Background <p>Chronic fatigue impacts individuals’ ability to engage in daily activities and their quality of life. The Activity Calculator is a method for fatigue management in relation to occupational participation that aims to: 1. Help patients to find a balance between what they want to do and what they are able to do. 2. Help patients to obtain congruence between occupational participation and their values and personal meaning.</p> Aims <p>Elaborate on a theoretical foundation for the Activity Calculator by addressing three concepts: occupational adaptation, patterns of daily occupational, and occupational balance.</p> Results <p>The three concepts are presented, and it is discussed how the concepts may serve as a theoretical foundation for the Activity Calculator when used in an occupational therapy intervention. The concept occupational adaptation is discussed as a transactional process, but also as an outcome as expressed by the two aims of the Activity Calculator.</p> Conclusion <p>The three addressed concepts seem to serve as a suitable theoretical foundation of the Activity Calculator.</p> Significance <p>The developed theoretical foundation makes it possible to evaluate the effects of the Activity Calculator in future research.</p>