Embodiments of the present disclosure are directed towards a flame retardant foam formulation. As an example, the flame retardant formulation can include a mono-phosphonate having a hydroxyl group, a polyisocyanate having a functionality in a range from 2.0 to 10.0, wherein the polyisocyanate is present in an amount to provide for an isocyanate index of from 100 to 320, a compound having active hydrogen groups capable of reacting with the polyisocyanate, a blowing agent, and a catalyst.