Originally Posted by Triguy

Here is a post I made regarding a similar post in the tri forum. Now, things are a little different if you have to adhere to UCI fit rules, but I think it's still relevant:

"I think that there is somewhat of a confusion on the exact nature of road to tri sizing. Many companies have now adapted size naming so that a person who rides a 54cm road bike will also ride a 54cm tri bike, however, the top tube of the two bikes will be different, the road bike being longer.

For instance Cervelo 54cm:
P2(tri) at 78* = 53cm top tube
S2(road) at 73* = 54.5 cm top tube

Felt 54cm:
B2(tri) at 78* = 52.8 cm top tube
F3(road) at 74.5* = 54.5cm top tube

So in effect your top tube will be shorter on your tri bike, but in many cases the size name will be the same. Obviously if your proportions are different than most people or your riding style is different than your bike selection may be different. One clear example is that if you are riding in UCI time trials, then you may need to size down a frame size because your seat will be farther back then if you're riding triathlons.

At ~6' you could probably be anywhere from a 54-58cm depending on the company and their fit characteristics."