SQL Query multiplication

In SQL, I am updating a Table : PlacedOrderDetails by mutiplying two columns and entering the mutiplied values to the Qty Column.

[Ship Qty] which is nvarchar(255)  
Mutiply with
[Size] which is nvarchar(255)

Resulted value goes to [Qty] which is float.

update PlacedOrderDetails          
set Qty = [Ship Qty] * (cast(substring(Size, 1, patindex('%[^0-9]%', Size + '/') - 1) as float) *   case when charindex('X', Size, 1) > 0 then 
             cast(substring(substring(Size, charindex('X', Size) + 1, len(Size)), 1, patindex('%[^0-9]%', substring(Size, charindex('X', Size) + 1, len(Size)) + '/') - 1) as float)             
             else  1 end )    
           WHERE Qty IS NULL

This is the Table value

Ship Qty            Size                      Qty
2                   100 EA                  200
1                   14.7 GM                   14

Issue with my query is, decimal value of size column treated as round number and decimal part is ignored.

How to fix to treat whole number and decimal numbers equally?