Body Fat Calculator From Waist Measurement

But we don't have to lean on my "trust me, bro": we can actually gut-check progress by using some of the research that's available.

In 2018, Woolcott and Bergman published a body fat estimation equation developed from data on more than 12,000 adults in the National Health and Nutrition Examination Survey and validated against DEXA. [1]

Unlike BMI, which only uses height and weight, their equation uses height and waist circumference:

We have Kristin's height (5'8") and her waist at both time points. When we plug in her numbers, the equation estimates her body fat went from about 29% to 26.5%: a drop of about 2.5%.

Now, I should point out that the equation has a margin of error of about ±3.5%. Meaning it could overestimate by up to 3.5 percentage points or underestimate by up to 3.5 percentage points.

Looking at Kristin's starting photo, I'd put her closer to about 25%. Likewise, her most recent photo suggests she's probably sitting around about 22–23%. Both of those still fall inside the equation's margin of error.

Regardless, the exact percentage doesn't matter too much. Whether you use the equation's numbers or the visual estimate, the change is roughly the same: about 2.5–3 percentage points.

Since her weight stayed stable at 136 lbs (~62 kg), that works out to roughly 3.5 lbs (~1.6 kg) of fat lost.

And since her weight didn't change, that fat had to be replaced by something:

If you're skeptically inclined, you might be thinking two things: can we really use waist measurements to predict fat mass changes, and could someone really gain 3+ lbs of muscle in four months?

The research would suggest yes.

On the first point: in a DEXA-tracked study of more than 300 adults, Lahav and colleagues found that changes in waist circumference closely mirrored changes in fat mass, with a correlation of r = 0.84. A correlation of 1.0 would mean a perfect match between the two variables. [2]