Memory consumed by String and StringBuilder is duplicate

I need to concatinate few strings. I am using StringBuilder for that as below

        StringBuilder result = new StringBuilder(length);

The length is the "length" of the string that i want at the end.

"Length" is of higher number. To get string i need to do

        return result.ToString();

When i am trying to analyze memory consumed by my application i see that "StringBuilder" and "String" are taking same amount of memory and its duplicate. As the length of the string is long, its occupying larger % of memory.

Is there a better way to solve this memory problem?