The anisotropy constant K of antiferromagnetic (111) textured IrMn films was determined from the blocking temperature TB of the high field rotational losses of NiFe∕IrMn(t) film systems of low thickness t⩽2.5nm. The losses are measured by torquemetry as a function of t and temperature T. TB has been calculated using a Stoner-Wohlfarth model adapted to the experiments. From TB=(0.04∕kB)Ktϕ2 (ϕ=grain size), we estimate a constant value of K=(3±1)×105J∕m3 for T=10–400K. The method is applicable also to other measurements of TB(t) and can be modified for other film systems.