How much muscle can you gain in a year?

Short answer

Less than you think. Lean mass rose about 1.5 kg in the first 10 weeks or so; 57 people training under a coach gained 2.8 kg in a year, and about 1.1 kg a year on average over 4 years. Gains slow down with experience.

How much muscle can you gain in a year?
'1 kg a month × 12 = 12 kg a year' — does this math hold?
When you start training it feels like you will add 1 kg of muscle every month, but the numbers measured in research are different. We looked at how much muscle people gained over the first months, over a year, and over several years.
The first 10 weeks or so
about 1.5 kg of lean mass
An analysis pooling 111 studies and 1,927 people found that after 4-24 weeks (about 10 weeks on average) of strength training, lean mass (muscle and everything except fat) rose 1.53 kg. A 12-week study of 48 people found 1.54 kg. That works out to about 0.6 kg a month.
But people varied a lot
From -2% to +59% over the same 12 weeks
585 people trained one arm the same way for 12 weeks. Changes in upper-arm muscle size ranged from -2% to +59%. People who barely grew and people who grew a lot came out of the same program.
Gains slow down with experience
Long-time lifters barely grew in 21 weeks
Doing the same program for 21 weeks, 8 beginners grew their thigh muscle 5.6%, while 8 long-time strength trainers changed by -1.8%. Following 9 national-team weightlifters for 2 years, muscle fiber size rose 5.9%, which was not a meaningful change.
Looking year by year
8 kg in 1 year, 4.3 kg over 4 years
57 US college football players training under a professional coach gained 2.8 kg of lean mass in 1 year. The 13 measured across all 4 years gained 4.3 kg in total, about 1.1 kg a year on average.
There is a ceiling
Without drugs it topped out near 81 kg for a 180 cm man
In a survey of 157 strength trainers, the 74 who did not use drugs had a 'fat-free mass ÷ height²' value that stopped at 25. For someone 180 cm tall, that is about 81 kg of fat-free mass. Some steroid users exceeded 30.
Bottom line: 12 kg a year does not add up
About 0.6 kg a month at first, around 1 kg a year with experience
In the first 10 weeks or so, lean mass rose about 1.5 kg. People already training gained 2.8 kg in a year, and about 1.1 kg a year on average over 4 years. Results varied widely on the same program, and the amount gained shrank as experience grew.