Chess, Ajedrez, Schach, Xadrez, Schaak, Шахматы, etc.
Floating-Point Rating Estimator

The previous, integer version is here.

New: Use browser bookmarks to save your work.

  Total Score
  Current Rating
# of Prior Games                       Age
Use Lower K values for high rated players (2200 and up) for estimating regular ratings in dual-rated events.