The FIDE Rating system is a numerical system in which
percentage scores are converted to rating differences and vice versa. Its
function is to produce scientific measurement information of the best
statistical quality. 
10.1 
The rating scale is an arbitrary one with a class interval set
at 200 points. The tables that follow show the conversion of percentage score
`p` into rating difference `d_{p}`. For a zero or 100% score dp is
necessarily indeterminate. The second table shows conversion of difference in
rating `D` into scoring probability `P_{D}` for the higher `H` and the
lower `L` rated player respectively. Thus the two tables are effectively
mirrorimages.
 The table of conversion from percentage score, p, into rating differences,
d_{p}
p 
d_{p} 
p 
d_{p} 
p 
d_{p} 
p 
d_{p} 
p 
d_{p} 
p 
d_{p} 
1.0 

.83 
273 
.66 
117 
.49 
7 
.32 
133 
.15 
296 
.99 
677 
.82 
262 
.65 
110 
.48 
14 
.31 
141 
.14 
309 
.98 
589 
.81 
251 
.64 
102 
.47 
21 
.30 
149 
.13 
322 
.97 
538 
.80 
240 
.63 
95 
.46 
29 
.29 
158 
.12 
336 
.96 
501 
.79 
230 
.62 
87 
.45 
36 
.28 
166 
.11 
351 
.95 
470 
.78 
220 
.61 
80 
.44 
43 
.27 
175 
.10 
366 
.94 
444 
.77 
211 
.60 
72 
.43 
50 
.26 
184 
.09 
383 
.93 
422 
.76 
202 
.59 
65 
.42 
57 
.25 
193 
.08 
401 
.92 
401 
.75 
193 
.58 
57 
.41 
65 
.24 
202 
.07 
422 
.91 
383 
.74 
184 
.57 
50 
.40 
72 
.23 
211 
.06 
444 
.90 
366 
.73 
175 
.56 
43 
.39 
80 
.22 
220 
.05 
470 
.89 
351 
.72 
166 
.55 
36 
.38 
87 
.21 
230 
.04 
501 
.88 
336 
.71 
158 
.54 
29 
.37 
95 
.20 
240 
.03 
538 
.87 
322 
.70 
149 
.53 
21 
.36 
102 
.19 
251 
.02 
589 
.86 
309 
.69 
141 
.52 
14 
.35 
110 
.18 
262 
.01 
677 
.85 
296 
.68 
133 
.51 
7 
.34 
117 
.17 
273 
.00 

.84 
284 
.67 
125 
.50 
0 
.33 
125 
.16 
284 


 Table of conversion of difference in rating, D, into scoring probability
P_{D}, for the higher, H, and the lower, L, rated player respectively.
D 
P_{D} 

D 
P_{D} 

D 
P_{D} 

D 
P_{D} 

Rtg Dif 
H 
L 
Rtg Dif 
H 
L 
Rtg Dif 
H 
L 
Rtg Dif 
H 
L 
03 
.50 
.50 
9298 
.63 
.37 
198206 
.76 
.24 
345357 
.89 
.11 
410 
.51 
.49 
99106 
.64 
.36 
207215 
.77 
.23 
358374 
.90 
.10 
1117 
.52 
.48 
107113 
.65 
.35 
216225 
.78 
.22 
375391 
.91 
.09 
1825 
.53 
.47 
114121 
.66 
.34 
226235 
.79 
.21 
392411 
.92 
.08 
2632 
.54 
.46 
122129 
.67 
.33 
236245 
.80 
.20 
412432 
.93 
.07 
3339 
.55 
.45 
130137 
.68 
.32 
246256 
.81 
.19 
433456 
.94 
.06 
4046 
.56 
.44 
138145 
.69 
.31 
257267 
.82 
.18 
457484 
.95 
.05 
4753 
.57 
.43 
146153 
.70 
.30 
268278 
.83 
.17 
485517 
.96 
.04 
5461 
.58 
.42 
154162 
.71 
.29 
279290 
.84 
.16 
518559 
.97 
.03 
6268 
.59 
.41 
163170 
.72 
.28 
291302 
.85 
.15 
560619 
.98 
.02 
6976 
.60 
.40 
171179 
.73 
.27 
303315 
.86 
.14 
620735 
.99 
.01 
7783 
.61 
.39 
180188 
.74 
.26 
316328 
.87 
.13 
over 735 
1.0 
.00 
8491 
.62 
.38 
189197 
.75 
.25 
329344 
.88 
.12 


