Chess Rating Calc

Chess Rating Calc is a small Macintosh utility for computing Elo chess ratings. It lets a player or tournament director enter prior ratings and game results to project the post-event rating change, replicating the standard Elo formula on a classic Mac desktop.

What Elo measures

The Elo system, devised by physicist Arpad Elo and adopted by FIDE in 1970, expresses a player's relative skill as a single number. Expected scores between two players are derived from the rating difference; actual results are then compared with expectations to produce a rating delta scaled by a K-factor.

Using the calculator

Enter your current rating, your opponent's rating, the game result (1, 1/2, or 0), and a K-factor appropriate for your federation or club. The application returns the expected score and your new rating. Some versions accept multiple opponents in a single tournament-style calculation.

Why a desktop tool

Before web-based rating calculators were common, club organizers and correspondence players relied on standalone utilities like this one to estimate ratings between official rating list publications, which historically appeared only quarterly or annually.

Compatibility

Distributed as a small classic Macintosh application; runs under System 7 through Mac OS 9. The archive is BinHex/StuffIt encoded in the typical shareware-era format.