It's basically the most inaccurate way of checking because it's based on comparing you to an average, and that average isn't formulated to accommodate anything that could affect those numbers.

For people who don't understand why this matters, bodybuilders are considered morbidly obese according to their BMIs.

It's just the fastest way of determining where you might sit on a scale. And while most people do fit within certain ranges for their heights and builds, your BMI is going to be widely inaccurate if your makeup falls even a little outside of the calculated norm.

It's somewhat useful as a quantitative measure i.e. If you're a researcher asked to get results from 10 000 people in 3 days.

The best way to determine how fat/thin you are is your BF index (figure out how much fat is in your body). There are different methods, but overall, the most effective method is the objective eye; take a picture of yourself in your undies and compare that picture to a few different body fat charts (easy to find on Google). Average those out and there you go.

A dexa scan will be more accurate, but seeing the numbers go up or down over time is the real goal and who has the money to go for dexa scans every month?