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BMI is a commonly used measure to determine whether or not a person is at a healthy weight. A person is said to be overweight if they are above 25 and underweight if below 18.5. This question asks what the scientific basis is for BMI ranges, but does not question the difference between the values within that range.

Between the upper and lower "healthy" limits is a rather large range of weights. Anecdotally, when I am at a BMI of 25 I still look and feel rather overweight (flabby, bulging belly, etc.). I've only ever been as low as around 21 or 22 BMI, at which point I am rather slim, and at the weight I'd need to be to achieve 18.5 I imagine I would be very skinny, too slender by my personal standards.

Is it really the case that someone at a BMI of 18.5 would be equally as healthy as someone at 21.75 (the midpoint) or 25? By healthy I mean equivalent risk of long term illnesses, early death, etc. (assuming other health-related factors to be the same).

If not, then is there a more accurate way to determine an optimally healthy weight (as opposed to merely a range of healthy weights)? Having been unable to find any information on this point, my target weight is a BMI of 21.75, but I would like to fine tune this if possible.

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    Look up body fat percentage. BMI is a universal number -- universally useless, misleading and stupid. – Carey Gregory Mar 22 '16 at 19:40
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    @CareyGregory there are certainly issues with it, but it is far from useless. It provides a simple and accessible way to track and set goals for weight and when used as a statistical measure, we can use it to identify correlations to health. – JBentley Mar 23 '16 at 6:13
  • Sure, assuming the person isn't physically fit and muscular, in which case the number is extremely misleading. A measure with such a huge limitation that has no way of expressing that limitation is so flawed as to be useless. – Carey Gregory Mar 23 '16 at 13:52
  • @CareyGregory You're looking at it from the point of view of an individual (and of course every individual is different). I am talking about statistics. When you look at a sufficiently large sample size, then you can make generalised correlations. In the same way that you can say "males are more likely to have a car accident than females" and that information is useful for calculating insurance premiums, but that doesn't mean it applies to a specific male who happens to be a very skilled and careful driver. – JBentley Mar 23 '16 at 15:06
  • Your second question was if there's a more accurate way to determine optimal weight. I would say yes, and it's not BMI. Body fat % works for all body types across all populations. BMI does not. – Carey Gregory Mar 23 '16 at 19:08
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BMI is calculated from this formula, only looking at the height and weight of a person.

However, muscle is bulkier than fat, so two people could have the same BMI but be at different health levels (for example, if people A and B are the same height and weight then they will have the same BMI. However, person A could be mostly muscle and in good shape they will be healthier than person B who has less muscle weight and more fat weight and isn't in as good of shape).

As stated in the comments above, Body Fat % is a better way to determine health.

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