RE: Those BMI machines they use at gyms are BS

I think gyms like to look cool by having lots of machines

This is exactly what it is. It's a selling point. While it might not be complete nonsense that it spits out, it certainly isn't accurate. If you ever find yourself in a store and one of these are for sale have a close look at the box and all the disclaimers that it has in the small print. They are required to tell the consumer that the machine isn't actually capable of giving accurate results.