What is a Healthy BMI? Get the Skinny on Body Mass Index.

Health Articles
August 27, 2015

In addition to providing you with the most accurate weight measurements, all Withings smart scales also calculate your Body Mass Index (BMI). Here’s what you should know about this key health indicator.

Who invented the Body Mass Index and how is it calculated?

The BMI was established by Belgian astronomer, mathematician, statistician and sociologist Adolphe Quetelet in the first half of the 19th century.

Your BMI is an equation mixing your height and your weight and the result is intended to assess if you have a healthy weight/height ratio.

The BMI formula:
BMI = weight / height2

What is a healthy BMI?

Based on Quetelet’s work, the World Health Organization (WHO) defined The International Classification of adults who are underweight, overweight and obese:

  • BMI under 18.5: Underweight
  • BMI from 18.5 to 24.9: Normal weight
  • BMI from 25 to 29.5: Overweight
  • BMI over 30: Obese

Note: If you are a top level athlete or a weight-lifter, the general BMI model isn’t going to be as accurate. We advise you to switch your body composition model to “Athlete” in the Health Mate app.

What are the risks for my health if I’m not in the “Normal” weight category?

On the one hand, people with a BMI higher than 25 have a higher risk of developing heart disease, hypertension, stroke and diabetes. On the other hand, a BMI lower than 18.5 may indicate an eating disorder or malnutrition.

Body Mass Index Isn’t Perfect

Many people have raised issues with BMI calculations. Some of these issues include a criticism that Body Mass Index ignores factors like the scaling of height, the fact that it can’t differentiate fat mass from muscle mass (our body composition scales give fat mass as well) and exactly where the line for overweight and obese should be set.

That said, Quetelet’s system has survived for generations, largely unchanged, and is still considered one of the best tools to help classify and assess the health of individuals and the general population.

Update 2021: Want to learn more? Check out our Health Insight page that will tell you lots more about what BMI means.