What is BMI and is it a reliable tool for health?

BMI includes height and total body weight.


There’s a lot of attention surrounding BMI — body mass index — and whether it should be used as a health measurement.

BMI is used as a guide for assessing body weight as a risk factor for health. Weight and height are plugged into a formula to categorize a person as underweight, normal weight, overweight or obese.

The argument against BMI is that it measures weight, not fat, which makes it inaccurate and thus that it contributes to body shaming.

On the positive side, it can be a valuable screening measurement with years of research surrounding it.

BMI has been used by doctors as part of annual physical exams for decades. It’s used to determine whether a person needs to lose weight for long-term health, as higher BMI seemed to correlate with higher body fat. Obesity is linked with several health issues, which is why BMI is monitored.

The simple calculation is weight in pounds multiplied by 703 and then divided by height in inches multiplied by height in inches. So a person who weighs 150 pounds and is 67 inches tall (150 X 703/68 X 68) has a BMI of 23.

This calculation expresses the weight to height relationship as a single number. According to the chart, a normal BMI is between 18.5 and 25, overweight is between 25 and 30, obese is over 30, and underweight is under 18.5.


The relationship between weight and height does not reflect many other differences in body composition affected by things such as muscle mass, age, gender, body frame, ethnicity and where a person carries excess body fat.

In the case of muscle mass, muscle weighs more than fat, so people with more muscle mass will have a higher BMI, possibly putting them into the overweight or obese range even though they might be healthier than people with a lower BMI and less muscle mass.

Women tend to have a higher fat percentage than men, and a person with a larger, heavier bone structure or a smaller, lighter bone structure might have a less accurate BMI.


There are other more accurate tools to assess a person’s healthy weight and to determine body fat percentage, such as waist circumference measurement, body scans and calipers.

As a starting point for anyone concerned about their weight and related health risks, BMI is an accessible and potentially valuable tool. It’s simple to calculate and relatively accurate. Studies have shown a correlation between body fat and future health risks. And BMI is so widely used that health professionals have data to help with assessments.

Bottom line

BMI is a reasonable assessment of body fat but doesn’t measure body fat directly. It’s useful in screening and tracking weight to identify potential health risks.

Environmental Nutrition is an independent newsletter written by nutrition experts. 

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