At mass events, pedestrians can experience the level of crowdedness as unsafe, unpleasant and stressful. To gain a better understanding of perceived crowdedness, the effects of personal, trip and event characteristics at an event are researched. Data collection was performed by a simultaneous survey and monitoring study at the TT Festival in Assen and at the Red light district in Amsterdam. A SEM model shows that perceived crowdedness is influenced by mainly by the density, quantified using Wi-Fi sensor data. Besides that, trip purpose and familiarity with the event influence perceived crowdedness as well. Furthermore, this research shows that perception of safety, comfort, atmosphere and attractiveness of the environment are also related to the perception of crowdedness.