A small fraction, f , of cosmic string loops can collapse to form Primordial Black Holes (PBHs). Constraints on the abundance of PBHs can therefore be used to constrain f . We update these calculations, taking into account the PBH extended mass function, and find f < 10 −31 (Gµ/c 2 ) −3/2 . This is roughly one order of magnitude tighter than previous constraints. The improvement from the tighter constraints on the abundance of PBHs is partly off-set by refinements to the theoretical calculation of the cosmic string loop formation rate.