You can use the following syntax to calculate a percentage by group in R:

library(dplyr)

df %>%
  group_by(group_var) %>%
  mutate(percent = value_var/sum(value_var))

The following example shows how to use this syntax in practice.

Example: Calculate Percentage by Group in R

Suppose we have the following data frame that shows the points scored by basketball players on various teams:

#create data frame
df <- data.frame(team=c('A', 'A', 'A', 'A', 'A', 'B', 'B', 'B', 'B', 'B'),
                 points=c(12, 29, 34, 14, 10, 11, 7, 36, 34, 22))

#view data frame
df

   team points
1     A     12
2     A     29
3     A     34
4     A     14
5     A     10
6     B     11
7     B      7
8     B     36
9     B     34
10    B     22

We can use the following code to create a new column in the data frame that shows the percentage of total points scored, grouped by team:

library(dplyr)

#calculate percentage of points scored, grouped by team
df %>%
  group_by(team) %>%
  mutate(percent = points/sum(points))

# A tibble: 10 x 3
# Groups:   team [2]
   team  points percent
        
 1 A         12  0.121 
 2 A         29  0.293 
 3 A         34  0.343 
 4 A         14  0.141 
 5 A         10  0.101 
 6 B         11  0.1   
 7 B          7  0.0636
 8 B         36  0.327 
 9 B         34  0.309 
10 B         22  0.2 

The percent column shows the percentage of total points scored by that player within their team.

For example, players on team A scored a total of 99 points.

Thus, the player in the first row of the data frame who scored 12 points scored a total of 12/99 = 12.12% of the total points for team A.

Similarly, the player in the second row of the data frame who scored 29 points scored a total of 29/99 = 29.29% of the total points for team A.

And so on.

Additional Resources

The following tutorials explain how to perform other common tasks in R: