Body Mass Index Measurement

This post deals with measuring Body Mass Index (BMI) for adults, children, and teens. Body mass index, or BMI, is a measure of body size. It combines a person’s weight with their height. The results of a BMI measurement can give an idea about whether a person has the correct weight for their height. 

BMI (Body Mass Index) is a screening tool that can indicate whether a person is underweight or if they have a healthy weight, excess weight, or obesity. If a person’s Body Mass Index is outside of the healthy range, their health risks may increase significantly.

Carrying too much weight can lead to a variety of health conditions, such as type 2 diabetes, high blood pressure, and cardiovascular problems.  In a previous post, we offered five simple exercises which can help a person achieve some of their weight goals.

A weigh that is too low can increase the risk of malnutrition, osteoporosis, and anemia. The doctor will make suitable recommendations.

Body Mass Index does not measure body fat directly, and it does not account for age, sex, ethnicity, or muscle mass in adults.

However, it uses standard weight status categories that can help doctors to track weight status across populations and identify potential issues in individuals.

BMI (Body Mass Index) in adults

Body Mass Index calculator shows healthy weight
A BMI chart or calculator can show a person if they have a healthy weight.

Calculating BMI involves measuring a person’s height and body weight.

 

Metric

  • To calculate BMI in metric units, use the following method: BMI = kg/m2
  • So, to calculate an adult’s BMI: Divide their weight in kilograms (kg) by the square of their height in meters (m2)

Since most people measure height in centimeters (cm), divide height in cm by 100 to get height in meters.

Imperial

  • When using imperial units, the formula is: BMI = lbs x 703/in2
  • In other words: Multiply a person’s weight in pounds (lbs) by 703. Then divide by their height in inches, squared (in2)

To avoid using the math, a person can use a calculator or a chart to find their BMI.

BMI calculator

Enter height or weight in either imperial or metric measurements to find your BMI.

1) Metric BMI Calculator

2) Imperial BMI Calculator

BMI charts

People can also work out their BMI using a chart. Click here to see a chart provided by the National Heart, Lung, and Blood Institute (NHLBI).

Locate your height in inches on the side of the chart, then look across to find your body weight in pounds. Scan to the top to see if the result corresponds to a normal weight, overweight, or obesity.

Understanding the results

The following table shows the standard weight status categories associated with BMI ranges for adults.

BMI Weight status
Below 18.5 Underweight
18.5–24.9 Healthy
25.0–29.9 Overweight
30.0 and above Obese

BMI of less than 18.5

A BMI of less than 18.5 indicates that you are underweight, so you may need to put on some weight. You are recommended to ask your doctor or a dietitian for advice.

BMI of 18.5–24.9

A BMI of 18.5-24.9 indicates that you are at a healthy weight for your height. By maintaining a healthy weight, you lower your risk of developing serious health problems.

BMI of 25–29.9

A BMI of 25-29.9 indicates that you are slightly overweight. You may be advised to lose some weight for health reasons. You are recommended to talk to your doctor or a dietitian for advice.

BMI of over 30

A BMI of over 30 indicates that you are heavily overweight. Your health may be at risk if you do not lose weight. You are recommended to talk to your doctor or a dietitian for advice.

BMI in children and teens

In adults, BMI values are not linked to age and are the same for both sexes.

However, measuring Body Mass Index in children and teens is slightly different. Girls and boys develop at different rates and have different amounts of body fat at different ages. For this reason, BMI measurements during childhood and adolescence take age and sex into consideration.

Doctors and other health professionals do not categorize children by healthy weight ranges because:

  • they change with each month of age
  • male and female body types change at different rates
  • they change as the child grows taller

Doctors calculate BMI for children and teens in the same way as they do for adults, by measuring height and weight. Then they locate the BMI number and person’s age on a sex-specific BMI-for-age chart. This will indicate whether the child is within a healthy range.

Calculator and charts for child and teen Body Mass Index

The Centers for Disease Control and Prevention (CDC) have produced a calculator that provides BMI and the corresponding BMI-for-age percentile on a CDC growth chart for children and teens.

First, click here for the calculator.

Next, use the charts to see if a child’s weight is suitable for their age.

Click here for the charts:

What do the results mean?

