# BMI Calculator

This BMI calculator can be used to compute the Body Mass Index (BMI) of a person given their body weight and height. The BMI category, BMI Prime, and amount of weight a person would need to gain or lose to reach a normal BMI range are outputs intended for adults aged 20 or older.

## What is Body Mass Index (BMI)?

A person's body mass index (BMI) is a general measure of a person's weight category, be it underweight, healthy weight, overweight, or obese. It is a measure of the ratio of a person's weight to their height. Although BMI is not an exact measure of body fat, it is moderately correlated with body fat, and is thus useful as a quick measure. Generally, a person with a higher BMI has more body fat, while a person with a lower BMI has less body fat. There exceptions however, since BMI is calculated using only weight and height, and does not take body composition into account. Thus, a person who has significant amounts of muscle can potentially be categorized as being overweight/obese even if their body fat percentage is lower than someone else of a similar height and weight. However, this is not particularly common, and BMI serves as a convenient rule of thumb for classifying an average individual's weight. It is also helpful as a screening method for determining whether an individual should undergo further testing, since BMIs under 20 and over 25 have been associated with higher all-cause mortality (death rate from all causes of death for a given population and time period).

## How to calculate BMI?

BMI is calculated by dividing mass in kilograms by the square of body height in meters. Thus, it has units of kg/m2. Given height measured in meters and mass in kilograms, computing the BMI involves basic arithmetic. For example, a person who is 1.75 m tall and weighs 70 kg has a BMI of,

 70 1.752
= 22.86 kg/m2

which is in the healthy range. Given a height in feet and inches and weight in pounds, we can calculate BMI by first converting the height to meters and weight to kilograms. After converting units, simply use the same formula above. The conversions are as follows:

1 lbs = 0.4535924 kg

1 inch = 0.0254 m

For example, a person who is 5 feet 4 inches tall (64 inches) and weighs 165 pounds is:

64 × 0.0254 = 1.63 m

165 × 0.4535924 = 74.84 kg

Their BMI is:

 74.84 1.632
= 28.17 kg/m2

which is in the overweight range.

## BMI ranges

The basic categories for BMI are underweight, normal, overweight, and obese, each of which corresponds to a specific range of BMI values. Also, the underweight and obese categories are further divided into subcategories. The table below shows the BMI categories and their corresponding range of BMI values.

CategoryBMI (kg/m2)
Underweight (severe thinness)< 16.0
Underweight (moderate thinness)16.0 - 16.9
Underweight (mild thinness)17.0 - 18.4
Normal range18.5 - 24.9
Overweight25.0 - 29.9
Obese (Class I)30.0 - 34.9
Obese (Class II)35.0 - 39.9
Obese (Class III)≥ 40.0

The above ranges were largely designed for people in the United States, though it is used to some degree globally. It is worth noting that BMI for kids (aged 2-20) is measured differently than it is for adults. Furthermore, certain Asian countries use different BMI ranges to more accurately represent their populations, since some differences exist based on ethnicity.

### BMI for kids

BMI for kids is measured in the same way as it is for adults: by dividing their mass in kilograms by their squared height in meters. However, instead of comparing the BMI to some fixed scale in order to determine weight category, the BMI of kids (aged 2-20) is instead compared to that for kids of the same sex and age to determine their BMI percentile. This is different from adults in that BMI for adults is not dependent on sex and age. The table below shows the various weight categories for kids based on BMI percentile:

CategoryBMI percentile
Underweight< 5th
Normal5th - 85th
Overweight85th - 95th
Obese> 95th

### BMI for Asian countries

Studies have found that the chance of developing certain diseases such as diabetes, heart disease, and other weight-related issues increases as BMI increases. Furthermore, at any given BMI, certain ethnicities have a significantly higher risk of developing these issues than other ethnicities. In particular, people of Asian descent have been found to have a higher risk of developing type 2 diabetes than white people. Similarly, Hispanic and black people also have a higher risk than white people, but to a lesser degree than Asian people. For this reason, as well as the fact that people from different populations and of different descent have different associations between BMI and factors such as body fat percent, certain countries and cities use different BMI ranges. Japan, Singapore, and Hong Kong are three such examples.

