The sensitivity σ of an optically pumped quantum magnetometer is measured directly by comparing the output frequencies of ⁸⁵Rb and ⁸⁷Rb magnetometers integrated in a single two-isotope cell. The result σ₈₇ = 59.6 ± 6 fT Hz^-1/2 agrees well with the value σ₈₇^light = 53.7 ± 2 fT Hz ^-1/2 obtained indirectly by measuring the ratio of the resonance steepness to the shot noise of light.