BMI Calculator
Body Mass Index (BMI) – Overview
Body Mass Index (BMI) is a quick calculation that compares a person’s weight to height to estimate if their body weight is in a healthy range. Even though it does not calculate body fat directly, it is widely accepted as a useful method for determining if a person is underweight, normal weight, overweight, or obese. Health professionals tend to utilize BMI as a first screening tool, as extreme values (both too low or too high) could be signs of potential health problems. Although not ideal, it is a valuable guideline for further medical assessment.
BMI Categories for Adults (WHO Standards)
The World Health Organization (WHO) offers the following BMI categories for adults (men and women 20 years and older):
| Classification | BMI Range (kg/m²) |
Severe Thinness Moderate Thinness Mild Thinness Normal Weight Overweight Obese Class I Obese Class II Obese Class III | < 16 16 – 17 17 – 18.5 18.5 – 25 25 – 30 30 – 35 35 – 40 > 40 |
BMI Guidelines for Children & Adolescents (Ages 2–20)
| Category | Percentile Range |
|---|---|
| Underweight | Below 5th % |
| Healthy Weight | 5th – 85th % |
| At Risk of Overweight | 85th – 95th % |
| Overweight | Above 95th % |
Health Risks of Being Overweight
Excess weight can raise the risk of developing a number of health issues. The CDC states that risks include:
- High blood pressure
- Unhealthy levels of cholesterol (high LDL, low HDL, high triglycerides)
- Type 2 diabetes
- Heart disease and stroke
- Gallbladder disease
- Osteoarthritis (joint damage)
- Sleep apnea and breathing problems
- Some cancers (breast, colon, kidney, liver, gallbladder, endometrial)
- Mental health (depression, anxiety)
- Decreased mobility and chronic pain
- Shorter life expectancy
Health Risks of Being Underweight
Conversely, being underweight also threatens serious health risks, including:
- Malnutrition and vitamin/mineral deficiencies
- Reduced bone strength (osteoporosis)
- Compromised immune function
- Children and adolescents’ growth and development delay
- Women’s fertility issues and pregnancy complications
- Greater surgical risk
- Generalized increased risk of premature death
In some cases, low weight without clear explanation can be indicative of underlying conditions such as eating disorders or chronic disease, hence the need for medical consultation.
Limitations of BMI
Even if a useful tool, BMI has obvious limitations:
- It cannot differentiate between muscle, fat, and bone.
- High-muscle-mass athletes can be labeled as overweight although they are extremely healthy.
- Older people can be of normal BMI but have excess body fat because they are not active.
- Women carry more body fat than men of the same BMI.
- Age and height growth and puberty in children influence the accuracy of BMI.
Although these limitations exist, BMI remains a valuable screening measurement for approximately 90–95% of individuals when used in conjunction with other health evaluations.
BMI Formula Calculation
BMI = 703 × mass (lbs) height2 (in) = 703 × 160 702 = 23.0
For example, a person who weighs 160 lbs and is 5’10” (70 in) tall has a BMI of 23.0.
SI, Metric Units:
BMI = mass (kg) height2 (m) = 72.57 1.7782 = 23.0
For example, a person weighing 72.5 kg and 1.78 m tall has a BMI of 23.0.
BMI Prime
| Classification | BMI | BMI Prime |
|---|---|---|
| Severe Thinness | <16 | <0.64 |
| Moderate Thinness | 16-17 | 0.64-0.68 |
| Mild Thinness | 17-18.5 | 0.68-0.74 |
| Normal | 18.5-25 | 0.74-1.0 |
| Overweight | 25-30 | 1.0-1.2 |
| Obese Class I | 30-35 | 1.2-1.4 |
| Obese Class II | 35-40 | 1.4-1.6 |
| Obese Class III | >40 | >1.6 |