A lean bulk aims to maximize levels of leanness, while still adding muscle mass. It takes roughly ~2500 kcal to build 1 lb of muscle. So you need an approximate 100 kcal surplus to gain 1 lb of muscle per month. However, your metabolism will adapt, so this number may be higher. I like to use a 150 kcal figure. How much muscle you can gain each month depends on multiple factors, largely your genetics and how much training experience you have. Here is a rough breakdown of the rate of muscle growth the average person can expect if they do everything right: • Beginner: 1–1.5% of body weight per month. • Novice: 0.75–1.25% of body weight per month. • Intermediate: 0.5–0.75% of body weight per month. • Advanced: Less than 0.5% of body weight per month. I've written more about this in my guide: How To Bulk Without Getting Fat