If you're new to lifting weights, things like how much you eat, how much protein you get, how old you are, or your BMI don’t really help predict how much muscle you’ll gain in the first few months.

If you're new to strength training, your bone size or density doesn't really predict how much muscle you'll gain in the first few months—people with all kinds of frames tend to gain muscle at similar rates.

If you're new to lifting weights, having more muscle at the start doesn't really mean you'll gain more muscle from training — it's not a strong predictor of results, especially once you account for whether someone is male or female.

If healthy adult men do resistance training for more than two weeks, they will reliably gain about 1.5 kilograms of muscle. This shows that lifting weights is a very effective way to build muscle overall, no matter how you measure it.

If healthy guys between 18 and 40 lift weights regularly for at least two weeks up to a year, they typically gain about 1.5 kilograms of muscle—no matter what kind of weight routine they follow.

If you're a healthy man between 18 and 40 years old, your starting height, weight, and age won't really change how much muscle you build from working out. Basically, everyone in this age group tends to gain muscle at roughly the same rate when they follow the same training plan, no matter their body size.