Kontact

There is no perfect math equation for this. The easiest thing is to look at the website of the brand and check the recommendations based on height.

In general, if the sizes are "classic", you can match things up like this:
5'4" - 50cm
5'6" - 52
5'8" - 54
5'10" - 56
6' - 58
6'2" - 60

This is roughly what you'll find on bike manufacturers' recommendations pages.

oldbobcat

With some bikes using extended head tubes for comfort, effective seat tube length hardly has any meaning now. My 58 cm H2 Trek Madone has not a single dimension that measures 58 cm, and using your method would designate it 60 or 61. About the only thing you can say about its size is, "It seems to fit like a 58 that was built in the late 1980s or '90s." My advice for riders who don't know where to start on sizing is to read the manufacturers' recommendations and geometry charts and know your basic body morphology. If your arm or leg length is not average for your overall height, understand that some models will be a difficult fit. Bike dealers are getting more sophisticated in sizing bikes to customers.

Iride01 

The OP has a bike from his/her teen years and wants to know if it's too small. It's thought by them to be a letter size, either X or XS.

My suggestion to the OP was that if nothing bothered them when riding it, then it really didn't matter whether it too small from what the manufacturer or other might suggest for the OP's current size.

The other stuff is just the usual bickering among the rest of us. I never stated that effective seat tube length is the one and only way to size a bike. It's just one of several.

The OP is 161cm and has a 72cm inseam. If it's a S or Small size frame, then it's probably the proper size or the OP is only a cm or two of it having been the correct recommended size. Still, to me, it doesn't matter what size it is. The OP either is able to ride it comfortably or is not able to ride it comfortably. Not anything different than those cyclist claiming to have properly size bikes but still can't seem to ride comfortably.

oldbobcat

Yep, whether its a S or XS, it should still fit. Most women stop growing in their later teen years anyway. If it's still a concern, she should ride down to here local bike shop and get a 10-minute eyeball fit instead of asking a bunch of strangers who can't even see how she sits on the thing.

puppychow

I had a problem understanding Giant’s frame sizing. I’m 5’10” but with relatively shorter legs for my height (29” inseam). There were three sizes M(5’7-5’11) M/L(5’9-6’1), and L(5’10-6’3) that could have technically fit

I ended up going with the M bc of my inseam and it seems to fit well!

Olgaimage

I was at a bike shop today and tried a Trek with frame size 52. The guy who helped me took his time to walk beside me while I was on my old bike and the new bike. He said that my bike is too small for me and the new bike is better. He also said that Trek-sizes are small and he always recommends going one size up when buying Trek.

Now I have another question. I have purchased extra short crank arms for my old bike 5 years ago and asked to have them moved to the new bike. At first, he said he wouldn't recommend moving used parts to the new bike. Then, after assessing the problem, he said it could be done, he just had to use some other chainrings than those that the bike came with from the manufacturer. He hasn't done it yet. Do you know if using the used crank arms could cause additional wear on the new bike compared to using new arms? This is the right arm:

Kontact

I wouldn't say Treks are generally small. Get whatever size fits you in whatever model. Some Nikes run smaller than other Nikes.

Square taper cranks like the one pictured won't work on some newer bikes because the frame's bb is too wide to accept a square taper BB.

But they won't hurt the bike just because they're used.

OregonJim

This might work for the hypothetical "average" person, but it doesn't work for me. I'm 5'8", and a 54cm frame would have me crushing my bits on the top tube as my feet dangled off the floor. I simply cannot even straddle a bike of that size. Long torso, short legs. For me, 44cm to 46cm is about perfect, and NO manufacturer would suggest that size based on my height. I've learned to use standover height as a go no-go measurement - everything else can be adjusted to fit (within reason).

Kontact

What happened with compact frames was that the standover height of classic sizes got much lower, so people with shorter legs for their height were able to straddle bikes they previously could not while still having more than adequate top tube length to not require a really long stem.

I ride a 50 and can stand over a 52, so I'm assuming your inseam must be 27" - which is indeed pretty short for someone 5'8". The point of the chart I posted is that body proportions are a bell curve, and most people 5'8 are going to have around 32" inseams, and nearly 90% will be within 2" of 32". Bike makers can't really control for people whose proportions are on the 1% ends of the curve, which is why some people choose a custom frame to get adequate fit.

Luckily for you, the actual proportions of something like a 44cm 700c road bike is nowhere near 5 sizes smaller in top tube length than a 54, so you probably can get away with one without a 200mm stem. But given the fact of your proportionally long torso, I would be shocked if you are using much shorter than a 140mm stem with good reach.

OregonJim

I can (barely) stand over a 19" (48cm) frame - with zero clearance. Last time I measured my inseam, it was 29", but I've shrunk a bit since then. I should probably check it again - your guess may be close. I was 6' tall in my 40's - now in my early 60's and I've lost 4 inches of height! Darned herniated discs... (not that those should affect my inseam :-)

Wildwood 

Would love to see a pic(s) with both bikes. Upgrading from a stiff aluminum frame from teenage years probably is not a bad decision from a comfort perspective, but the frame size + fit issue is a different question. The bike pictured with lots of spacers under stem, maybe was a bad fit to begin with for a teen still growing a bit.

I think the OP should level the saddle, put a longer stem on that bike and ride it for a short while. Cost invested = less than $50. Then, if uncomfortable, get a 2nd opinion from a different bike shop.
__________________________
edit = ignore below paragraph. Location Denmark and not a Trek store (presumably)
Just wondering if my opinion of Trek store employees trying to sell bikes rather than simply solving a minor fit issue holds true. I have only been to 4 Trek stores since 2007, the experience level of the employees I spoke with was minimal (at the best). Was told at 2 locations that 5+year old carbon frames with round tubes needed to be replaced = . Not one had heard of a Calfee, or asked anything about the bike or my riding style. Maybe new employee training has been modified. I hope so.

edit: Sorry, for mini-rant.

Josh BWS

Hi guys, I have old MTB bike, elevated chain stay type. It has C-C 42 cm, is it considered small bike or regular? My height is 175 cm

Kontact

That's a 17" frame, which is medium or medium/small.