BMI (Body Mass Index)

What BMI Measures

BMI doesn't measure body fat. It's arithmetic: weight in kilograms divided by height in meters, squared, producing a single number regardless of what that weight is made of.^[1] Adolphe Quetelet, the Belgian mathematician and astronomer who developed the formula in the mid-19th century, wasn't building a clinical tool. He was studying patterns across large populations as part of his broader work in social physics, not assessing any one person's health.^[1] That population-level design is built into what the number can and can't tell you.

How It's Measured

Calculating BMI takes nothing more than a height and a weight, which is part of why it spread so widely. No scan, no specialized equipment, just a formula anyone can run.^[1] That same simplicity is the tradeoff. Two people with an identical BMI can carry very different amounts of muscle, bone, and fat, because the formula has no way to tell those components apart. StatPearls notes that BMI tends to overestimate body fat in people with a lot of lean mass, like athletes.^[1]

Why People Track BMI

BMI's original use case was research, and that's still where it does the most work: it's cheap to collect at scale, so it lets scientists compare body-weight patterns across huge groups of people over time. Some people log it in an app for the same reason they log weight, as one more data point in a longer trend, without treating any single reading as a verdict on their body.

For a closer look at what BMI leaves out, see Body Fat Percentage, which covers how fat mass itself gets measured.

How Go Go Gaia Tracks BMI

Go Go Gaia doesn't calculate BMI itself. It syncs the value Apple Health already calculates from your logged height and weight, then charts it over time alongside your cycle phase and other biometrics, so it sits in context instead of standing alone.

What the Research Shows

In June 2023, the American Medical Association adopted a policy addressing how BMI should and shouldn't be used in medicine. Its press release notes that BMI doesn't account for differences across race and ethnic groups, sexes, genders, and age span, and that the formula was built largely from data collected on earlier generations of non-Hispanic white populations.^[2] The AMA's recommendation is to use BMI alongside other measures, not as a standalone number.^[2]

A widely cited 2015 critical review in Nutrition Today puts the core problem in concrete terms: "a person can have a high BMI but still have a very low fat mass and vice versa," because the formula has no way to separate the two.^[3] The same review concludes that a BMI-based category "should not be used exclusively" to counsel an individual or design a treatment plan, precisely because the measure was built to describe populations, not diagnose people.^[3]

Related Terms

Frequently Asked Questions

Educational information based on published sources. Not medical advice. For personal concerns, please consult your doctor.

Does BMI measure body fat directly?

No. BMI is a ratio of weight to height, calculated with a simple formula. It correlates with body fat at a population level, but it doesn't measure fat tissue directly and can't tell fat mass apart from muscle or bone in any one person.

What are BMI's documented limitations?

The American Medical Association's 2023 policy notes that BMI doesn't account for differences across race, ethnicity, sex, gender, and age span, and that it was built largely from data on earlier generations of non-Hispanic white populations. A 2015 critical review in Nutrition Today makes a related point: two people can share a BMI and have very different amounts of actual fat. Both sources note BMI was built from population data and wasn't designed to diagnose an individual.

How does Go Go Gaia get my BMI?

Go Go Gaia doesn't calculate BMI itself. It syncs the value Apple Health already calculates from your logged height and weight, then charts it as a trend alongside your cycle phase and other biometrics.