This tool shouldn't be taken too seriously, and if anything should simply be used as a way to motivate and measure self-improvement. I originally built this thing because I noticed there are a lot of people searching for stuff like "arm size", "average arm size", "average chest size men", etc. and there really isn't any good place to find out what reasonable expectations of those things should be. Meanwhile, there's a treasure trove of CDC and military data that makes the actual, data-backed answer to those kinds of questions pretty easy to find.
Regarding accuracy and relevant populations, the civilian data (with the exception of chest, shoulder, and hip measurements) all comes from either the 2011-2014 CDC anthropometric reference, or the 2003-2006 version. The data used was from the 20-30 age range. Hip measurements came from a 2005 Australian Government study of Australian Air Crew and potential Air Crew members (i.e. 20-30 something Australian men and women).
The military data all comes from the 2012 Anthropometric Survey of US Army Personnel. The chest and shoulder data for civilians was extrapolated from a combination of the US military data and a 2004-2005 study of Dutch adults aged 20-30. The extrapolation process basically involved adjusting the Dutch measurements for differences in average height (the Dutch are tall!), then adjusting for differences in measuring methods between that and the US military data (US military measured with a full breath).
Body fat estimates are based on the Navy bodyfat formula and average neck size is assumed if you don't provide one.Data sources