Fix a finite field K of order q and a word w in a free group F on r generators. A wrandom element in GL N (K) is obtained by sampling r independent uniformly random elements g 1 , . . . , g r ∈ GL N (K) and evaluating w (g 1 , . . . , g r ). Consider E w [fix], the average number of vectors in K N fixed by a w-random element. We show that E w [fix] is a rational function in q N . Moreover, if w = u d with u a non-power, then the limit lim N →∞ E w [fix] depends only on d and not on u. These two phenomena generalize to all stable characters of the groups {GL N (K)} N .A main feature of this work is the connection we establish between word measures on GL N (K) and the free group algebra K-module with a well-defined rank. We show that for, where C is the number of rank-2 right ideals I ≤ K [F] which contain w − 1 but not as a basis element. We describe a full conjectural picture generalizing this result.In the process, we prove several new results about free group algebras. For example, we show that if T is any finite subtree of the Cayley graph of F, and I ≤ K [F] is a right ideal with a generating set supported on T , then I admits a basis supported on T . We also prove an analogue of Kaplansky's unit conjecture for certain K [F]-modules.