HOW GRADES ARE CALCULATED
Points are allocated in respect of each game. For a win you score the opponent's Grade plus 50, for a draw the opponent's Grade, and for a loss the opponent's Grade minus 50. There is a proviso that if your opponent's Grade differs from yours by more than 40 points it is assumed to be exactly 40 above (or below) yours. This is to prevent a player increasing his Grade by losing to a much stronger player, or decreasing his Grade by beating a much weaker player. If the opponent is ungraded, his Grade is estimated, using all available information.
The Grade is calculated by dividing the total number of points scored by the number of games played. If there are at least 30 games in the current period, then the Grade is based on these games alone. If there are not, results are brought forward from the previous period to make the total up to exactly thirty. If there are not 30 games in the two seasons together, results are taken from the season before that. Games are never taken from further back than this; the maximum is two prior grading periods.
Results are brought forward in two different ways, depending whether the Grade is Rapid or Standard. With Rapidplay, any games brought forward from a previous period will be the most recent games in that period. This is possible because the dates of Rapid games are (almost) always known. With Standardplay, unfortunately, this is not the case. So, instead, the required number of (notional) games is brought forward at the average score for the period.
For juniors under the age of 18, an enhancement is added to their Grade to take into account their expected improvement over the year. This enhancement is six points for juniors of age 15 to 17, eight for juniors of age 11 to 14, and ten for juniors under the age of 11. The enhancement is added after all calculations are done, and is included in the published Grade.