BMI Calculator
A BMI (Body Mass Index) calculator is a useful tool for determining your body fat based on your height and weight. It’s a widely used indicator of whether your weight is in a healthy range. To calculate your BMI, you’ll need to know your weight in kilograms and your height in meters. You can then use an online BMI calculator or a formula to determine your BMI category: underweight, normal weight, overweight, or obese. A healthy BMI is typically between 18.5 and 24.9. Using a BMI calculator can help you track your progress and make informed decisions about your health.
BMI introduction
Based on a person’s height and weight, BMI calculates their leanness or corpulence and is used to determine tissue mass. It’s commonly used to determine whether an individual is at a healthy body weight for their height. To be more precise, the BMI value is used to determine a person’s classification as underweight, normal weight, overweight, or obese based on where the number falls in the range.
These BMI ranges are subject to regional and age-based variations, and they are occasionally further classified into extremely underweight and extremely obese subcategories. Although BMI is not a perfect estimate of healthy body weight, it may be a helpful tool in determining if further testing or treatment is necessary because being overweight or underweight can have serious health consequences. View the table below to view the many BMI-based categories that the calculator uses.
BMI table for adults
This is the recommended body weight for adults by the World Health Organization (WHO), based on BMI measurements. Men and women who are 20 years of age or older can utilize it.
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 |
Risks associated with being overweight
Being overweight increases the risk of a number of serious diseases and health conditions. Below is a list of said risks, according to the Centers for Disease Control and Prevention (CDC):
- High blood pressure
- Higher levels of LDL cholesterol, which is widely considered “bad cholesterol,” lower levels of HDL cholesterol, considered to be good cholesterol in moderation, and high levels of triglycerides
- Type II 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)
- Low quality of life
- Mental illnesses such as clinical depression, anxiety, and others
- Body pains and difficulty with certain physical functions
- Generally, an increased risk of mortality compared to those with a healthy BMI
As can be seen from the list above, there are numerous negative, in some cases fatal, outcomes that may result from being overweight. Generally, a person should try to maintain a BMI below 25 kg/m2, but ideally should consult their doctor to determine whether or not they need to make any changes to their lifestyle in order to be healthier.
Risks associated with being underweight
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. Underweight women also have a higher chance of miscarriage in the first trimester
- Potential complications as a result of surgery
- Generally, an increased risk of mortality compared to those with a healthy BMI
In some cases, being underweight can be a sign of some underlying condition or disease such as anorexia nervosa, which has its own risks. Consult your doctor if you think you or someone you know is underweight, particularly if the reason for being underweight does not seem obvious.
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, body fat, and activity level, 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, according to the CDC:
- 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.
BMI formula
Below are the equations used for calculating BMI in the International System of Units (SI) and the US customary system (USC) using a 5'10", 160-pound individual as an example:
USC Units:
BMI = 703 × mass (lbs)/height2 (in) = 703 × 160/702 = 22.96 kg/m2
SI, Metric Units:
BMI = mass (kg)/height2 (m) = 72.57/1.782 = 22.90 kg/m2
BMI Prime
BMI prime is the ratio of a person's measured BMI to the upper limit of BMI that is considered "normal," by institutions such as the WHO and the CDC. Though it may differ in some countries, such as those in Asia, this upper limit, which will be referred to as BMIupper, is 25 kg/m2.
The BMI prime formula is:
BMI prime = BMI/25
Since BMI prime is a ratio of two BMI values, BMI prime is a dimensionless value. A person who has a BMI prime less than 0.74 is classified as underweight; from 0.74 to 1 is classified as normal; greater than 1 is classified as overweight; and greater than 1.2 is classified as obese. The table below shows a person's weight classification based on their BMI prime: