Abstract: A modular fan unit of a diagonal and/or axial fan having a housing that includes an inlet opening and an outflow opening. The housing accommodates a fan, that conveys the air at least in the axial direction and is divided perpendicularly to an axis (X-X) of the fan into at least two housing halves that are joined together. The housing forms a chamber in which the blades of the fan rotate around an axis and has a circumferential edge area in the vicinity of which the housing has a thickness B extending in the axis direction of the fan. A motor holding part has a dimension R as the maximum extension in the edge area and the chamber has an extension in the axis direction of the fan at a height of a dimension L, whereas the extension of the air inlet part has a dimension T in the area of the chamber. A ratio of the dimension B to the dimension R with respect to the ratio of the dimension L to the dimension T is within the range from 0.9 to 1.2.