How accurate are Thermocouples?

There are many types of thermocouple (K,T,J,E,N,R,S & B). Each type has its own accuracies as governed by the IEC. There are typically 3 accuracy tolerances for each type of thermocouple, with Class 1 being the most accurate. For type K thermocouples the Class 1 accuracy is 1.5°C or 0.4% of reading (whichever is greater).