Understanding the Body Mass Index (BMI)

  • 25 Apr 2023
  • 3 mins

Originally invented by Belgian mathematician Adolphe Quetelet in the 1830s, BMI has since been used by medical professionals to define weight categories for many years and to determine whether or not a person is of a healthy weight. Resident doctor at HealthConnexion Dr. Grace Soo explains how to make sense of this measurement of an individual’s height to weight ratio.

Calculating BMI

Most healthcare institutions in Singapore follow the metric system which calculates weight in kilograms (kg) and height in meters (m).

Calculating BMI under the metric system can be done with this formula: Weight (kg) ÷ [Height (m)]2.

For example, an individual who weighs 68 kg and is 170 cm tall would have his BMI calculated as such: 68 kg ÷ (1.7 m)2 = 23.52.

Under the foot-pound-second system widely used in the United States (US), BMI calculation will require an additional step of multiplying the weight-height ratio by a conversion factor of 703: Weight (pounds) ÷ [Height (inches)]2 x 703.

The same individual’s BMI would be calculated after converting 68 kg to 150 pounds (lb) and 170 cm to 66 inches (in.) as: [150 lb ÷ (67 in.)2 ] x 703 = 23.49.1

Interpreting BMI

According to the Centers for Disease Control and Prevention in the US, BMI should be interpreted differently depending on whether a person is a child or an adult, where adults are defined as aged 20 or older.

For adults, the following reference is used universally regardless of a person’s gender and body type.

BMI Weight Status
Below 18.5 Underweight
18.5 - 24.9 Healthy Weight
25.0 - 29.9 Overweight
30.0 and Above Obese

Source: Centers for Disease Control and Prevention

Interpreting BMI for children and teenagers is slightly different. BMI measurements are still determined using the same formulas but obesity is determined by percentile among children of the same age and gender, rather than by fixed categories.

This is done because the body fat compositions of children and teenagers fluctuate rapidly as they grow. The percentiles used might vary slightly from country to country.

Dr. Soo stresses the importance of maintaining a healthy weight to reduce risk of developing chronic diseases. Individuals in the overweight or obese category are at higher risk of developing high cholesterol, high blood pressure, and high blood sugar, three conditions that are also known collectively as the “3-Highs.”

Over time, these conditions can also contribute to other chronic health issues such as:

  • Diabetes
  • Coronary heart disease
  • Bone and joint disorders
  • Certain cancers

How Useful Is BMI?

Using BMI alone as a health indicator comes with limitations. Nevertheless, medical practitioners still find it useful as a starting point for assessing an individual’s overall health.

Critics of the BMI formula argue that by lumping muscle and fat mass together in a single weight measurement, high performance athletes with muscular bulk will be considered to be unhealthier than less active individuals who happen to fall within the healthy BMI range.

Dr. Soo says, “This might cause very muscular athletes to be categorized as obese under the BMI system even though they are in fact healthy. So, BMI is not the only parameter, we also use the waist circumference to determine the level of abdominal obesity.”

At the same time, the majority of us are not very muscular athletes and can therefore use BMI as a rough indicator of overall health.2

“Though body fat percentage differs between individuals with the same BMI, the correlation between BMI and body fat is still fairly strong for the average individual,”3 explains Dr. Soo.

Waist Circumference

Along with BMI, measuring one’s waist circumference has been mentioned in achieving good health outcomes.

Waist circumference accounts for the amount of tissue around a person’s belly, providing a better indicator of fat distribution.4 It has been suggested that waist circumference can reflect well an individual’s heart disease risk.5

According to the World Health Organization, waist circumference and BMI can be used in a complementary way.6 In addition, linking waist circumference and risk factors for health “is not an easy task and should be done scientifically using proper methods.”7

For an example of considering waist circumference, the following is a set of standards outlined by the British Heart Foundation.

Relationship between health risk and waist circumference for Men

Waist circumference Health risk
Below 94 cm or 37 in. Low risk
Between 94-102 cm or 37-40 in. High risk
Above 102 cm or 40 in. Very high risk

Relationship between health risk and waist circumference for Women

Waist circumference Health risk
Below 80 cm or 31.5 in. Low risk
Between 80-88 cm or 31.5-34.6 in. High risk
Above 88 cm or 34.6 in. Very high risk

Addressing a High BMI

Dr. Soo advises that if your BMI and waist circumference lie outside of the healthy range, staying active and maintaining a good diet can help with not just lowering your cholesterol levels and blood pressure, but boosting your overall mood as well.

HealthHub suggests that healthy adults should engage in a minimum of 150 minutes of moderate-intensity aerobic activity such as brisk walking, leisure cycling, leisure swimming, doubles tennis, or line dancing each week.

Those who are capable of engaging in vigorous-intensity aerobic activity such as jogging, running, swimming, inline skating, or rope skipping can do so for a minimum of 75 minutes per week instead.

Eating carbohydrates, proteins, and fruits and vegetables according to My Healthy Plate’s quarter-quarter-half combination respectively as recommended by HealthHub can go a long way in ensuring that your body receives the essential nutrients it needs.

It is also important to stay hydrated throughout the day with plain water while avoiding dehydrating beverages such as coffee, tea, soda, and alcohol. That is not to say that there is never room for dessert, as long as taken in moderation.

1 Figures have been rounded up to the nearest whole number during conversion resulting in a slightly different BMI.

2 https://www.hsph.harvard.edu/obesity-prevention-source/obesity-definition/obesity-definition-full-story/
3 https://www.cdc.gov/healthyweight/assessing/bmi/adult_bmi/index.html
4 Ross R, Neeland IJ, Yamashita S, Shai I, Seidell J, Magni P, Santos RD, Arsenault B, Cuevas A, Hu FB, Griffin BA, Zambon A, Barter P, Fruchart JC, Eckel RH, Matsuzawa Y, Després JP. Waist circumference as a vital sign in clinical practice: a Consensus Statement from the IAS and ICCR Working Group on Visceral Obesity. Nat Rev Endocrinol. 2020 Mar;16(3):177-189. doi: 10.1038/s41574-019-0310-7. Epub 2020 Feb 4. PMID: 32020062; PMCID: PMC7027970.
5 Powell-Wiley TM, Poirier P, Burke LE, Després JP, Gordon-Larsen P, Lavie CJ, Lear SA, Ndumele CE, Neeland IJ, Sanders P, St-Onge MP; American Heart Association Council on Lifestyle and Cardiometabolic Health; Council on Cardiovascular and Stroke Nursing; Council on Clinical Cardiology; Council on Epidemiology and Prevention; and Stroke Council. Obesity and Cardiovascular Disease: A Scientific Statement From the American Heart Association. Circulation. 2021 May 25;143(21):e984-e1010. doi: 10.1161/CIR.0000000000000973. Epub 2021 Apr 22. PMID: 33882682; PMCID: PMC8493650.

6 https://www.who.int/europe/news-room/fact-sheets/item/a-healthy-lifestyle---who-recommendations
7 Ibid.

Contributed by

Dr. Grace Soo
Farrer Park Hospital