Edwin Sanchez shows a variety of methods to calculate percentages of the whole in T-SQL:
Calculating percentages in SQL Server is like slicing a pie. You need to know the total size (the denominator) and the size of the slice you want (the numerator). To get a percentage, you divide the slice size by the total size and multiply by 100.
Read on for a variety of methods to calculate this. I wouldn’t use all of the methods myself, as I have certain predilections against subqueries in the SELECT clause, but they do get the job done.