To obtain a full BCF A grade you will need to play at least 30 games in the year (June to May). If you play fewer games your grade may be categorised B, C, D, or E, with games from previous seasons possibly being taken into account. For an E grade you need to have played just a minimum of 10 games over the last 3 seasons, at least one of which is in the last year. The full grading list indicates whether a grade is categorised as A, B, C, D or E, but otherwise the grades are treated equally. However the the possible inclusion of previous years games in grade calculations other than for category A will result in a players grade changing less than they might anticipate based purely on their seasons performance.
To calculate, first obtain points for each game on the basis of opponents grade +50/+0/-50 if you win, draw or lose the game respectively. For these purposes your opponents grade is deemed to differ from yours by at most 40 points. Thus if a player graded 150 takes on one graded 100, the players grades are taken as 140 and 110 respectively for the purposes of calculating their opponents points. A players new grade is the average of these game points, subject to an enhancement for juniors and estimation of strength of ungraded opponents.
Subject to the same constraints, an equivalent calculation is
Average of opponents grades + percentage score over the season - 50