Conventional uniaxial creep tests on massive specimens and small punch creep tests on thin discs were performed on P91 steel at temperatures from 823 to 873 K. The shapes of corresponding time dependences, i. e., creep curves and time dependence of the disc deflection, are very similar. A simple linear relationship between the applied stress in creep tests and the acting force in small punch tests leading to identical times to rupture was empirically derived. The proportionality factor reaches a value close to values observed in other creep-resistant ferritic steels.