What is BMI? How to calculate it, what it means, and what you should know

BMI (Body Mass Index) is a body-mass measure used to check whether a person’s weight is in a suitable range for their height.

BMI cannot fully describe someone’s health, but it is a simple, fast, and useful tool for tracking weight over time.

What is BMI?

BMI stands for Body Mass Index. It was developed by Belgian mathematician Adolphe Quetelet in the 19th century.

The index is based on weight (kg) and height (m).

BMI helps estimate body fat and assess risk of weight-related conditions such as:

• Cardiovascular disease
• Type 2 diabetes
• High blood pressure
• Fatty liver disease
• Some types of cancer

BMI formula

The formula is simple:

BMI = weight (kg) ÷ [height (m)]²

Example

Weight: 70 kg
Height: 1.75 m

BMI = 70 ÷ (1.75 × 1.75)

BMI = 22.86

WHO BMI classification

Asian BMI classification

Many health organizations use lower cutoffs for Asian populations because metabolic risk can rise at lower BMI levels.

What BMI is considered healthy?

For most adults, BMI from 18.5 to 24.9 is often considered healthy.

For many Asian adults, 18.5 to 22.9 is often treated as a more ideal range.

BMI is only a reference index and should not be the only measure of health.

Is a low BMI dangerous?

BMI below 18.5 may indicate underweight.

People who are underweight may face higher risk of:

• Malnutrition
• Weakened immunity
• Vitamin and mineral deficiencies
• Osteoporosis
• Long-term fatigue
• Loss of muscle mass

If BMI stays very low for a long time, talk with a doctor or nutrition professional.

What are the effects of a high BMI?

A high BMI is often linked to excess body fat.

This can raise the risk of:

• Type 2 diabetes
• High blood pressure
• Heart disease
• Stroke
• Fatty liver
• Sleep apnea
• Joint pain

Keeping BMI in a suitable range can help reduce these risks substantially.

Limitations of BMI

Despite being popular, BMI has important limitations.

1. It does not separate muscle and fat

Two people with the same BMI can have very different body-fat percentages.

Example: a bodybuilder and a sedentary person may both have BMI 28 with very different fat levels.

2. It does not show where fat is stored

Visceral (abdominal) fat is often more harmful than subcutaneous fat, but BMI cannot tell them apart.

3. It is less suitable for some groups

BMI may be less accurate for:

Athletes
People who lift weights heavily
Pregnant women
Older adults
Children (use age- and sex-specific BMI charts)

What else should you track besides BMI?

For a fuller health picture, combine BMI with:

• Waist circumference
• Body fat percentage
• Muscle mass ratio
• Blood pressure
• Blood glucose
• Cholesterol
• Daily activity level

How to maintain a healthy BMI

Helpful habits include:

• Eat plenty of vegetables and fruit
• Get enough protein
• Limit fast food and sugary drinks
• Exercise at least 150 minutes per week
• Sleep 7–9 hours each night
• Drink enough water
• Track weight regularly

Small habits kept consistently work better than short extreme changes.

Frequently asked questions (FAQ)

Conclusion

BMI is a simple, easy-to-calculate index that helps assess the relationship between weight and height.

For a fuller view, combine BMI with waist size, body fat, diet, activity, and regular health checks.