The following categories explain the meaning of the results:

Weight status category Percentile range
Underweight Below the 5th percentile
Healthy weight 5th percentile to less than the 85th percentile
Overweight 85th to less than the 95th percentile
Obesity Equal to or greater than the 95th percentile

How doctors use BMI

BMI is not accurate enough to use as a diagnostic tool, but it can screen for potential weight problems in adults and children.

If someone has a high or low BMI, a doctor or other healthcare professional might then consider other factors, such as:

  • skinfold thickness measurements, which indicate how much fat is in the body in adults and children
  • evaluations of diet and physical activity
  • discuss any family history of cardiovascular disease and other health problems
  • recommend other appropriate health screenings

The doctor or healthcare professional can then make diet and exercise recommendations based on these results.

Health risks of extra weight

Excess weight has the following effects on the body:

  • It increases how hard the heart has to work.
  • It raises blood pressure, blood cholesterol and triglyceride levels.
  • It lowers high-density lipoprotein (HDL), or good cholesterol levels.
  • It can make diabetes and other health problems more likely.

According to the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), carrying extra weight can increase the risk of the following conditions:

  • hypertension, or high blood pressure
  • dyslipidemia, which involves high LDL cholesterol, low HDL cholesterol, or high levels of triglycerides
  • type 2 diabetes
  • coronary heart disease
  • stroke
  • gallbladder disease
  • osteoarthritis
  • sleep apnea and respiratory problems
  • some cancers, including endometrial, breast and colon cancer

Carrying extra weight as a child or teenager can also pose significant health risks, both during childhood and into adulthood.

As with adult obesity, childhood obesity increases the risk of various health problems, including cardiovascular disease, diabetes, and sleep apnea.

The American Heart Association (AHA), point out that children with a high BMI also have a higher risk of:

Benefits of a healthy weight

Walking with the family helps Body Mass Index levels
Walking with family or friends can be an enjoyable way of keeping fit and preventing unwanted weight gain.

Apart from reducing the risk of the health conditions, maintaining a healthy weight offers additional benefits:

  • fewer joint and muscle pains
  • increased energy and ability to join in more activities
  • improved regulation of bodily fluids and blood pressure
  • reduced burden on the heart and circulatory system
  • improved sleep patterns

Other measures of a healthy body

BMI is a useful tool, but it cannot identify whether a person’s weight is made up of muscle or fat.

For example, an athlete with a lot of muscle tissue may have a higher BMI than a person who is not very active. But, this does not mean that the athlete is overweight or unhealthy.

In addition, cardiovascular disease and high blood pressure are more likely to occur in people who have additional fat — known as visceral fat — around their middle rather than their hips.

Other measures of body size include waist-hip ratio, waist-to-height ratio, and body composition, which measures body fat and lean body mass. These measurement systems focus more on the amount of fat a person has and its distribution around the body.

Together with BMI, these additional measures can help to assess more accurately the health risks associated with an individual’s weight.

Takeaway

Body Mass Index can be a useful screening tool for predicting certain health risks. However, people should use it with caution, as it does not take other factors — such as activity levels and body composition — into account.

For children and teens, it is important to include their age and sex when taking a BMI measurement, because their bodies continuously change as they develop.

BMI calculators and charts

BMI Calculators and Charts enable health professionals to evaluate the optimum weight of individuals. Body mass index, or BMI, is one way a person can check if their weight is healthy or not. BMI takes both height and weight into consideration.

Carrying too much or too little weight can increase a person’s risk of health problems, either now or in the future.

BMI is not the only factor that affects this risk. Other tools for assessing whether a person has a healthy weight include waist-to-hip ratio, waist-to-height ratio, and body-fat percentage.

However, BMI is a useful starting point. This page provides some tools for people to work out their BMI.

BMI calculators

These BMI calculators and charts can give an indication of whether a person’s weight may affect their risk of health problems.

We are publishing the calculators here courtesy of The Calculator Site. There are two calculation options available: Metric and imperial.

1) Metric BMI Calculators

2) Imperial BMI Calculators

BMI charts

To use the charts below, find your weight in pounds along the top and your height in feet and inches down the side. Then look across to find your BMI.

There are two charts. If a person’s weight is 200 pounds (lb) or under, they should use the first chart. If their weight is over 200 lb, they should look at the second one.

The shaded areas correspond to BMI values that indicate either a healthy weight, excess weight, or obesity.

In addition, researchers and clinicians divide obesity into three categories.

  • Class I: BMI is 30 to 34.9
  • Class II: BMI is 35 to 39.9
  • Class III: BMI is 40 and above

The charts are an adaptation of the Adult body mass index (BMI) chart. created by the University of Vermont, in the United States.


