StringBuilder consumes less memory than String ?

Since you failed to provide any context information about how you came to this conclusion, all we can say is "No, it does't". A string is a data structure that has a small amount of header information and array of bytes (usually 2 bytes per character) that hold the data for the string.

A StringBuilder holds an array of characters and has a bunch of code associated with it, so, in general, a StringBuilder cannot take up less memory than a string.

Oh! When you actually convert the StringBuilkder to a string so you can use it as such, it takes up about TWICE the memory of the target string. Once to hold the string instance and another to hold the StringBuilder and its data.