Comment by hafabnew

Neat!

From their JS:

  readinglvl = getReadingLevel(paragraphs, sentences, words, chars);
  [..]
  function getReadingLevel (p, s, w, c) {
      var r = Math.round((4.75 * (c / w)) + (0.5 * (w / s)) - 21.43);
      return r;
  }

So it's a slightly modified http://en.wikipedia.org/wiki/Automated_Readability_Index (uses 4.75 instead of 4.71 .