# GD&T What is the difference between circularity and cylindricity?

Here is an image from Shigley's Mechanical Engineering Design about circularity... Now here is another image from the same textbook about cylindricity... If circularity applies along the entire length of the cylinder then they seem like they achieve the exact same thing. What am I missing? What is a situation where these two would not be the same?

I'm going to present a highly contrived example of a shape which would conform to the circularity constraint, and not cylindricity:

You can see how in the front view, every cross section is a perfect circle, but it's clearly not a cylinder.

Examples and/or discussion of when you would choose one over the other are out of scope for this question, but suffice to say that circularity is the less limiting of the two, so is used in most cases where cylindricity is not a design-critical additional requirement.

Often the straightness of the cylinder will be controlled by other dimensional constraints or tolerances, making the use of cylindricity redundant, and/or overconstraining.

• Thanks, Mike! I must have been in a hurry :) Commented Jun 17, 2020 at 16:07
• So I see how the image you posted is not a cylinder but what I am questioning is that Shigley’s definition of circularity makes it seem like your image would not qualify as circular either. For circularity, every slice(cross-section) in the surface must be between two concentric circles. Are these concentric circles not consistent throughout the entire length of the surface? Based on the images I posted, I hope you understand my confusion. Commented Jun 17, 2020 at 22:44
• The point is that circularity applies to cross sections independently, but cylindricality applies to the complete structure. For example a banana could be circular, but not cylindrical (unless it was a straight banana). Commented Jun 17, 2020 at 23:11
• After thinking about it, I believe I understand. For circularity, the high and low points of cross-sections must be between two concentric circles but the concentric circles do necessarily have the same radii throughout the length of the surface. For cylindricality, the concentric circles are the same size throughout the length of the surface. EDIT: thanks I didn’t think about the cross-sections being independent of each other for circularity. Commented Jun 17, 2020 at 23:15
• That is close, but not quite right. For circularity, even if the radii of the circles at the different cross sections are the same, the centers of the circles need not all lie on the same straight line. For cylindricality, the centers of the circles at different cross sections are all on the same line. Commented Jun 17, 2020 at 23:22