BackgroundMost people’s tooth brushing performance is deficient, even when they are encouraged to brush to the best of their abilities. The aim of the present study was to explore the nature of this deficit by comparing best-possible vs. as-usual brushing.MethodsUniversity students (N = 111) were randomly assigned to receive one of two instructions: “brush your teeth as usual” (AU-instruction) or “brush your teeth to the best of your abilities” (BP-instruction). Video analyses assessed brushing performance. The marginal plaque index (MPI) assessed after brushing was used as an indicator of brushing effectiveness. A questionnaire assessed subjectively perceived oral cleanliness (SPOC).ResultsParticipants in the BP group brushed their teeth longer (p = 0.008, d = 0.57) and used interdental devices more often (p < 0.001). No group differences emerged in the distribution of brushing time among surfaces, the percentage of brushing techniques used beyond horizontal scrubbing, or the appropriate use of interdental devices (all p > 0.16, all d < 0.30). Plaque persisted at the majority of the sections of the gingival margins, and the groups did not differ in this respect (p = 0.15; d = 0.22). SPOC values in the BP group were higher than those in the AU group (p = 0.006; d = 0.54). Both groups overestimated their actual oral cleanliness by approximately twofold.ConclusionsCompared to their usual brushing effort, study participants increased their effort when asked to brush their teeth in the best possible manner. However, that increase in effort was ineffective in terms of oral cleanliness. The results indicate that people’s concept of optimized brushing refers to quantitative aspects (e.g., longer duration, more interdental hygiene) rather than qualitative aspects (e.g., considering inner surfaces and gingival margins, appropriate use of dental floss).Trial registrationThe study was registered in the appropriate national register (www.drks.de; ID: DRKS00017812; date of registration: 27/08/2019 - retrospectively registered).Supplementary InformationThe online version contains supplementary material available at 10.1186/s12903-023-03127-3.