Originally Posted by Iride01

Well I wasn't talking specifics for the OP, maybe I should have specified that. All of the size I mentioned were not intended to be recommended sizes.

However I won't disagree that you could try to figure out the imaginary seat tube length by your method. Some manufacturers even show that in the bike geometry tables. But for a long time we've known that sizing a bike by the seat tube measurement isn't the perfect way to tell.

Arguably I will agree that it gives a decent basis to pick a bike. But probably no more than going by anything else. I rode bikes that ran the gamut from 64 cm frames to 56 CM frames during my adult life. Most years 35+ spent on a 64 cm, most miles on a 59 and 60 cm, and currently on a 56 cm bike. All comfortable for the riding I was doing at the time on them. I'm 5'-11" with a 34.5" inseam.

Also, when the manufacturers list their bikes in letter sizes you can't do an imaginary seat tube length if the mfr doesn't tell you it in their geometry tables. IE. Look at this TCR Advanced SL

https://www.giant-bicycles.com/us/tc...sl-disc-1-2021

They don't draw the imaginary horizontal top tube or even give a measurement that will suggest the imaginary seat tube length. Yet here we are, looking at a M (medium) size frame that the manufacturer say might be the one for the 5'-10" OP. As with many bikes people fall in the overlap of bike sizing. And even when they don't, a size bigger or smaller probably isn't going to be bad for most.