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The final cable length listed is achieved when needle tips and cable are joined together ie measure your circular needles tip-to-tip once joined to the cable.
For example if you want a 80cm circular needle, you would join a pair of 12.8cm regular tips to an orange cable (which measures 56cm by itself) and that would be just over 80cm. If you had the special short tips (which are 10cm long) your finished circular needle would measure approximately 76cm.
If using the 40cm cable, it is recommended to use the special short KnitPro needle tips. Using standard length tips will make the length from tip-to-tip approximately 46cm, but will also limit the angle you can get between the tips and may make knitting very awkward.