Body mass index chart: Weight from 95–245 pounds

bmi chart underweight to overweight
Adult BMI chart showing ranges “under healthy weight: BMI

Body mass index chart: Weight from 250–400 pounds

obese bmi chart
Adult BMI chart showing ranges “obese I: BMI 30–34.9,” “obese II: BMI 35–39.9” and “obese III: BMI? 40.”

These figures are only a guide. The BMI Calculators will not determine whether a person has an ideal body weight, but it can help to show if an individual’s weight is increasing their risk for disease.

A person who is very fit, for example, an Olympic athlete, may have a high BMI.

This does not necessarily mean that they are overweight. The excess weight, in this case, may be due to increased muscle mass.

BMI categories

The following table shows the standard weight status categories associated with BMI ranges for adults:

BMI Weight status
Below 18.5 Underweight
18.5–24.9 Healthy
25.0–29.9 Overweight
30.0 and above Obese

BMI of less than 18.5

A BMI of less than 18.5 indicates that you are underweight, so you may need to put on some weight. You are recommended to ask your doctor or a dietitian for advice.

BMI of 18.5–24.9

A BMI of 18.5–24.9 indicates that you are at a healthy weight for your height. By maintaining a healthy weight, you lower your risk of developing serious health problems.

BMI of 25–29.9

A BMI of 25–29.9 indicates that you are slightly overweight. You may be advised to lose some weight for health reasons. You are recommended to talk to your doctor or a dietitian for advice.

BMI of over 30

A BMI of over 30 indicates that you are heavily overweight. Your health may be at risk if you do not lose weight. You are recommended to talk to your doctor or a dietitian for advice.

Health risks

A healthy weight can help prevent a range of diseases and health conditions.

People with a BMI of 30 or more have a higher risk than others of diseases such as heart disease, type 2 diabetes, sleep apnea, high blood pressure, colorectal cancer, for example. Some of these can be life-threatening.

Having a BMI of under 18.5 can increase the risk of malnutrition, osteoporosis, anemia, and a range of problems that can result from various nutrient deficiencies. It can also be a sign of a hormonal, digestive, or other problem.

Varying cutoff points

Evidence suggests that the associations between BMI, percentage of body fat, and body fat distribution may differ across populations, due to variations in race and ethnicity.

A Brazilian study, published in 2017, looked at the correlation between BMI and body-fat percentage in 856 adult men and women.

They concluded that to predict obesity-type body-fat percentage:

  • The standard BMI threshold of 29.9 kg/m2 was appropriate for men.
  • A more suitable cutoff point for women appeared to be 24.9 kg/m2.

In 2017, Korean researchers pointed out that people in the Asia-Pacific region often have a higher risk of type 2 diabetes and cardiovascular disease at a BMI below the existing WHO cutoff point.

In Korea, they added, there is evidence that almost twice as many people have features of metabolic obesity but a normal weight compared with the U.S.

In 2010, results of a study published in The International Journal of Obesity found that Asian Americans within the healthy weight range were more likely to have symptoms of metabolic syndrome than their non-Hispanic white counterparts.

The following table, published in 2006 by the World Health Organization (WHO), shows some comparisons and cutoff points that may apply.

Doctors may use these variations when treating or advising specific people.

Classifications BMI (kg/m2)
principal cutoff points
BMI (kg/m2)
additional cutoff points
Underweight
Severe thinness
Moderate thinness 16.00–16.99 16.00–16.99
Mild thinness 17.00–18.49 17.00–18.49
Normal range 18.50–24.99 18.50–22.99
23.00–24.99
Overweight ?25.00 ?25.00
Pre-obese 25.00–29.99 25.00–27.49
27.50–29.99
Obese ?30.00 ?30.00
Obese class I 30.00–34.99 30.00–32.49
32.50–34.99
Obese class II 35.00–39.99 35.00–37.49
37.50–39.99
Obese class III ?40.00 ?40.00

Takeaway

BMI is a useful tool that gives a general idea about whether a person’s weight is healthy or not. However, it is a simple tool that does not tell the whole story about people’s individual weight and health risks.

Anyone who is concerned about their weight should speak to a doctor, who may also consider the individual’s body-fat distribution and the ratio of their waist size to their height. A health professional will also be able to offer advice to suit every individual.