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Regarding BMX bikes, is there an easy way to determine what top tube length is most suitable for a person based on their height?

In my experience, most LBS only want to talk about BMX size in terms of wheels ("it's a 20 inch" or "it's an 18 inch) since they're mostly considered to be kid's bikes.

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    It's probably worth finding a rider-owned LBS then, or at least where they don't put all bmx bikes in the kid's corner. Also note that a person's height is only one aspect to consider for TT length. Another one is what you are going to ude the bike for: generally dirt jumpers might prefer a longer TT than for riding pure street. Then again, seat tube angle also plays a role in 'effective' TT length so in the end you'd ideally try some different models and see what fits best as there is no one-to-one relationship between rider size and TT length.
    – stijn
    Oct 6, 2015 at 9:54
  • In general, standard diamond frame bikes are sized by seat tube length, and top tube length is in some fixed proportion to the seat tube length (because we know that all people have the same ratio of leg length to trunk length, right?). But BMX bikes, of course, are not standard diamond frames and not sized the same way -- there are several variables to play with. I think you have to seek out some web resource, or a good reference book. Oct 6, 2015 at 11:43

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This is the tool you're looking for:

https://www.ebicycles.com/bicycle-tools/frame-sizer/bmx-bike

It's not a linear calculation, but it would be fair to say that a rough estimate is: start with a 22.5" top tube for a 6' adult male, and add/subtract half an inch of top tube for every inch of height. bmx calculator

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    It super matters if you're racing or riding park/ramp. This guide sucks.
    – jqning
    Oct 6, 2015 at 23:06
  • @jqning Agree. I'm about 190 (6' 2" or something like this), I ride a 21" frame and for street/park stuff it only feels a tad too short (I'd probably move to 21.5").
    – K. Minkov
    Mar 26, 2021 at 15:25

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