Calculating the size of clustered index rows depends on whether the keys are fixed- or variable-length. Use the first formula if all keys are fixed-length. Use the second formula if keys include variable-length columns.
For fixed-length columns, calculate:
5 + (Sum of bytes in the fixed-length index keys) = Clustered row size
For fixed- and variable-length columns, calculate:
5 + (Sum of bytes in the fixed-length index keys) + (Sum of bytes in variable-length index keys) = Subtotal
(Subtotal) ((Subtotal / 256) + 1) + (Number of variable-length columns + 1) + 2 = Clustered index row size
The values 5 and 2 in the previous equations are overhead that SQL Server uses for internal storage.