threshold of 23 (instead of 25) for these groups.
- BMI is a population-level screening tool; pair it with waist circumference, body fat percentage, or DEXA for a complete individual assessment.
$$\text{BMI} = \frac{\text{Weight (kg)}}{\text{Height (m)}^2}$$
What is BMI (Body Mass Index)?
Body Mass Index (BMI) is a screening metric that estimates whether a person's weight is appropriate for their height. It is calculated by dividing body weight in kilograms by the square of height in meters: $$\text{BMI} = \frac{\text{weight (kg)}}{\text{height (m)}^2}$$
The World Health Organization classifies BMI into four primary categories: below 18.5 is underweight, 18.5–24.9 is normal weight, 25.0–29.9 is overweight, and 30.0 or above is obese. Obesity is further subdivided into Class I (30–34.9), Class II (35–39.9), and Class III (≥40).
For adults aged 65 and over, research consistently shows that the optimal BMI range shifts upward to approximately 23–27.5. A slightly higher BMI in older adults is associated with better survival outcomes, greater bone density, and improved resilience during illness - the standard thresholds do not apply directly to this population.
BMI (Body Mass Index) explained to a beginner
BMI is a simple formula: divide your weight in kilograms by your height in meters squared. A person who is 70 kg and 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 70 ÷ 3.06 ≈ 22.9 - "normal weight" by WHO standards.
The formula was invented as a quick population-level screen, not a precise individual health tool - which is why a stocky, muscular rugby player and a sedentary person with identical numbers can have completely different health profiles. Think of BMI as the first signal in a longer investigation, not the final answer.
When to use BMI (Body Mass Index)
Use BMI as a rapid, population-level screening tool to flag individuals who may warrant further metabolic risk assessment. It is appropriate for tracking weight trends in clinical and epidemiological settings. Do not use BMI alone to diagnose obesity or assess cardiovascular risk - pair it with waist circumference, body fat percentage, or DEXA for a complete picture.
Worked examples for BMI (Body Mass Index)
This table quickly gives you the overview you need to understand BMI (Body Mass Index) and its most important comparisons.
| Category | BMI range | Clinical interpretation |
|---|---|---|
| Underweight | < 18.5 | Risk of malnutrition, osteoporosis, immune suppression |
| Normal weight | 18.5 – 24.9 | Lowest all-cause mortality risk in general adults |
| Overweight | 25.0 – 29.9 | Increased risk of metabolic syndrome, type 2 diabetes |
| Obese Class I | 30.0 – 34.9 | Substantially elevated cardiometabolic risk |
| Obese Class II | 35.0 – 39.9 | High risk; weight-related comorbidities likely |
| Obese Class III | ≥ 40.0 | Very high risk; severe functional impairment possible |
Common pitfalls
BMI cannot distinguish fat mass from muscle mass, so a heavily muscled athlete may be classified as overweight or obese despite very low body fat. It also ignores fat distribution - two individuals with identical BMIs but different waist measurements can have vastly different cardiometabolic risk profiles. BMI is less accurate at the individual level than it is as a population-level statistic.
A 2016 UCLA study found that approximately 54 million Americans classified as "overweight" or "obese" by BMI were actually metabolically healthy, while approximately 21 million people in the "normal weight" BMI range had poor cardiometabolic health markers - illustrating BMI's well-documented limitations as an individual diagnostic. (Tomiyama et al., International Journal of Obesity, 2016)
Frequently asked questions about BMI (Body Mass Index)
Is BMI an accurate measure of health?
BMI is a useful screening tool at the population level but has well-documented limitations at the individual level. It does not measure body fat directly, does not account for fat distribution, and does not reflect metabolic health markers such as blood pressure, blood glucose, or lipid profiles. It is best used as one input alongside other metrics.
What is a healthy BMI for adults?
For most adults aged 18–65, the WHO-defined healthy range is 18.5–24.9. For adults over 65, evidence supports a slightly higher range of 23–27.5 as optimal for survival and functional health. Ethnic-specific thresholds also exist - South Asian and East Asian populations have higher cardiometabolic risk at lower BMI values, and some guidelines recommend a lower overweight threshold of 23 for these groups.
Does BMI apply to athletes?
Standard BMI thresholds are unreliable for athletes with high muscle mass. A competitive bodybuilder or rugby player may have a BMI of 28–32 with body fat below 12%, which would be misclassified as overweight or obese. For athletic populations, body fat percentage and FFMI provide far more meaningful assessment.
Quiz: how well do you know BMI?
1. How is BMI calculated?
2. According to the WHO classification table, which BMI range is "Normal weight"?
3. For adults aged 65 and over, what does the definition section say about the optimal BMI range?
4. What key limitation does the pitfalls section identify about BMI?
5. For athletes with high muscle mass, what does the FAQ recommend as a more meaningful alternative to BMI?