Road Bike Size

Well, I'm 5'10 and I ride a 54, which is probably a medium (reach 381, stack 552). But I have proportionally longer legs and a shorter torso.

I would say a small for you, but really you need to try before you buy, or else get a bike fit. I know that's not very helpful, but you are taking a stab in the dark otherwise. Bear in mind that it's always better to go too small than too large.

Iso you can have another data point on the curve.

6ft, relatively long leg / shorter torso. 

I'm on a 58cm frame.  Both road bikes I've had + the gravel bike.  (I do have to fit shorter stems though- they  always come set up for knuckle draggers wirh arms like gibbons. 

Similar height, just ever so slightly taller at 173cm (about 5foot 7). I have a 29” inside leg.

I ride a 52 in most manufacturers sizes, which is typically a size small.

Ive had a couple of bike fittings, frame size typically comes out at 51 or 52.

I did once buy a bike from the internet, so no chance to ride it before hand. That was a medium…..was also ok. But I ride road bikes a lot and I’ve never really had any issues with different setups.

I'm 5ft 8in and about a 28in inside leg, always medium for me in road bike frames - and I prefer ones with a little more length in the top tube for stability.

Good time to be buying secondhand, what's your budget?

Most things on internet are saying a medium does this sound like a starting point ? 

No. Small.

I'm a chunk taller than you and ride a small. (i could probably have got away with a medium, but would have been too tall at the front.)

Most things on internet are saying a medium does this sound like a starting point ? 

No.  A 52 or 50 would be a better starting point.  Without any knowledge you can at least sit on them, get how it feels to reach the bars/hoods etc,.  

I would have thought you would fit more like a 52cm depending on the specific bike.  Road bikes are simultaneously easier to get the correct size because there are typically a lot more in the range and harder because one 52cm will not be the same as another due to differing seat angles, rule of thumb is that for every 1deg steeper the seat angle is, add 1cm to the top tube length for comparison. e.g. a 52 cm frame with 73.5 angles will fit roughly like a 53cm frame with 72.5 angles (imagine the BB and headtube have been kept static but the seattube has been rotated forward 1cm).

It varies between brands. Off the top of my head, a cannondale 56 is more like a trek 58 if you're between sizes (i.e. the 56 might feel small and the 58 big, but a cannondale 58 is even bigger and a trek 56 is even smaller), but that's only in the racing bikes, in the sportive/endurance bikes the treks are even shorter / taller whereas Cannondale are more consistent.  Giant are more like MTB sizing.  Cube and Focus are both wierd, ignore them unless you can actually test ride it. Specialized are very 'over the front' with long chainstays and steep head angles which makes them less 'neutral', the wrong size feels even more 'wrong' if it puts your weight in the wrong place. 

I'd suggest it's hard to go wrong with something like a 51cm Cannondale or 50/52 depending on the model and whether you perceive yourself to have long or shorter legs than average.  Sometimes you just need to sit on a bike and ride it to see if it works for you. As long as you don't overpay then small bikes are easy to sell as there's hundreds of racing teenagers growing through them. 

I’m 5’7 and manufacture say a 54cm for my size but I always went for a 52cm. When I got my Crux I went for the 54cm as recommended but in hindsight wished I stayed with 52cm as for me that just felt a lot better. 

I’d say the same as Kerley either a 50cm or 52cm but it be best to go to a shop and try a few. 

I've a 30" inside leg and when frames were measure in cm rode a 55cm Trek, nowadays it's an ML giant.

Applying the theory that a 2" shorter leg would probably mean a 2" smaller frame then you're looking at 50cm but as my saddle's not on the frame and you probably don't want to be all hunched up, a 52cm frame sounds about right for you.

That said, nothing beats sitting on a bike to make sure it fits.

Go smaller with a longer stem. Puts more weight over the front wheel for better handling. You’re a 52 and/or small. You can fit a medium but it will need a shorter than the standard 100 stem and not handle as well. 

I'm 178cm with short legs 82cm cycling inseam, so ~72cm BB to saddle top. Cube frames are plain weird, my Attain GTC Disc is "58cm" but no tube is 58cm long! 388mm reach, 610mm stack. Seat tube is 54cm long and given many road bikes come with 350mm posts that can have 250mm exposed, a frame with ~48cm seat tube would give me more comfort.

