Introduction Bedtime routines are one of the most common family activities. They affect children’ wellbeing, development and health. Despite their importance, there is limited evidence and agreement on what constitutes an optimal bedtime routine. This study aims to reach expert consensus on a definition of optimal bedtime routines and to propose a measurement for bedtime routines. Method Four-step DELPHI process completed between February and March 2020 with 59 experts from different scientific, health and social care backgrounds. The DELPHI process started with an expert discussion group and then continued with 3 formal DELPHI rounds during which different elements of the definition and measurement of bedtime routines were iteratively refined. The proposed measurement of bedtime routines was then validated against existing data following the end of the DELPHI process. Results At the end of the four round DELPHI process and with a consistent 70% agreement level, a holistic definition of bedtime routines for families with young children between the ages of 2 and 8 years was achieved. Additionally, two approaches for measuring bedtime routines, one static (one-off) and one dynamic (over a 7-night period) are proposed following the end of the DELPHI process. A Bland-Altman difference plot was also calculated and visually examined showing agreement between the measurements that could allow them to be used interchangeably. Discussion Both the definition and the proposed measurements of bedtime routines are an important, initial step towards capturing a behavioural determinant of important health and developmental outcomes in children.