A method for determining a pulse duration T90 of a 90° pulse in a nuclear magnetic measuring method. A signal generator has a known generator resistance RS, wherein a coil has a coil impedance ZL with a coil resistance RL and a coil reactance XL, wherein a coupling circuit has an adjustable matching capacitance CM and an adjustable tuning capacitance CT, and wherein the medium has a Larmor precession having an angular Larmor frequency ωP. The time needed for determining the pulse duration T90 of the 90° pulse is reduced by the matching capacitance CM and the tuning capacitance CT being set so that the angular resonance frequency ω0 corresponds to the angular Larmor frequency ωP and by power matching being present between the signal generator and the coil. The coil resistance RL is determined and the pulse duration T90 is determined as a function of the coil resistance RL.