#### BMI ranges in Japan

The following BMI ranges are used in Japan and are based on a study conducted by the Japan Society for the Study of Obesity.

CategoryBMI (kg/m2)
Underweight (thin)< 18.5
Normal18.5 - 24.9
Obesity (Class 1)25.0 - 29.9
Obesity (Class 2)30.0 - 34.9
Obesity (Class 3)35.0 - 39.9
Obesity (Class 4)≥ 40.0

#### BMI ranges in Singapore

Based on studies that show that many Asian populations have higher proportions of body fat and are therefore at increased risk of cardiovascular disease and diabetes, the Health Promotion Board of Singapore categorizes BMI using the following ranges:

CategoryBMI (kg/m2)
Underweight< 18.5
Normal18.5 - 22.9
Mild to moderate overweight23.0 - 27.4
Very overweight to obese≥ 27.5

#### BMI ranges in Hong Kong

The Hospital Authority of Hong Kong categorizes BMI using the following ranges:

CategoryBMI (kg/m2)
Underweight (unhealthy)< 18.5
Normal range (healthy)18.5 - 22.9
Overweight I (at risk)23.0 - 24.9
Overweight II (moderately obese)25.0 - 29.9
Overweight III (severely obese)≥ 30.0

## How good is BMI as an indicator of health?

BMI is a good indicator of body fat for the majority of the population, and by extension, it is a good indicator of health in the context of diseases or health risks that occur as a result of excess body fat; these include cardiovascular disease, diabetes, hypertension, stroke, and much more. In general, the higher a person's body fat, the more at risk they are for many negative health outcomes. Similarly, a person whose BMI is too low is more at risk of malnutrition, osteoporosis, decreased immune function, developmental issues, and more.

However, BMI is not a perfect indicator of body fat, as it does not take body composition into account. As such, it is most useful as a screening method to quickly and simply categorize a person's weight. Once a qualified physician categorizes a person's body weight, they can then use their discretion along with information provided by the patient to determine whether the patient is at risk. If necessary, the physician can then order more tests or intervene as necessary. In general, the following are true, given the same BMI:

• Women tend to have more body fat than men
• Certain ethnic groups have higher body fat (such as Asians)
• Older people tend to have more body fat
• Athletes tend to have less body fat

Overall, while BMI is a good indicator of health in the context of overweight and obesity, it is important to realize that it is still just a general measure, and cannot be used to definitively indicate a person's health status. Ideally, a person should consult a physician in order to assess their health, and should use BMI mainly as a guide or a frame of reference.

## BMI Prime

BMI prime is a measure of the ratio of a person's BMI to the upper limit of their optimal BMI. In the US definition of BMI, the upper limit is 25 kg/m2. In Asian countries, the upper limit used should be 23 kg/m2 instead, since Asians tend to have higher body fat at the same BMIs as other ethnic groups. As an example, a person in the US who has a BMI of 23.2 kg/m2 has a BMI prime of:

 23.2 25
= 0.928

Thus, they have a BMI that is ~7% below the upper limit. BMI prime is useful in that it allows us to quickly determine how much a person's BMI differs relative to the upper limit of normal BMI for the population. It also allows us to compare the BMIs of populations whose upper limit optimal BMIs differ (e.g. 25 in the US vs. 23 in Asian countries).

BMI prime is a dimensionless quantity since it is a ratio of two BMI values. The table below shows the various weight categories and their corresponding BMI primes.

CategoryBMI PrimeBMI
Very severely underweight< 0.6< 15.0
Severely underweight0.6 - 0.6415.0 - 16.0
Underweight0.64 - 0.7416.0 - 18.5
Healthy0.74 - 1.018.5 - 25
Overweight1.0 - 1.225.0 - 30.0
Moderately obese1.2 - 1.430.0 - 35.0
Severely obese1.4 - 1.635.0 - 40.0
Very severely obese> 1.6≥ 40.0

Although it makes it easier to compare different populations, BMI prime still shares the same limitations as BMI in that it does not take body composition into account. As such, it should be treated in the same way as BMI: as an indicator of health rather than as a conclusive measure.