This calculator was inspired by this AWIS blog post on gender biases in recommendation letters. The blog post and the scientific paper it is based on also explain why this gender bias is important. I am grateful to Dr. Karen James for bringing it to my attention and leading me to examine my own written recommendations.
I made this tool in 2013, for no other reason than to show that building tools could drive more engagement than writing papers. Since then it's become very popular, but I have no opinion on the science/sociology that powers the tool.
A slightly more current version of the tool is maintained on GitHub.