Noise is the most detrimental factor of the indoor environment in open-plan offices. Speech is the most distracting source of noise. Distraction and speech privacy can be estimated objectively by determining speech intelligibility between workstations. Previously used measurement methods have focused on two neighbouring workstations. However, noise complaints are not restricted to the nearest workstation. The aim of this paper is to suggest a new method to measure and characterize the acoustical conditions of the whole office space, including both short and long distances from the speaker. The method should result in compact and expedient single-number quantities to improve the utilization of the method in building design. The measurement is carried out between workstations along a line consisting of at least 4 workstations. Measurements are made for background noise level, spatial reduction of Speech Transmission Index, STI, and spatial reduction of the SPL of normal effort speech. An omni-directional loudspeaker is used. The acoustic performance of offices could be logically described by three single number quantities: distraction distance, rD, spatial attenuation rate of A-weighted SPL of speech, DL2, and SPL of speech at a distance of 4 metres, Lp,S,4m. The method has been validated in 16 offices which varied significantly in room geometry, furniture and absorption. The differences between offices were unexpectedly large. Recommendations are presented for new target values which are already adopted in two Finnish guidelines.