We study the back stable K-theory Schubert calculus of the infinite flag variety. We define back stable (double) Grothendieck polynomials and double K-Stanley functions and establish coproduct expansion formulae. Applying work of Weigandt, we extend our previous results on bumpless pipedreams from cohomology to K-theory. We study finiteness and positivity properties of the ring of back stable Grothendieck polynomials, and divided difference operators in K-homology.