Hemp cord sizes are measured in millimeters. The thinnest hemp cord available is .5mm ( half of a millimetre) and the thickest  3mm. Thicker hemp rope (4mm and upwards)  is made up of   multi strands twisted together versus one solid  strand of cord. All measurements  are approximate to give you an idea of thickness but they do vary between products and brands.  The measurements provided are set by industry standards and are meant as a guide only. Be aware that the cord you receive may not be the exact  measurement stated according to your ruler. That is because it is just  not possible to measure every spool and ball to  check the width due to   variation between batches and the fact that different factories have different machines and manufacturing processes.