The Body Mass Index (BMI) is a measure to identify an abnormal weight to height proportion and a gross estimation of body fat that has been used for almost two centuries and that is correlated with several diseases and conditions. You can check what your BMI is by using the calculator on this page a few paragraphs below. For more information about what the body mass index is, what are the flaws of BMI and more on this topic keep reading.
What is BMI?
The Body Mass Index (BMI) measures your estimated body fat taking into account your body weight and your height and is used to determine if your weight is within a healthy range or if poses you at higher risk of developing certain conditions and diseases.
The standard BMI should not be used for anyone below 20 years of age, nor in pregnant or lactating women. It provides an estimate based on just two variables, without considering other factors such as age, ethnicity, gender, body composition, fat distribution. Your BMI should be interpreted and not looked at as a single number: while it provides very useful information, you need to look at the bigger picture; your doctor will help you understand what the BMI result you obtained means in your specific case and what you can do about it. Your medical history, family history, lifestyle and other measurements together with the BMI will help in assessing your health risks.
What is my BMI? BMI Calculator
The BMI formula
If you want to know how to calculate your BMI here’s the formula for both metric and imperial systems:
- Using the metric system, the formula for BMI is weight in kilograms (kg) divided by height in meters (m) squared.
- Using the imperial system, the formula for BMI is weight in pounds (lbs) divided by height in inches (in) squared and multiplied by a conversion factor of 703.
Here’s the formula written in mathematical language and a sample BMI calculation using my own weight and height.
My calculator above this paragraph will calculate the result for you as well as will convert your height from cm to m (metric) and from feet and inches into inches (imperial) for your convenience
What is a healthy BMI?
In the general population, a BMI between 18.5 and 24.9 is considered a healthy BMI. The other Body Mass Index ranges are:
- Below 16.0: severe underweight
- Between 16.0-16.9: moderate underweight
- Between 17.0-18.5: mild underweight
- Between 25.0-29.9: overweight
- Between 30.0-34.9: obesity class I
- Between 35.0-39.9: obesity class II
- Above 40: obesity class III
As said before, the BMI does not take into account several factors and variables; for example, professional athletes with high muscle mass may have a body mass index that falls into the overweight or obese category, even though their body fat percentage is low and their disease risk is low.
Moreover, in the Asian population the BMI categories established by the World Health Organization (WHO) are different from those for the general population: this is because from the data collected over the years, it appears that Asians generally have a higher body fat percentage when compared to Caucasians of the same age, gender and BMI.
In general, a healthy BMI range for the Asian population is considered to be between 18.5 and 23.0 but depending from the specific region of the Asian continent it may be slightly higher or lower.
Male BMI vs Female BMI
There is no difference in the way the BMI is calculated among men or women, nor there is any difference in how the result is evaluated. Nevertheless, when it comes to body fat percentage (BF%) it is normal for women to have a higher BF% than men, due to human physiology and differences among the two sexes such as hormonal functions and pregnancy energy demands.
BMI in children and teenagers
For teens and children between 2 and 19 years of age, the BMI is calculated using the same formula the adults use, but the result is interpreted based on percentiles for the same age and gender reference group.
In general, a child in the 5th percentile is considered underweight and one in the 95th percentile is considered obese.
- 5th percentile means that 95% of the people in the same age and gender reference group will have a higher BMI.
- 95th percentile means that 95% of the people in the same age and gender reference group will have a lower BMI.
BMI in different ethnic groups
Healthy BMI ranges have been found to vary according to a person’s ethnic group.
- Asians tend to have higher fat percentage compared to Caucasians with the same BMI.
- Africans and African American tend to have lower fat percentage compared to Caucasians with the same BMI.
BMI in the elderlies
As we age, we lose lean body mass, a process known as sarcopenia. For this reason, older adults will have higher fat percentage when compared to younger adults with the same BMI.
What are the possible health effects of having a high BMI?
Being overweight or obese puts you at a greater risk of developing several diseases. Among the many conditions there are:
- Coronary artery disease
- Type 2 diabetes
- Gallbladder disease
- Sleep apnea
- Several different cancers, including colon, kidney, endometrial and breast cancer.
What are the possible health effects of having a low BMI?
Being underweight is a health risk as well and poses you at risk for the following conditions:
- Hair Loss
- Delayed growth and development
- Weakened immune system and higher risk of infection
- Fragile bones and osteoporosis
- Delayed healing from trauma or surgery with increased risk of complications
- If the low BMI is caused by malnutrition, micronutrient and macronutrient deficiencies may also cause a wide range of diseases and conditions.
Alternatives to BMI: other ways to assess body weight and health risks
Body Mass Index is an easy and quick test. It does not require expensive equipment or specialized tool, nor it needs a professional to be calculated. It allows large population studies and comparisons between large groups of people. For this reason, it is a very useful tool for a general population screening, for research purposes and to predict risks in large populations. For an individual assessment, BMI should be complemented with or substituted by other anthropometric measurements as well as other medical tests.
The following are alternative measures that may help improve the health risk prediction in an individual:
- Waist Circumference (WC). WC is taken with a measuring tape wrapped around your waist, roughly midway between the bottom of your ribs and the top of your hips; you should be standing and breath out naturally when taking the measurement. A waist circumference greater than 94 cm (37 in) in men or 80 cm (31.5 in) in women suggests an increased risk for disease, while a waist circumference greater than 102 cm (40 in) and 88 cm (34 in) suggests a substantially increased health risk.
- Waist to Hip Ratio (WHR). A waist/hip ratio greater than 0.90 in men or 0.85 in women suggests a substantially greater health risk.
- Skinfold caliper measurement.
- Bioelectrical Impedance
- Hydrostatic Weighting.
- Dual Energy X-ray Absorptiometry (DEXA)
Each alternative method to the BMI has its advantages, disadvantages, flaws and limitations.
Complementary exams and tests to evaluate health status in conjunction with BMI
For a BMI above 30 it may be advised to perform the following tests:
- Medical and family history
- Physical exam
- Waist Circumference
- Fasting blood glucose and glycated hemoglobin
- Lipid panel
- Thyroid hormones
- Hepatic Panel
- Electrocardiogram (ECG)
For a BMI below 18, one or more of the tests below may be advised:
- Medical and family history
- Physical Exam
- Full blood count
- Urea and Electrolytes
- Liver function panel
- Thyroid hormones
- Psychiatric evaluation for eating disorders
- Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies.
WHO Expert Consultation – The Lancet, Jan 2004
- Excess Deaths Associated With Underweight, Overweight, and Obesity.
KM Flegal – Journal of the American Medical Association, Apr 2005
- Association of bodyweight with total mortality and with cardiovascular events in coronary artery disease: a systematic review of cohort studies.
A Romero-Corral – The Lancet, Aug 2006
- Body-mass index and cause-specific mortality in 900 000 adults: collaborative analyses of 57 prospective studies.
PSC Collaborators – The Lancet, Mar 2009
- Physiology, Body Mass Index.
A Zierle-Ghosh – StatPearls, Jul 2021
- Healthy Weight, Nutrition, and Physical Activity.
US Centers for Disease Control and Prevention (CDC)
- Healthy weight
UK National Health Service (NHS)