# Question about BMI

**URL:** <https://boards.straightdope.com/t/question-about-bmi/933696>\
**Category:** Miscellaneous and Personal Stuff I Must Share\
**Created:** [February 25, 2021, 1:43am UTC](https://boards.straightdope.com/t/question-about-bmi/933696 "2021-02-25T01:43:05Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![Leaper](https://avatars.discourse-cdn.com/v4/letter/l/4bbf92/32.png) [@Leaper](https://boards.straightdope.com/u/Leaper)\
**Post date:** [February 25, 2021, 1:43am UTC](https://boards.straightdope.com/t/question-about-bmi/933696/1 "2021-02-25T01:43:05Z")

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Let’s take a hypothetical person. Let’s say this hypothetical person’s BMI is 23.2. If 25 is the lower limit for the next danger category up, how close is this nonexistent and totally hypothetical person to getting to that next level? Because in terms of raw numbers, 1.8 certainly sounds close and scary, but I don’t know how fine these things are, not being a math type. Any clarification out there?

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**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\
**Post date:** [February 25, 2021, 2:39am UTC](https://boards.straightdope.com/t/question-about-bmi/933696/2 "2021-02-25T02:39:59Z")

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BMI itself merely has units of kg/m², if I understood what you are talking about. So, if this hypothetical person simply gained weight then it would take about an 8% increase to get from 23.2 to 25. That is, the number is proportional to the person’s mass, assuming the same height.

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**Author:** ![Monty](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/monty/32/47_2.png) [@Monty](https://boards.straightdope.com/u/Monty)\
**Post date:** [February 25, 2021, 5:11am UTC](https://boards.straightdope.com/t/question-about-bmi/933696/3 "2021-02-25T05:11:45Z")

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Should that be kg/m2 or kg/m3?

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**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\
**Post date:** [February 25, 2021, 5:25am UTC](https://boards.straightdope.com/t/question-about-bmi/933696/4 "2021-02-25T05:25:17Z")

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> [@Monty](#):
>
> kg/m2 or kg/m3

If taller people had exactly the same scaled-up shape as shorter people, the cube of height would be correct. In fact, a typical taller person is slimmer than a typical shorter person. The empirical power relationship is roughly 2.5.

I haven’t seen any good answer to why they don’t use 2.5. BMI is known to be wrong at the extremes of height, it overestimates the obesity level of very tall people.