| BMI's are inherently bad. they don't take into consideration body frame or current muscle-to-fat ratios. for example, last year, i worked out hard for 6 months and increased my bench from 135x8x3 to 175x10x3, yet i didn't gain or lose a single pound. So, based on the BMI, i was still exactly the same, but that's impossible based on my power gains.
A proper test will require using calipers in at least 5 areas, not an online test.
I think good goal BMI for guys is around 14-16 and for women 18-20 ... keeping in mind that these are not "perfectly" fit people, but are in good shape. Also, too low is just as bad, if not worse than too high. | 
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