Can the BMI be a good measure of ideal weight?


BMI or body mass index is a scientific measurement used by doctors to try and determine whether or not a person is obese or is at risk of becoming obese. BMI compares a person’s height to their weight and estimates how much body fat they have. 

The problem with this test is that it cannot actually measure the percentage of body fat and it does not take into account the differences in people such as sex, race, and body type.

BMI is calculated by dividing your weight in pounds by your height in inches squared, and then you multiply the results by a conversion factor of 703. For an example, if a person was 5' 5" (65 inches) tall and weighed 150 pounds you would take 150 and divide it by 65 (inches) squared and then multiply by 703 for a total BMI of 24.96.

The number you score on the BMI height and weight chart will place you into a category. There are four categories on the BMI scale:

  • Underweight = BMI below 18.5
  • Normal weight = BMI between 18.5 and 24.9
  • Overweight = BMI between 25.0 and 29.9
  • Obese = BMI 30.0 and above

Conclusion: Not always

BMI is not a suitable measure of whether or not you are obese and certainly not of whether or not someone is healthy overall. Most doctors will tell you that BMI is meant to be one of several screening tools used to determine overall health. BMI does not take into account sex, age, or body condition. BMI is especially inaccurate in children because they grow so quickly and the range they should be in for their height and weight changes constantly.

One example of how BMI screening can be inaccurate is with sportspeople. Athletes have lots of muscle mass and muscle weighs up to four times more than fat does. Because muscle weighs more than fat an athlete’s BMI will probably come up in the obese range when they are really healthy. BMI is also inaccurate for the elderly because as you age you lose bone and muscle mass and there is no allowance for that with the BMI formula. BMI is known to be highly inaccurate for certain groups. For body builders, pregnant women, and professional athletes the BMI will overestimate body fat and possibly place them in the overweight or obese range when they are not. For the elderly, those with physical disabilities, and people with eating disorders the BMI can underestimate the amount of body fat. 

The BMI test doesn’t cost anything or take any special equipment so it is widely accessible to all. Just because your BMI is high does not necessarily mean that you are overweight or obese, but it is something that you should discuss with your doctor.

Find out more about the Body Mass Index (BMI):

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BMI Alternative

Hi Joan

Rather than you using BMI to measure a change in body shape/size I would suggest you use a tape measure and measure your waist at belly button height, your chest at nipple height and arms at the widest part of the bicep (with your arm muscles flexed and elbows at 90 degrees).

Do this on a weekly basis and that should give you a clearer indication of your progress.

Good luck.

Alternative to using BMI

Hi iBodz

I am 65 years old and want to check my progress from the exercise I have been doing, how would you recommend I do it if BMI is not a good measure?