 
10.2 
Determining the Rating `R_{u}` in a given event of a
previously unrated player. 

10.21 
First determine the average rating of his competition
`R_{c}`. (GA `94)
 In a Swiss or Team tournament: this is simply the average rating of his
opponents.
 The results of both rated and unrated players in a round robin tournament
are taken into account. For unrated players, the average rating of the
competition `R_{c}` is also the tournament average `R_{a}`
determined as follows: (GA `94)
 Determine the average rating of the rated players `Rar`.
 Determine p for each of the rated players against all their opponents. Then
determine dp for each of these players. Then determine the average of these dp =
`dpa`.
 `n` is the number of opponents.
R_{a} = R_{ar} 
d_{pa} x n/(n+1) 

10.22 
If he scores 50%, then R_{u} = R_{c}. (GA `94)


10.23 
If he scores more than 50%, then R_{u} = R_{c}
+ 12.5 for each half point scored over 50%. (GA `94) 

10.24 
If he scores less than 50% in a Swiss or team tournament (GA
`94):
R_{u} = R_{c} + d_{p}. 

10.25 
If he scores less than 50% in a roundrobin (GA `94):
R(u) = R(c) + d(p) x n/(n+1). 
10.3 
The Rating R_{n}
which is to be published for a previously unrated player is then determined by
taking the
weighted average of all his R_{u} results. e.g.
A player has R_{u} results of 2280 over 5 games, 2400 over 10 games and
2000 over 5 games:
R_{n} = [ 2280 x 5 + 2400 x 10 + 2000 x 5 ]
/ 20 = 2270. 

10.31 
Where a player`s first performance(s) is less than 1801, the
result(s) is ignored. 

10.32 
R_{n} for the FIDE Rating list (FRL) is rounded off to
the nearest 1 or zero.
Only R_{n}>= 1801 are considered. 
10.4 
If an unrated player receives a published rating before a
particular tournament in which he has played is rated, then he is rated as a
rated player with his current rating, but in the rating of his opponents he is
counted as an unrated player. 
10.5 
Determining the rating change for a rated player:


10.51 
Determine R_{c}
For a Swiss, team or individual match this is the average
rating of the player`s opponent(s).
A difference in rating of more than 350 points will be counted
for rating purposes as though it were a difference of 350 points (cf 10.58).
For a roundrobin (GA `94):
R_{c} = 
R_{a} (n+1)  R



n

where 
R 
= 
rating of the player 
n 
= 
number of opponents 
R_{a} 
= 
average rating of the whole tournament as calculated in 10.21b
when unrated players are competing.  

10.52 
See B.01./9.4. for an Honorary Rating which regulation also
applies for 10.21b. 

10.53 
R is the published rating of the rated player RR_{a} =
D. Use 10.1b. to determine P_{D}. 

10.54 
W is the score achieved and W_{e} is the expected
score. (GA `94)
W_{e} = P_{D} x n for a Swiss or individual
match.
DR = K (W  W_{e}).
DR is the rating change.
 K is the development coefficient.
K = 25 for a player new to the rating
list until he has completed events with a total of at least 30 games. K = 15
as long as a player`s rating remains under 2400. K = 10 once a player`s
published rating has reached 2400, and he has also completed events with a total
of at least 30 games. Thereafter it remains permanently at 10.


10.55 
To determine R_{n} for a rated player,
determine DR for each tournament in which he has
played. R_{o} is the old rating. SDR is
the summation of DR for each event in which the player
has participated. R_{n} = R_{o} + SDR 

10.56 
Rn is rounded off to the
nearest 1 or 0. 
