How is obesity diagnosed?

    Written by:
    Morgan Pfiffner, Marina Shkayeva

    Fact-checked

    by:

    Nick Milazzo
    Last Updated: May 16, 2025

    Obesity is diagnosed using a body mass index (BMI) of 30 or higher, and specific categories range from overweight to class III obesity. However, because BMI does not account for factors like muscle mass, additional measures such as body fat percentage may also be considered in the evaluation.

    Obesity is defined as having a body mass index (BMI) of greater than or equal to 30 kilograms per meters squared (kg/m2; “meters” meaning a person’s height). The categories are designated as follows:[1]

    BMICategory
    25–29.9overweight
    30–34.9class I obesity
    35–39.9class II obesity
    ≥40class III obesity

    While BMI is a generally useful measurement at the population level, because it only accounts for total body mass, individuals who are tall and/or muscular may have a BMI that qualifies as “obese”. As such, it’s useful to factor other measures like body fat percentage into an evaluation of whether someone has obesity.[2]