Which I'll ask here because I hope to be flamed less than the other forums.
Here goes, When sizing a bike in cm, such as a 54 cm frame, what is being measured.
For instance, my old bike is an 18 inch frame, the seat tube is 18 inches long. But if you convert that to cm it would be 46. Now my wifes bike is a 50 cm frame, and it is way to small for me. So my question is what does that 50 represent?
This is something I really should know already, but I don't.
Asbestos under pants are on.

The problem is that there really isn't a standard. And with the introduction of frames with sloping top tubes, seat tube measurements aren't that helpful anyway. Typically you see MTBs measured in inches from center of BB along seat tube to top of top tube and road bikes are measured in centimeters from center of BB long seat tube to center of top tube.

Some will measure frames with sloping top tubes from center of BB to a point that would be the top tube if the top tube was not sloping. Top tube length is typically a better indicator of fit than seat tube given the nature of frames nowadays.

Don't know what your old bike is but MTB "Type" bikes are normally measured in inches and road/ comfort/ hybrids in Cms. You can convert inches to Cms as you have done and come up with the seat tube length.---But there is a difference between MTB and Road bikes in other dimensions. The top tube on an MTB (Size for size) is longer. although both frames are sold in Seat tube length- it is the top tube length that denotes how well a frame will fit you.

So your old 18" frame that fits you is a 46cm. So you would think that a 50cm road bike would make for a bigger fit- but it doesn't.

Looking at the Giant Specs and geometry--An 18" MTB Frame (46 cms) will have a top tube length of 23.4"- That equates to TT in metric of 594mm.

The road bike of 46.5cms has a top tube length of 530mm.

Perhaps it would be better if the two types of bike were both measured in the same scale but I think it would cause problems. I ride a 51 cm road frame. If I were to get an MTB frame in that size- I might be stretching for the bars a bit. Hence I ride a 15" MTB Frame instead of the 20" that 51cm equates to in Seat tube

Now if you just ride an odd sized road bike- then all the above is rubbish so disregard it.

Attached are my MTB and My main Road ride. Both fit me and I would not like to ride either bike in a different size.

Even in the days of level top tubes, the measurement could have different meanings, depending on the manufacturer's preferred method of measuring. Some measured center-to-center, which was from the center of the bottom bracket to the centerline of the top tube where it met the seat lug (the intersection of the centerlines of the top tube and the seat tube). Others measured center-to-top, which measured from the same bottom bracket centerline to the top of the top tube at the seat lug. Now, with sloping top tubes, the numbers (IMO) are all suspect and have to be examined individually on a bike-by-bike basis.

I won't repeat what has already been said except recap that and add that three ways exists to measure size by seatpost - center of bottom bracket to center of top tube, center to top of top tube, and center to top of seat tube (Fuji is one that did this a lot until recently). Sloping top tubes throw this off but practically all manufacturers include data on the virtual seat tube size by extrapolating what the size would be if the top tube was horizontal. If that isn't availble, you can approximate the size by using the top tube length since the majority of bikes in recent years use a "square" methodology in sizing (the top tube and seat tube lengths are the same).

Like CAC notes there is no real standard. Some bikes with sloping top tubes come small, medium and large.

Some questions for you:

What kind of bike are you interested in?

What's your size?

Here's a video that covers basically fitting a road bike:

You might enjoy reading Sheldon Brown's Revisionist Theroy of Bicycle Frame Size. https://www.sheldonbrown.com/frame-sizing.html

BTW, the only flaming you'll get from me is the misleading title to your thread. I thought this was gonna be about erectile dysfunction, 'roids, severe halitosis, or being ridden down by a bunch of girl scouts...

Or is it okay to tuck your jersey inside the waist of shorts? Or maybe wearing full length socks with cycling shoes.

That I find disturbing just thinking about. Shame on you for bring it up here.

Back to the OP's question. It is my opinion that the measurements in cm are not true as stated by others but used for suggesting a size. Some manufacturers use XS, S, SM, M etc... describing a frame size. However, many buyers are leery of that and want to see a # that associates the bike size with something they already can equate to. Remember when we were young and got our little league team baseball caps in the spring and they had sizes in them? Now-a-days the fitted hats are S, M, Lg, XL and not something like 6 ¾ or 7 ¼.

Assuming the traditional sizing standards of seat tubes from center of BB to center or top of top tube, the differences you see are because MTB/hybrid sizing allows for more top tube clearance than road bikes. In traditional sizing, I ride a 19" (48.3cm) MTB and a 58cm (22.8") road bike. Nowdays, with sloping top tubes and such, there are so many different ways bikes are measured that it is hard to tell the size of a bike without comparing several different measurements. You can't be sure if it is an actual measurement or a virtual measurement.

Watch out or we'll get the mods to ban you!

Size from manufacturer to manufacturer and sometimes even in product lines within a specific manufacturer are not directly comparable. Or, so say several dealers and mfg reps. From personal experience I know that from three different manufacturers I take three different sizes for bikes that fit. So, there is only one way to know what the right size is for you; ride the bike.

It is your money and your fun. Spend it as you wish. Just ride whatever you are thinking of buying enoiugh before you buy to have a reasonable chance of being happy with your purchase. Oh yes, don't worry about the Bike Snobs who think you have to dress a certain way or wear certain clothes, have certain equipment, or whatever. It is your ride, not theirs.

Definitely not with shorts. You can't reach the pockets----------------




BIBS? Now that would look different and get you noticed.

A image is worth a thousand words ...

You left out "Prevention of accidental bisection by idiotically long retractable dog leashes"

By the way, in addition to "sizing" being, at best a somewhat measure that has questionable usefulness I forgot to post one thing. Asbestos underpants are definitely a BAD idea. Not only are they stiff they have a tendency to chafe those parts of the body that may be especially sensitive. Or, so I've been told.

Thanks for all the responses, sometimes a picture is what it takes.
BTW, what is wrong with full length socks? Some times, that's the only option.

There's always pantyhose.

Nothing, if you're in the British army and posted to one of the subtropical colonies.