The tritium start-up inventory required by a tritium self-sufficient DEMO-class fusion reactor is subject to a wide margin of uncertainty, with estimates in the literature varying from less than 1 kg to almost 20 kg for a ~2 GW fusion reactor. If ITER is successful, it is conceivable that multiple DEMO-class devices may be developed in parallel; the European DEMO machine, the Chinese Fusion Engineering Test Reactor (CFETR) and others could potentially require several kilograms of tritium each in the 2050s.