An adjustable duct start collar may include a base with a flange on a first end of the base and a plurality of tabs on a second end of the base. The base may be constructed of a material of substantially uniform thickness and include at least two slots in the base. The first of the slots may be positioned substantially parallel to a second of the at least two slots. Each of the slots of the at least two slots being able to independently receive the flange, whereby when the base is curved into a cylindrical shape with the flange positioned in the first of at least two slots, the curved base will form a cylindrical shape with a first diameter and when the flange is positioned in the second of the at least two slots, the curved base will form a cylindrical shape with a second diameter.