© 2015 Instytut Matematyczny PAN. Let A be a Banach operator ideal. Based on the notion of A-compactness in a Banach space due to Carl and Stephani, we deal with the notion of measure of non-A-compactness of an operator. We consider a map XA (respectively, nA) acting on the operators of the surjective (respectively, injective) hull of A such that XA(T) = 0 (respectively, nA(T) = 0) if and only if the operator T is A-compact (respectively, injectively A-compact). Under certain conditions on the ideal A, we prove an equivalence inequality involving XA(T?) and nAd (T). This inequality provides an extension of a previous result stating that an operator is quasi p-nuclear if and only if its adjoint is p-compact in the sense of Sinha and Karn.