What Beema said They take a few different measurements, and average them out to give an overall size to work out where bub is on the chart. For instance, my bubby had his head as 10th centile, but his abdominal and femur measurements were not on the charts, which brought his 'overall' size to about 5th centile (I think? It was almost a year ago LOL). Obviously there is way more to it than that, but that is a lay person's way of looking at it