An identifying code in a graph G is a subset of vertices with the property that for each vertex v ∈ V (G), the collection of elements of C at distance at most 1 from v is non-empty and distinct from the collection of any other vertex. We consider the minimum density d * (S k) of an identifying code in the square grid S k of height k (i.e. with vertex set Z × {1,. .. , k}). Using the Discharging Method, we prove 7 20 + 1 20k ≤ d * (S k) ≤ min 2 5 , 7 20 + 3 10k , and d * (S3) = 7 18 .