Realistically, how much muscle can I gain in one year?

[RapidFail]

Oh yeah

But, who cares? What does it matter in regards to any particular other person and what they weigh?

And you know that BMI considers height, yes?

Hearns at 160 looked like he needed to eat a sammich

lol at he would have died getting down to 160. No.

6'1 and 155 lbs is 'small'. This started with a comment about "at x height you should weigh at least y", within the context of bodybuilding/weight training/muscle/strength. It's certainly up to the individual what they want to weigh and why, but, something like 6'1 155 is small as is 5'9 139

The only reason I mentioned the boxers was to point out that different bone structures make a huge difference in potential lean weight. A light framed guy may look ripped at 5'9" and a lean 160lb, while a thick-boned but still lean guy may look like he doesn't lift. I understand exactly how how BMI works. I had an 'ideal' BMI when I was skinny-fat.

I used to be much skinnier - when I finished high school I was 6'0" and 135lb and ate like a horse, but I had the racehorse metabolism to go with it. I still had a little belly fat and no 6-pack at that weight. I reckon I've got about 5lb more fat and 15lb more muscle now than I did then.

[Farley1324]

The only reason I mentioned the boxers was to point out that different bone structures make a huge difference in potential lean weight. A light framed guy may look ripped at 5'9" and a lean 160lb, while a thick-boned but still lean guy may look like he doesn't lift. I understand exactly how how BMI works. I had an 'ideal' BMI when I was skinny-fat.

I used to be much skinnier - when I finished high school I was 6'0" and 135lb and ate like a horse, but I had the racehorse metabolism to go with it. I still had a little belly fat and no 6-pack at that weight. I reckon I've got about 5lb more fat and 15lb more muscle now than I did then.

If two guys are the same height with the same bodyweight, bodyfat, and muscle mass, one wont look lean and ripped while the other looks like he doesn't lift.

[RapidFail]

In my example, the ecto guy has a lighter skeleton and would therefore have more muscle at the same weight and body fat.

I'm using extreme examples, of course, but factors such as bone breadth, muscle attachments, fat distribution, limb length, width of shoulders and pelvis etc mean that guys can look very different at similar heights, weights and body fat percentages.

[BlackJojo]

Thanks for all the advice

[LukeEverhart]

I'm 21, 5'9 and 139 lbs. I'm not 100% certain what my bodyfat percentage is but I've never had abs before. I want to become really shredded this year, but every time that I've tried working out in the past I had no idea what I was doing. What's a good program to help me gain muscle and burn fat at the same time? And what should be my goal weight for the end of the year?

Short Answer on Subject Q: 15#-20# (of LBM not total bodyweight) in 1st year, assuming neither gifted nor a hard gainer
Long Answer:
As a beginner, it is possible (and indeed typical) to both lose bodyfat and gain muscle mass at the same time. The more advanced the trainee, the more difficult that proposition is: after the 1st 6 months or so of training it requires an unrealistic degree of micromanaging for most. However, that doesn't mean that straddling both goals and adopting a plan with mixed goals is a good idea. Choose one of the other as the initial "phase 1" goal and rest assured that as a noob you'll still see some incidental, modest progress on the other aspect.
The idea that a beginner has to "bulk" to achieve the low hanging fruit of beginner gains is nonsense. If you're metabolically healthy (no insulin resistance, etc) even a eucaloric diet (maintenance calories) will facilitate healthy gains providing 1) that the macronutrients (protein, carbs, fat) are redistributed appropriately, and 2) that you aren't already in a fairly lean state (13%> bodyfat roughly). Bodyfat represents an abundance of calories to satisfy the increased energy expenditure needs of a noob trainee. It does NOT, however, supply any of the building blocks for that muscle (amino acids aka protein); so, it's essential that you prioritize protein in your macronutrients.
The most credible range for protein needs is 0.7grams/lb to 1.0grams/lb and a beginner should consume the top of that range (1.0/lb). (Advanced trainees can actually get away with less as protein synthesis improves the longer one trains.) The "lb" is lean body mass, not total bodyweight. However, in lieu of a bodyfat analysis all trainees without a 6-pack can default to bodyweight less 10% to err comfortably on the high side.
On muscle gain goals: the majority of the population (excluding the margins of "gifted" & legit "hard gainers") can expect 15#-20# of actual lean body mass, not total body weight but hydrostatic or DEXA tested lean body mass, in the first year of proper, dedicated training. This LBM gain will be somewhat front-loaded, typically about half of that 15#-20# in the 1st 4months with the remaining half accruing the remaining 8 months of the year. Those seeing less are usually sacrificing muscle mass gains in the pursuit of strength gains with such programs as Starting Strength, Fierce 5, & Stronglifts, which will yield results in both strength & mass but are very suboptimal approaches to hypertrophy/muscle mass. [Strength & hypertrophy only very loosely correlate. There is a reason the aforementioned programs are often used by wrestling coaches & similar to assist their athletes in increasing strength while maintaining bodyweight for competition. I've personally witnessed a 5'9" 149# guy deadlift 615# in good form (he tripled his deadlift in 2 years of training while adding a grand total of... 6# of lean body mass (it was consistent with goals however) & incidentally, the U.S. record for his weight (148# class) is a hair over 660#)).]
Decide on whether you're going to prioritize getting those abs or focus on hypertrophy for your phase 1 as the goal will determine the optimal program. (But as said, incidental progress should be evident on the non-goal aspect as a healthy beginner.)