Shackles are sized based on which measurement?

Shackles are primarily sized based on the diameter of the steel in the bow section. This diameter is critical because it directly influences the shackle's weight capacity and performance under load. The larger the diameter, the more load a shackle can safely handle, ensuring it meets the requirements of various lifting and rigging operations.

The diameter also affects the strength of the shackle, as a thicker bow can resist deformation and failure under stress. In many scenarios, the working load limit of a shackle is specified in relation to this diameter, making it essential for ensuring that the shackle is fit for the intended application.

Other measurements, like the length of the shank, total height, or weight capacity, might be relevant in certain contexts but do not define the size of the shackle in the same way that the diameter of the steel in the bow section does.