What Is Body Mass Index (BMI)?
BMI is a measurement of a person's leanness or corpulence based on their height and weight. It is widely used as a general indicator of whether a person has a healthy body weight for their height. The value obtained from BMI is used to categorize whether a person is underweight, normal weight, overweight, or obese.
While BMI is not a direct measure of body fat, it correlates with more direct measures of body fat such as underwater weighing and dual-energy X-ray absorptiometry (DXA). Due to its low cost and ease of measurement, it remains the primary screening tool used by clinicians worldwide.
BMI Formula
Use kilograms and meters, or the U.S. conversion if you start from pounds and inches:
Worked Example
An adult who is 175 cm tall and weighs 70 kg has a height of 1.75 m. Height squared is 1.75 × 1.75 = 3.0625. BMI = 70 / 3.0625 = 22.9. On the WHO adult scale, 22.9 is in the normal-weight band of 18.5–24.9. The weight that stays inside that band at 175 cm is about 56.7–76.3 kg.
The same check in U.S. units: 5 ft 9 in is 69 inches, and 160 lb gives BMI = 703 × 160 / 69² = 23.6, also in the normal band. Five feet nine inches is 175.3 cm, so the height table below lists a kilogram range a fraction higher than the exact 175 cm example. A result in this band is a screening result. It does not measure body fat, and it is not the scale used for children.
Healthy Adult Weight by Height
The weights below are the WHO adult range, BMI 18.5–24.9, for ages 20 and older. They are rounded to the nearest pound. Athletes, pregnancy, and ages 2–19 need a different reading, described further down this page.
| Height | Metric | Healthy weight |
|---|---|---|
| 5'0" | 152.4 cm | 95–128 lb (43.0–57.8 kg) |
| 5'3" | 160.0 cm | 104–141 lb (47.4–63.8 kg) |
| 5'6" | 167.6 cm | 115–154 lb (52.0–70.0 kg) |
| 5'9" | 175.3 cm | 125–169 lb (56.8–76.5 kg) |
| 6'0" | 182.9 cm | 136–184 lb (61.9–83.3 kg) |
If BMI and how you look disagree, compare it with the body fat calculator or the ideal weight calculator. Those tools use different inputs than height and weight alone.
BMI Categories for Adults (WHO Standards)
The World Health Organization (WHO) provides standardized categories for adults age 20 and older. These thresholds are linked to the risk of metabolic and cardiovascular diseases:
| Classification | BMI Range (kg/m²) | Health Risk |
|---|---|---|
| Severe Thinness | < 16.0 | Very High |
| Moderate Thinness | 16.0 – 16.9 | High |
| Mild Thinness | 17.0 – 18.4 | Moderate |
| Normal | 18.5 – 24.9 | Low |
| Overweight | 25.0 – 29.9 | Moderate |
| Obese Class I | 30.0 – 34.9 | High |
| Obese Class II | 35.0 – 39.9 | Very High |
| Obese Class III | ≥ 40.0 | Extremely High |
Asian BMI Standards
Research has shown that many Asian populations have a higher percentage of body fat and higher health risks at lower BMIs than Western populations. The WHO has acknowledged that many countries in Asia have adopted lower thresholds:
- Normal weight: 18.5 – 22.9 kg/m² (vs. 18.5 – 24.9 for general population)
- Overweight: 23.0 – 27.4 kg/m² (vs. 25.0 – 29.9)
- Obese: ≥ 27.5 kg/m² (vs. ≥ 30.0)
Our calculator's "Asian BMI Standards" toggle applies these verified regional thresholds, providing a more accurate health risk assessment for South and East Asian populations.
BMI for Children and Teens (Ages 2–20)
Because children's body compositions change rapidly with age and vary between boys and girls, BMI for children is interpreted using age-and-sex-specific percentiles from the CDC growth charts:
Underweight
Consult pediatrician for nutritional assessment.
Healthy Weight
Ideal growth and metabolic range.
Overweight
Elevated risk of early metabolic issues.
Obese
Significant long-term health risks.
Advanced Metrics: BMI Prime & Ponderal Index
Beyond standard BMI, this calculator provides two advanced anthropometric metrics:
- BMI Prime: The ratio of your measured BMI to the upper limit of the normal range (25 kg/m²). A value of 1.0 means you are exactly at the limit. Values above 1.0 indicate overweight status. This facilitates quick comparison across different population standards.
- Ponderal Index (PI): Also known as Rohrer's Index. Unlike BMI, it cubes height in the denominator, providing better accuracy for people at extreme heights (very tall or very short individuals).
Risks Associated with Being Overweight
According to the CDC, being overweight or obese increases the risk of:
- High blood pressure and higher LDL cholesterol
- Type 2 diabetes and coronary heart disease
- Stroke and gallbladder disease
- Osteoarthritis and sleep apnea
- Certain cancers (endometrial, breast, colon, kidney)
- Clinical depression and reduced quality of life
Risks Associated with Being Underweight
Being underweight also carries health risks including:
- Malnutrition, vitamin deficiencies, and anemia
- Osteoporosis and weakened immune function
- Reproductive issues and hormonal imbalances in women
- Growth and development issues in children
Limitations of BMI
BMI is only an estimate that cannot take body composition into account. It cannot distinguish between muscle mass and fat mass. Athletes and bodybuilders may be classified as overweight or obese despite having low body fat. For a more complete picture, consider using a Body Fat Calculator or Waist-to-Hip Ratio Calculator alongside BMI.
Frequently Asked Questions
What is a healthy BMI range?▼
How is BMI calculated?▼
Is BMI accurate for athletes?▼
What is BMI Prime?▼
How is BMI interpreted for children?▼
What is the BMI for 5 feet 9 inches and 160 pounds?▼
What is a healthy weight for 5 feet 9 inches?▼
Disclaimer: This calculator is for informational purposes only and does not constitute medical advice. BMI should be considered one part of a comprehensive health assessment. Consult a healthcare professional for personalized guidance.
Data sources: WHO (World Health Organization), CDC (Centers for Disease Control and Prevention).