What size mountain bike frame do I need if I ride a 56cm bike?
 

54 is way too small. 55 is too small. 56 is perfect. I can ride 57. 58 is definitely too big. What size mountain bike frame (inches) should I be looking for? My saddle height is 785mm.

Re:
 

I am pretty much exactly the same fit as you... same frame, same saddle height. I also have a mtn bike. It has 27.5 wheels and a large frame.... I think a medium would fit me better. but the large is ok.

I would test ride a few bikes.

It’s hard to say without knowing which specific mtb you’re looking at.

Also it depends on how you ride and what time of terrain.

I ride a 55cm road frame and am perfectly comfortable on a size L mtb with short stem and wide bars. I have long legs for my height (57 st 55 tt on the road bike). I’m not exactly doing Red Bull trails at blistering speed though.

Need more info

Totally dependent on manufacturer and whether you are looking for an XC or trail bike fit, IMO. I'm 56/57 on most drop bar bikes, and I ride a large or 19" in some manufacturers, an XL or 21" in others. My XC rig has 440mm of reach. My trail bike is 451mm. Also make sure you have enough seat post extension for a decent dropper, if that's the flavor you are looking for.

there's no right correlation because road bikes are fit (more or less) statically whereas mtb fit is highly dynamic, so a numbers to numbers translation doesn't work well.

it's complicated by the different suspension setups and bike types. an xc bike and an enduro bike will feel different in the same size, even from the same mfg.
i'd start with a M or L and go from there.
best way is if you can demo a bike because parking lot donuts don't always say much either.

Mtb
 

Start with a medium MTB as a baseline.

Basically I want a 29er for XC, I do a lot of climbing which means going back down hills too, so I figure I’ll need a dropper post.

I'm basically the same seat height and I ride a Medium Niner hardtail. But a Medium in a Trek would probably be too small for me. So you'll need to check to the manufacturer's geo chart.

I'd say start with a large and see how it feels. As others mentioned, different manufacturers measure and size differently, so it really depends.

And no, you don't need a dropper! If the trails are really steep and technical, you may feel more comfortable with one, but they can be ridden just fine without it. Mellower trails do not need one at all. Just adds cost, weight, and maintenance.

I also ride a 56 and prefer a 56.5 TT with a short stack. So thinking that I like an aggressive position, got a size M 17.5 full suspension short travel mtb and was miserable riding the thing.

Rode a couple large full suspensions recently and felt like I was in the bike rather than on top of it and had no issues getting it to do exactly what I wanted. So upgraded to a medium Stumpjumper ST Comp this fall and am way happier. Thought it was pretty telling since I live across the street from the Fells (no really, actually across the street) and barely went riding there. Already been on a few rides there despite terrible weather we've been having and just getting the bike recently.

Also modern mtbs fit a bit differently than they used to, if you straddled the line between M and L before with a trial bike, you'll most likely want to go with a larger size due to how they ride now.

It kind of sounds like I’d be happier on a bigger frame, so maybe “large” would be the most appropriate, but considering that the sizes name doesn’t necessarily indicate any particular measurement, what should I look for in the geometry chart that would indicate whether a particular manufacturers medium or large is what I need?

18".

Beside getting fit @ your LBS I recommend using Competitive Cyclist's fit calculator --> https://www.competitivecyclist.com/S...ulatorBike.jsp

Seat tube length is almost meaningless for MTB fit, even though it's what a lot of manufacturers list for bike size. I'd use reach (somewhere between 440 mm and 455 mm) or maybe effective top tube (somewhere between 620mm to 640mm) to get in the ballpark. Unless you have really end-of-the-bell-curve proportions, you should be able to adjust from there.

Narrow down some targets and get some dirt time on those bikes if at all possible. It's pretty challenging to find a "best" MTB just based on numbers. Much harder than road bikes.

Yeah, agree with above -- ride a couple. XC, trail, and long travel all ride and fit differently.