BMI Calculator
The Body Mass Index (BMI) calculator provides an estimate of your body fat based on your height and weight. It's a widely used tool to identify potential weight problems in adults.
BMI Introduction
The Body Mass Index (BMI) is a measurement of a person's leanness or corpulence based on their height and weight, and is intended to quantify tissue mass. It is widely used as a general indicator of whether a person has a healthy body weight for their height. Specifically, the value obtained from the calculation of BMI is used to categorize whether a person is underweight, normal weight, overweight, or obese depending on what range the value falls between.
These ranges of BMI vary based on factors such as region and age, and are sometimes further divided into subcategories such as severely underweight or very severely obese. Being overweight or underweight can have significant health effects, so while BMI is an imperfect measure of healthy body weight, it is a useful indicator of whether any additional testing or action is required.
How BMI is Calculated
BMI is calculated using a person's weight and height. The formula differs between the metric and imperial systems:
Metric Formula
For example, if you weigh 70 kg and are 1.75 meters tall:
BMI = 70 / (1.75)² = 70 / 3.0625 = 22.86
Imperial Formula
For example, if you weigh 154 pounds and are 5'9" (69 inches) tall:
BMI = 703 × 154 / (69)² = 703 × 154 / 4,761 = 22.77
BMI Categories
For Adults (20 years and older)
This is the World Health Organization's (WHO) recommended body weight based on BMI values for adults. It is used for both men and women, age 20 or older.
| Classification | BMI Range (kg/m²) |
|---|---|
| Severe Thinness | < 16 |
| Moderate Thinness | 16 - 17 |
| Mild Thinness | 17 - 18.5 |
| Normal | 18.5 - 25 |
| Overweight | 25 - 30 |
| Obese Class I | 30 - 35 |
| Obese Class II | 35 - 40 |
| Obese Class III | > 40 |
For Children and Teens (2-20 years)
The Centers for Disease Control and Prevention (CDC) recommends BMI categorization for children and teens between age 2 and 20.
| Category | Percentile Range |
|---|---|
| Underweight | < 5% |
| Healthy weight | 5% - 85% |
| At risk of overweight | 85% - 95% |
| Overweight | > 95% |
Health Implications of BMI
Risks Associated with High BMI
Being overweight increases the risk of a number of serious diseases and health conditions. Below is a list of said risks:
- High blood pressure
- Higher levels of LDL cholesterol ("bad cholesterol"), lower levels of HDL cholesterol ("good cholesterol"), and high levels of triglycerides
- Type 2 diabetes
- Coronary heart disease
- Stroke
- Gallbladder disease
- Osteoarthritis, a type of joint disease caused by breakdown of joint cartilage
- Sleep apnea and breathing problems
- Certain cancers (endometrial, breast, colon, kidney, gallbladder, liver)
- Mental illnesses such as clinical depression, anxiety, and others
- Body pains and difficulty with physical functions
- Generally, an increased risk of mortality compared to those with a healthy BMI
Risks Associated with Low BMI
Being underweight has its own associated risks, listed below:
- Malnutrition, vitamin deficiencies, anemia (lowered ability to carry blood vessels)
- Osteoporosis, a disease that causes bone weakness, increasing the risk of breaking a bone
- A decrease in immune function
- Growth and development issues, particularly in children and teenagers
- Possible reproductive issues for women due to hormonal imbalances that can disrupt the menstrual cycle
- Potential complications as a result of surgery
- Generally, an increased risk of mortality compared to those with a healthy BMI
Limitations of BMI
Although BMI is a widely used and useful indicator of healthy body weight, it does have its limitations. BMI is only an estimate that cannot take body composition into account. Due to a wide variety of body types as well as distribution of muscle, bone mass, and fat, BMI should be considered along with other measurements rather than being used as the sole method for determining a person's healthy body weight.
In adults:
BMI cannot be fully accurate because it is a measure of excess body weight, rather than excess body fat. BMI is further influenced by factors such as age, sex, ethnicity, muscle mass, and body fat, among others. For example, an older person who is considered a healthy weight, but is completely inactive in their daily life may have significant amounts of excess body fat even though they are not heavy. This would be considered unhealthy, while a younger person with higher muscle composition of the same BMI would be considered healthy.
In athletes, particularly bodybuilders who would be considered overweight due to muscle being heavier than fat, it is entirely possible that they are actually at a healthy weight for their body composition. Generally:
- Older adults tend to have more body fat than younger adults with the same BMI.
- Women tend to have more body fat than men for an equivalent BMI.
- Muscular individuals and highly trained athletes may have higher BMIs due to large muscle mass.
In children and adolescents:
The same factors that limit the efficacy of BMI for adults can also apply to children and adolescents. Additionally, height and level of sexual maturation can influence BMI and body fat among children. BMI is a better indicator of excess body fat for obese children than it is for overweight children, whose BMI could be a result of increased levels of either fat or fat-free mass (all body components except for fat, which includes water, organs, muscle, etc.). In thin children, the difference in BMI can also be due to fat-free mass.
That being said, BMI is fairly indicative of body fat for 90-95% of the population, and can effectively be used along with other measures to help determine an individual's healthy body weight.
When to Use Other Measurements
Due to BMI's limitations, healthcare providers often use additional measurements to assess health risks:
- Waist Circumference: Measures abdominal fat, which is a better predictor of certain health risks.
- Waist-to-Hip Ratio: Compares waist circumference to hip circumference.
- Body Fat Percentage: Directly measures the proportion of fat in the body.
- Skinfold Measurements: Uses calipers to measure subcutaneous fat at specific body sites.
- Bioelectrical Impedance: Estimates body composition by measuring the resistance to a small electrical current.