A Refinement of BMI
It’s usually not the number of pounds you weigh that indicates a risk for disease, but rather the percentage of those pounds that are made up of fat rather than muscle. BMI — body mass index — came into use in the 1970s when Ancel Keys proposed it as a reasonable proxy for body fat percentage, but it was preceded by around 25 years by the observation by French physician Jean Vague that abdominal obesity was a good predictor of serious medical problems such as diabetes.
The Harvard School of Public Health recommends that physicians take a waist measurement rather than relying on BMI to diagnose overweight or obesity, because people can have abdominal obesity even at relatively low weights.
Should I Use Waist Size Instead of BMI?
Yes — partly because BMI is not a very good tool for individual tracking. BMI is more useful when we are talking about big groups of people, or as a cheap, simple screening tool to decide what to do next, like getting lab tests.
When you have a fat-loss goal, tracking body fat is ideal, but it is difficult or expensive to track well — electrical readings on weight scales are OK for trending but not very descriptive. And you may choose to limit how much you use the scale, anyway, especially if negative experiences with weight loss in the past have left you anxious about stepping on it.
Waist size will definitely help you much more than scale weight or BMI as a simple method for tracking fat-loss progress at home.