In a method for determining an absolute number of electron spins in an extended sample (3) with the assistance of an apparatus for measuring magnetic resonance, the extended sample (3) is disposed within a measurement volume (2) of a radiofrequency RF resonator (1) of the apparatus during an electron spin resonance measurement (ESR). The method has the following steps: determining a spatial sensitivity profile f of the RF resonator (1) over the measurement volume (2); determining a resonator sensitivity constant c by means of a comparison to the measurement volume (2) of small calibration sample having a known number of electron spins at a particular position within the measurement volume (2); measuring a magnetic resonance signal RS of the extended sample (3) in the apparatus with a known spatial distribution of extended sample (3) within the measurement volume (2); weighting the magnetic resonance signal RS with the integral of the spatial sensitivity profile f of the RF resonator over the partial volume of the measurement volume (2) occupied by the extended sample (3); and determining the number of electron spins NS in extended sample (3) as a quotient between the weighted resonance signal and the resonator sensitivity constant c. The method facilitates a simpler determination of the absolute number of electron spins in the sample.