Room acoustic models are often used to predict parameters that describe acoustic conditions in various spaces, also in open-plan offices. An open-plan office of 81 m² was experimentally studied using 22 different room acoustic conditions. The conditions were built using typical absorption materials, office screens, and furniture. The acoustic conditions were measured according to ISO 3382-3. The measurements were modelled using two methods: A. room acoustic simulation software based on ray-tracing, and B. a simple empirical model. The precision was assessed by the difference of predicted and measured single-number values (SNVs) of ISO 3382-3. For method A, the modelled and measured SNVs were very similar in conditions with low sound absorption and low screen height. The agreement was worse in conditions with high sound absorption on the ceiling and walls, and over 1.6-m-high sound-absorbing screens. For method B, the agreement of SNVs was on the same level in most of the conditions. Consequently, SNVs with method B were more precise than with method A in conditions with high screens and high sound absorption, but less precise in conditions with low sound absorption and lower screens.