Ive sold my Boardman hybrid as I fancy a nice road bike to commute on instead.

There is a nice Planet x carbon road bike in the classifieds in Large but Im not sure if it will be to small or not as the top tube is around 4 cm shorter than my full susser.

Looking at the planet x website it says large is the right size for me but the top tub length thing is worrying me!

should I be comparing bike sizes like that or is there another way.

Buying seconhand is going to get me a good deal but without the chance to test it.

not discounting the fact that ‘top tube’ measurement can mean different things to different manufacturers, don’t forget that on Flat bar bike (MTB/hybrid)

TT + stem length – bar sweep back

Is where your hands end up, on a road bike its more like

TT + stem length + bar reach + hood length

eg:

take MTB 60mm stem (gah, mixed units!) so reach is TT + 2.5in

vs

on road bike it’s 100mm stem (shortish on large), making TT + 4in but then you could have 70-100mm of bar reach putting another 3-4 inches on, before you even add the reach of the hoods.

but as always, try before you buy!

*ignoring bar width for now although that obviously does affect final body position on both types.

You can’t really compare because they’re completely different bikes, but having said that…

Road bikes generally have longer stems than MTB, which would make up the difference in TT length: My MTB has an ETT of 615mm, but a 35mm stem, while my road bike is 550mm with a 110mm stem, so works out slightly longer.

Edit: Beaten!

As a very rough guess (they’re completely different beasts, obviously), that sounds about right – I ride mtbs with about 590- 610mm top tubes and short 50mm-ish stems, and road bikes between 545-565mm ETTs, 100-120mm stems, and short reach bars (80mm). Remember you’ve got a long stem (90mm+ normally) and the reach of the drops (75mm+) to add to the road bike figure, plus saddle to bar drop – it’ll most likely feel plenty long enough, but you’ve really got to ride it to find out (and know what it should feel like!)

edt: beaten, twice 😥

You don’t. You don’t even compare road bikes sizes as the geometry etc will be different.
Why not just look at the website and see what’s recommended+?