In order to provide manufacturers of carding machines and relevant accessories with theoretical references, how cylinder radius, setover and heel-toe difference affect cylinder-flat gauge of a carding machine was theoretically studied. The relationship between cylinder-flat gauge and cylinder radius, setover and heel-toe difference was geometrically discussed. Numerical calculation and illustration about the relationship were made with MATLAB in accordance with practical settings. A general formula about the relationship is derived. A concept, the small-gauge zone length, has been defined for the first time, and some relevant results thus obtained. Given setover and heel-toe difference, the greater the cylinder radius, the greater the average gauge. If a smaller overall cylinder-flat gauge is desirable, it is not necessary to emphasize the tangential direction of the heel of clothed surface to the cylinder. Their intersection within a small zone is acceptable. In many cases, small-gauge zone can reduce average gauge which may be helpful to the carding action; given cylinder radius and setover, the smaller the heel-toe difference, the more helpful to reduce the overall gauge; given cylinder radius and heel-toe difference, the small-gauge zone length will increase with the increase of setover, so does the difference between the smallest gauge and outlet gauge.