If 28" is cycling inseam, that's ~72cm, for your legs you could go as low as ~38cm seat tube with a 350mm post to get BB to saddle top ~62cm... But I suspect the top tube would be rather short and would need a long stem (well over 110mm) to get the bars in a good spot that's out of the way of your knees!

Just to add at 5’9 with proportionally longer legs / shorter torso and arms I rode a medium / 54cm /53cm depending on the brand. So it sounds like you might want a small / 52cm ish size frame. If you’re buying secondhand then go local and get a good sit on the bike to see how it feels. I’d be less inclined to buy unseen from eBay without sitting on the bike. 

5ft 4 here and ride an XS Giant Revolt. For MTB I'm a small. 

5’ 7” on a 52cm (small) Scott, 110mm stem.

Just to add a layer of confusion I also have a 54cm Cervelo which fits fine with a 100mm stem, but if I wanted it racier I’d probably size down.

I’m 5’10 with short legs.  I’ve had 2 medium Genesis which fit well, I’ve had a Specialized (2nd hand) that I bought at 56 off the size chart recommendation, it was too big.  I’m on a 54 Cannodale now after buying new and going to a good shop (shout out to Sigma) who spent time with me testing bikes and getting the fit right. If you can try to ride some bikes, find one that’s comfortable and look up their geo charts and use that as a guide as sizing varies per manufacturer. 

add 1cm to the top tube length for comparison.

Except top tube length hasn't been used in proper sizing discussions/comparisons for at least 2 decades now.

Stack, reach.

Correct your fore/aft position using saddle rails.

my Attain GTC Disc is "58cm" but no tube is 58cm long!

Not unusual TBH. 58 would be the nominal length of the seat tube, generally assuming it was attached to a horizontal top tube.

Stack, reach.

+1

I tend to end up with a shorter seat tube than manufacturers recommend and stick a long seat post in

Sizing between brands is not very consistent at all so trying to choose based on S/M/L or even seat tube length can be a bit of a minefield. I agree with everyone who says get hold of a bike (or two) to try out to get a sense of what will fit you. Once you find a bike that feels decent you can find it on  https://geometrygeeks.bike/ to get the stack/reach figures. Then when you find a bike you might want to buy, you can check the site to see how the stack/reach compare and you're much more likely to get a decent fit.

I prefer to look at the stack and reach because adjusting for those is harder (changing stems, bars or spacers) than adjusting for seat tube length (less than a minute to change saddle height).

I prefer to look at the stack and reach because adjusting for those is harder (changing stems, bars or spacers) than adjusting for seat tube length (less than a minute to change saddle height).

I don't disagree, but be careful especially when looking at small bikes as they'll often have made a decision between 

Long reach, slack head angle, short stem and large offset forks - but no toe overlap

Short reach, normal head angle, long stem, normal fork offset - but with toe overlap (but cheaper to not have to make a special fork for the small sizes).

There's a solid argument for just finding the manufacturers size guide and going with it unless you know you're weirdly proportioned. 

Effective too tube is important too. 

My bike in Small and XS both have the same reach. The XS has a slightly steeper seat angle which shortens the effective toptube. 

Posted by: mjsmke

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My bike in Small and XS both have the same reach. The XS has a slightly steeper seat angle which shortens the effective toptube

This is where it starts to get complicated if you have non-standard proportions. E.g. if you've got a long torso then you probably won't want a small bike with a steep seat tube angle (say 74 degrees) because you might end up cramped. On the same model you may find that the medium fits you better, especially if it has a shorter reach (Giant Defy would be one example, which comes in S, M, ML and L). That's obviously where sitting on them both really helps.

Toe overlap is a complete non-issue on road bikes. Every single road bike I've owned has had toe overlap.

Manufacturer quoted sizes are only a very rough indication (especially numbers, which might not correlate to any tube length or other dimension on the bike these days). Ideally you need to check the stack & reach figures for any models you're looking at (which probably means you googling rather than expecting the seller to provide) but even then it depends a lot on you as a rider (flexibility, what sort of riding you're doing etc.). It's also generally better to err on the small side rather than large side though when it comes to a frame (within reason).

Just for another perspective

5'11" with long legs. I'm always a large in MTB's. When I bought my gravel bike (Boardman), everything said medium. I went with medium and glad I did, large would have been too stretched

5ft8 and bought a 54 felt, having a hard time getting comfy on it, not impossible but takes some work, which is weird as i also have a nukeproof digger and that bike is huge but no issues with comfort

So, i'd definitely err on the smaller side, and at your  height that definitely isn't a 54/medium