Author
Bowtie Team
Updated on
2025-07-29
Disclaimer: This article is translated with the assistance of AI.
Ever spotted ‘BMI’ on your health check-up report? Curious about how it’s calculated and the health risks of high or low values? Dive in with the Bowtie team for all the insights!
BMI Calculator
What is BMI?
BMI (Body Mass Index) is also known as the “body weight index,” used to assess if your weight is ideal and to measure the degree of obesity.
In the 19th century, Belgian mathematician Adolphe Quetelet proposed the “weight divided by height squared” index. In 1972, researcher Ancel Keys named Quetelet’s formula “BMI” and cited it as a measure for overweight and obesity, which has been used ever since.
How to Calculate BMI? What’s Normal?
The BMI calculator at the start of this article is based on a simple formula:
BMI = Weight (kg) / Height (m)²
The World Health Organization divides BMI standards by region. For the Asia-Pacific region (including China, Hong Kong, and Taiwan), it follows the 2000 Western Pacific report to classify weight levels. For Asian adults, a BMI of 23 or higher is “overweight”; 25 or higher is “obese.” The WHO BMI standards do not differentiate by gender. Details are as follows:
| BMI Index | Weight Category |
| < 18.5 | Underweight |
| 18.5 – 22.9 | Normal |
| 23 – 24.9 | Overweight |
| ≥ 25 | Obese |
Example: Height 1.6m, Weight 55kg
BMI = 55kg / (1.6m x 1.6m) = 21.48
Weight Category = “Normal”
Since children and adolescents under 18 experience significant changes in height and weight, the above BMI calculation isn’t suitable. The World Health Organization has separate standards to assess if children and teens are obese or overweight. For details, check the BMI reference for children and adolescents aged 5-19
Additionally, Hong Kong’s Department of Health defines childhood obesity based on the 1993 growth survey. By their definition, weight adjusted for height exceeding 120% of the median weight is considered obese. This applies to boys with height ≤175cm or girls ≤165cm. For details, visit the Department of Health Student Health Service
What Health Issues from High or Low BMI?
Whether your weight is too low or too high, it can lead to various health problems, and in severe cases, even death.
If you’re overweight, leading to obesity, it can negatively impact your health and increase the risk of the following diseases:
- Diabetes
- High blood pressure and cardiovascular diseases
- Gallbladder diseases
- Arthritis
- Cancers, such as colon or breast cancer
- Mental or psychological disorders like depression
If you’re underweight, malnutrition can increase the risk of the following diseases:
- Impaired immune function
- Respiratory diseases
- Digestive system diseases
- Cancer
- Osteoporosis
According to the Department of Health’s population health survey in November 2017, more than half of people aged 15 or above had a BMI of 23 or higher, classifying them as overweight or obese—an 11% increase from the 2003-2004 survey.
Obesity can easily lead to serious health issues (like strokes and heart disease), so it’s even more reason to get insured! However, many insurance companies add ” loading ” (extra premiums) for overweight individuals. Bowtie does the opposite—we encourage healthy weight loss with our Term CI “loading reduction” mechanism.
If you’re loaded due to being overweight, once you slim down to normal levels, you’ll be exempted and only pay standard premiums.
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BMI vs Body Fat: Key Differences?
BMI is more suitable for population-level statistics to measure risks of morbidity and mortality, but it doesn’t effectively reflect an individual’s muscle-to-fat ratio. Everyone’s body fat varies, so even for the same age and gender, BMI can differ. For athletes with more muscle or pregnant women, BMI might overestimate body fat; for older adults or those with disabilities who have less muscle, it might underestimate it. That’s why measuring body fat is equally important.
Body Fat Percentage refers to the ratio of fat weight to total body weight, providing a more accurate measure of body fat content. According to Beth Israel Lahey Health Winchester Hospital , the standard body fat percentages for men and women across different age groups are as follows:
| Age | Male | Female |
| 20-39 | 8-19% | 21-32% |
| 40-59 | 11-21% | 23-33% |
| 60-79 | 13-24% | 24-35% |
What is the “New BMI”?
Some in the academic community question whether Quetelet’s BMI formula, designed with a simple system without computers, calculators, or any electronic devices, should be updated using modern technology to create a more complex formula. This could help assess body health from a three-dimensional perspective.
In 2013, Oxford University mathematics professor Nick Trefethen proposed a new BMI formula:
BMI = 1.3 x weight (kg) / height (m)^2.5; or
BMI = 5734 x weight (lbs) / height (in)^2.5
Trefethen admits that the human body structure is very complex and may not be fully captured by a single formula, but he believes the new BMI calculation can more accurately assess body shape and constitution.
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