copied and pasted from the edexcel website:The calculation of grades for unitised specifications is quite complicated and involves the use of what is called a uniform mark scheme (UMS). When the uniform marks are added up they give a total which is then converted to a grade. Although we give an explanation below of how a grade is calculated, do not worry too much about this; it is the final grade that is important for entry to college or university, or your chosen career.
When modular courses were first introduced, a problem occurred with the total number of marks that each subject had and so the examination boards decided to use what is called the Uniform Mark Scale. The following maximums were agreed for the qualifications listed:
Qualification Maximum uniform marks
Advanced Subsidiary GCE 300
Advanced GCE 600
For modular GCSE subjects (Mathematics, Science and Religious Studies), there is no agreed total of uniform marks and these subjects have different totals. Each unit of a specification is valued at a certain percentage of the whole scheme of assessment. The most common percentages are 15%, 17.5% and 20%. All units will, of course, add up to 100% and to the maximum shown above. An example follows:
Unit Percentage Maximum uniform marks
Unit 1 15% 90
Unit 2 15% 90
Unit 3 15% 90
Unit 4 15% 90
Unit 5 20% 120
Unit 6 20% 120
Totals 100% 600
The uniform mark obtained for each unit of a specification is shown on the results slip, and when added up, you obtain a total number of uniform marks for that specification which determines the grade for that subject, as follows:
For AS specifications:
Uniform marks
Grade A 240-300
Grade B 210-239
Grade C 180-209
Grade D 150-179
Grade E 120-149
For A level specifications:
Uniform marks
Grade A 480-600
Grade B 420-479
Grade C 360-419
Grade D 300-359
Grade E 240-299
The UMS grade boundaries for units are as follows:
GCE max 90 UMS
A 72
B 63
C 54
D 45
E 36
GCE max 100 UMS
A 80
B 70
C 60
D 50
E 40
GCE max 120 UMS
A 96
B 84
C 72
D 60
E 48
From your results slip, you can see not only if you did better in some units and not so well in others, but also how close you were to an overall grade boundary.
Link to Edexcel