What Is Body Mass Index (BMI)?
Body mass index is a measure that just focuses on weight and height. BMI was developed around 1850 by the Belgian mathematician Adolphe Quetelet.
Calculation of BMI
The formula for BMI is your weight in pounds times 703 divided by the square of your height in inches. For example, if your weight is 180 pounds and your height in 6 ft (72 inches), your BMI is 24.41, based on the following calculation:
- Step 1: 180 pounds x 703 = 126,540
- Step 2: 72 inches x 72 inches = 5,184
- Step 3: 126,540 divided by 5,184 = BMI 24.41.
Now you can see where you fall within the BMI range categories. Using the above example, with BMI of 24.41, you are considered to be of normal weight.
Body Mass Index Range
|Less than 16.0||Severely underweight|
|16.0 to 18.5||Underweight|
|18.5 to 25.0||Normal weight|
|25.0 to 30.0||Overweight|
|30.0 to 35.0||Moderately obese|
|35.0 to 40.0||Severely obese|
|Over 40||Morbidly obese|
Calculate Your BMI
To calculate your BMI you can use the BMI calculator below by inputting your weight in pounds and your height in inches (just remember 1 foot = 12 inches).Alternatively you can also use the charts below to see what your Body Mass Index is, and where you fall in terms of obesity (click to enlarge).
Source: Vermont Department of Health
Shortcomings of the BMI Measurement
BMI does not distinguish:
- between small and large body frame
- between male or female
- fat from muscle
- fat distribution
Small vs Large Body Frame
If you have a large body frame, you will naturally need more body mass to support your frame. BMI does not provide a range based on your body frame size.
If you have a large body frame, one way to determine if you are in a healthy BMI range is to shift the ranges by 10% (Source:Wikipedia). Add 10% for a large body frame and deduct 10% for a small body frame. Using this method, if you have a large body frame, you can use the following ranges to determine if you are overweight.
BMI Ranges for People with a Large Body Frame
- BMI below 20.35: you are considered to be underweight.
- BMI from 20.35 to 27.5: you have a normal weight.
- BMI from 27.5 to 33: you are considered overweight.
- BMI from 33 to 38.5: you are moderately obese.
- BMI from 38.5 to 44: you are severely obesity.
- BMI above 44: you are morbidly obese.
How to Figure Out Your Body Frame Size
According to the National Institutes of Health, you can determine your body frame size by measuring your wrist with a tape measure. Use the following chart to see where you fall in terms of body frame size.
- 5 Feet 2 Inches (157.5 cm) or Below
- Small: wrist size less than 5.5 inches (14 cm)
- Medium: wrist size 5.5 to 5.75 inches (14 to 14.6 cm)
- Large: wrist size over 5.75 inches (14.6 cm)
- 5 Feet 2 Inches to 5 Feet 5 Inches (157.5 to 165 cm)
- Small: wrist size less than 6 inches (15.2 cm)
- Medium: wrist size 6 to 6.25 inches (15.2 to 15.9 cm)
- Large: wrist size over 6.2 inches (15.9 cm)
- 5 Feet 5 Inches (165 cm) or Above
- Small: wrist size less than 6.25 inches (15.9 cm)
- Medium: wrist size 6.25 to 6.5 inches (15.9 to 16.5 cm)
- Large: wrist size over 6.5 inches (16.5 cm)
- 5 Feet 5 Inches (165 cm) or Above
- Small: wrist size 5.5 to 6.5 inches (14 to 16.5 cm)
- Medium: wrist size 6.5 to 7.5 inches (16 cm to 19 cm)
- Large: wrist size over 7.5 inches (19 cm)
Men vs Women
Most men have bigger frames than women, and are more muscular, so for those men, having a higher BMI than a woman of the same height is expected. However the BMI ranges do not differentiate between men and women.
Fat vs Muscle
Athletes tend to have higher muscle mass and weigh more, and sometimes fall under the overweight category. For these people, it does not necessarily mean that they need to lose weight or they are unhealthy. They just have lots of muscle (not fat) that make them weigh more. So they are not the same as someone who is the same weight but has hardly any muscle.
If you are athletic and are falling in the overweight category using BMI, make sure to also use the other methods (body fat percentage, waist-to-height-ratio, waist-to-hip ratio). If they are all within or below the acceptable range, there is probably no health reason for you to lose weight.
On the other hand, if you’ve been exercising and have gained weight, don’t be too quick to assume that all of that is muscle. Some people end up being very hungry and eating more after exercise, especially if they’ve just started to incorporate an exercise routine. You may want to check your body fat percentage to make sure you are not gaining more fat than muscle!
Body Mass Index does not capture the one very important indicator of potential health risks in the future, which is where the fat is distributed in your body. According to one study, the risk of death of people with normal weight but with high belly/abdominal fat was 2.08 times greater than that of people with normal BMI and waist-to-hip ratio.
So even if you are in the healthy category based on your BMI, if you have a high concentration of fat in your belly area, you are at double the risk of death than people who have a regular belly size.
Still a Good Starting Point
Keeping the above shortcomings in mind, BMI is still a good starting point to figure out how much you should weigh and your current obesity level, whether you are male or female. According to Weight Watchers, for both male and female, when the BMI increases above 25, the number of health issues also rise.
However again BMI has its limitations, so to get a more accurate and comprehensive view of where you are in terms of your health, make sure to calculate other ratios (Waist to Height Ratio & Waist to Hip Ratio & Body Fat Percentage).