Body Mass Index or BMI calculator is a screening tool that we use to indicate whether a person is underweight, overweight, or obese. This tool uses your weight and height to determine if your weight is healthy. Being overweight or obese can increase your risk of developing many health conditions like cardiovascular problems, high blood pressure, and diabetes. Being underweight can increase your risk of developing anemia, osteoporosis, or malnutrition.
Body mass index or BMI calculator is not a diagnostic test as it cannot determine the exact body fat percentage of a person. It does take into account age, sex, ethnicity, or muscle mass, so it may not be a reliable indicator in some cases. While BMI cannot be used to diagnose disease, it can give your doctor a fair indication of the issues that you may face.
How to calculate BMI?
We calculate Body Mass Index (BMI) using a person’s height and weight. It is a simple mathematical formula that helps to categorize your weight as underweight, normal, overweight, or obese.
The Metric BMI formula
BMI = Weight (kg)/ [height (m)]2
For example, if your weight is 70 Kg, and height is 1.7 meters your BMI is as follows –
BMI = 70 kg/ (1.7)2 = 24.2
The Imperial or English BMI formula
BMI = Weight (lbs.) / [height (in)]2 x 703
For example if your weight is 150 lbs. and your heights is 65 inches your BMI is as follows –
BMI = [150/ (65)2] x 703 = 24.96
You can also go to the website of the National Institute of Health (NIH) and calculate your BMI using the BMI calculator there (1). You can estimate your BMI using tables in which you can match height in inches to weight in pounds. The National Institute of Health website also provides BMI tables (2).
Find your height on the left side of the chart, then search for your weight across that row. Now scan across the top to check the corresponding BMI.
What does the result mean for adults?
For people older than 20 years, BMI is interpreted using standard weight categories. These categories are the same for both men and women.
Between 18.5 and 24.9
Between 25 and 29.9
For example, if a person’s height is 5’5”, here are the weight ranges and the corresponding BMI ranges and the weight status for that person:
110 lbs. or less
111 lbs. to 149 lbs.
18.5 to 24.9
150 lbs. to 179 lbs.
25 and 29.9
180 lbs. or more
Can you calculate BMI for children the same way?
Even though the same formula is used to calculate the BMI in teens and children, it is measured differently. The amount of your body fat changes with age, and it also differs between boys and girls. The CDC BMI-for-age growth charts take these differences in age and sex into account and show BMI in children as a percentile ranking (3, 4).
Children with BMI at 95 or above percentile of children of the same age and sex would be considered obese, while children below the 5th percentile are considered underweight. Children whose weight is between 5th to 85th percentile are regarded as healthy weight, and children between 85th to 94th percentile are considered to be overweight.
The Centers for Disease Control and Prevention website has a Child and Teen BMI calculator. This calculator provides BMI and the corresponding BMI-for-age percentile based on the CDC growth charts for children and teens (aged 2 to 19 years) (5).
Is BMI a good indicator of body fat measurement?
While there is a strong correlation between BMI and body fatness, it has its limitations. Two people with the same BMI can have different levels of fat in their bodies. A person’s gender, ethnicity, and athletic ability can make a difference in the total body fat in their body.
- At the same BMI, women have more body fat than men.
- At the same BMI, Asians have more body fat than Whites.
- At the same BMI, older people have more body fat than younger people.
- At the same BMI, athletes have less body fat than people who are not athletes.
What are the other ways to measure body fat?
There are several different ways to measure body fat. These methods measure your body composition to assess what percentage of your body mass is fat and what percentage of it is the non-fat mass that includes muscle, bone, water, organs, and tissues.
1. Skinfold calipers
Skinfold calipers are used to measure the thickness of the fat under your skin at different body locations. This method requires practice and basic knowledge of anatomy to get the correct readings.
2. Bioelectric impedance
Bioelectric impedance devices detect how your body responds to electrical currents. Fat, muscle, and water impede the current differently, which helps the device to predict your body composition.
3. Dual-energy X-ray absorption scan
DXA uses X-rays of two different energies for estimating your body fat percentage. DXA scan is also used to measure bone density and can only be performed in hospital settings.
4. Hydrostatic weighing
In this method, you are weighed when submerged underwater after exhaling as much air as possible from your lungs. This technique measures your body composition based on its density.
5. 3D body scanners
The 3D body scanners generate a 3D image of your body using infrared sensors. The equation of the scanners then estimates the fat percentage of the body based on your body shape. These scanners are not commonly available but are gaining popularity.
6. Electrical Impedance Myography
EIM devices send currents through smaller regions of your body to estimate the body fats of those specific areas. This device is placed on specific body regions like skinfold calipers. Limited knowledge is available about the accuracy of this device.
Health risks related to high BMI
If you are overweight or obese (BMI over 25) you have an increased risk of developing the following health conditions:
- Hypertension or high blood pressure
- High LDL cholesterol, low HDL cholesterol, or high levels of triglycerides
- Coronary heart disease
- Type 2 diabetes
- Gallbladder disease
- Sleep apnea
- Certain cancers (endometrial, breast, colon, kidney, gallbladder, and liver)
- Body pain
- Mental illnesses like depression and anxiety
Health risks of low BMI (less than 18.5)
If you have a low BMI, then you are malnourished and may develop
- Compromised immune function
- Respiratory disease
- Digestive diseases
Benefits of maintaining a healthy BMI
There are many benefits of maintaining a healthy BMI (18.5 to 24.9)
- Much lesser joint and muscle pains
- Higher energy levels
- Improved regulation of bodily fluids
- Healthy heart and circulatory system
- Improved sleep patterns
- Reduced risk of developing type 2 diabetes or high blood pressure
- Reduced risk of cancer
BMI is a helpful tool for measuring the risk of certain health conditions. Doctors can use it to check for weight problems in children and adults. However, just using BMI is not enough. They have to consider other factors like body mass composition, diet, and family history of health problems. Body mass composition can be checked with skinfold calipers, bioelectric impedance, DXA scan, hydrostatic weighing, and 3D scanners.