You measure the success of a client event on multiple levels. Not everything can be expressed directly in euros, but there are clear indicators.
Quantitative: turnout rate (aim for 60-70%) and Net Promoter Score after the event. Also count the number of follow-up appointments, client retention rate in the months afterwards and any new business.
Qualitative: the atmosphere and energy during the event, the quality of the conversations, spontaneous feedback from clients and whether the event is discussed in your guests' network.
Tip: send a short evaluation to your guests within 48 hours. Keep it to 3-5 questions. And schedule a follow-up moment with your most important clients within two weeks: the event is the start of the conversation, not the end.