(Narcissistic disclaimer: No, that's not a picture of me.)
BMI (Body Mass Index) has long been a measure of "health" because it correlates weight with height.
A new ratio—waist circumference to height, or the Waist-Height Ratio (WHR)—is being shown to correlate better with disease prevention. Short answer: Waist circumference should be less than half one's height.
For example, I am six feet (72 inches) tall and I wear a size 36" (waist size) pant. But my waist measurement is actually pushing 37". Therefore, I am over the maximum ratio of <1:2 (<36:72). Not good.
Obviously, I have known this for some time simply because I store fat in my belly. I can look at my belly and see that it is too big—not obese, but not trim. I don't know why, given my diet (very little fat), a vegan diet of whole foods, etc. Too few calories and too little movement, and the wrong kinds of exercises, have resulted in accumulated fat.
I have read several studies recently about the dangers of sitting—something I do most of every day as a writer/editor in a home office. I bought a pedometer a week ago to measure the number of steps I take every day and it averages between 2,500 and 3,000. Health guidelines say we ought to average 10,000 steps per day. When I was in another city recently on family business and was moving all day long, I discovered I had lost weight when I returned. So lack of movement in my daily vocation is a significant problem for me. (Studies show that people who sit most of the day live shorter lives than people who "move" most of the day.)
I began rowing on my Concept 2 rower two weeks ago (after a long layoff), about 5,000 meters per session to start, and am riding my bike less. An hour of stationary bike riding produces lots of sweat but very little abdominal or upper body stress, which rowing does. At the same time, I returned to exercises that will hopefully strengthen my core and gradually cut down on body fat. And have begun walking as well—I did 12,547 steps yesterday and am up to 5,578 today after a morning walk.
So, a WHR of <1:2 is my goal. Dr. Greger has a brief overview video on WHR here.