This calculator is presented for educational purposes only. You, as pilot in command, are solely responsible for assuring correct data and proper loading of your aircraft prior to flight.
Empty weight and C.G. data comes from your AC current weight & balance records.
Station arms, load limits, and airspeed limits come from the Airplane Flight Manual (before 03-01-79) or Pilot's Operating Handbook [POH] (after 03-01-79) [pop-up more detail], or the FAA Type Certificate Data Sheet (TCDS). Downloading the TCDS is recommended to learn more about your AC.
This page, by D. Williams of the UK, is similar to what we're doing here, but his scheme of using CSS to make an active plot of the calculated load on the limits graph is elegant! We grabbed some of his CSS code and experimented with it.