NEW Grade boundary chart
Until the first examination session for the 2018 Subject Guide, in May 2020, we will not know with any precision what the grade boundaries will be. These will only be decided properly once the senior examiners in each component have had chance to analyse and assess the overall performance under the new system. This means applying the new Criteria and Grade Descriptors to real student responses to real examination papers in the case of Papers 1 and 2; and to performance in the tasks expected in the Oral Internal Assessment. (By the way, grade boundaries for Oral Internal Assessment are fixed since the task involved in the component doesn't change from one session to another.)
It is very unlikely that there will be any grand change from the kind of grade boundaries that have been normal under the current system. Apart from the introduction of the Listening Comprehension element in the new Paper 2, the rest of the testing mechanisms are virtually the same as the present ones. In addition, the IB will wish to maintain a certain consistency of standards in the change from one assessment system to another.
It is worth considering that three existing assessment elements have been removed - the Section B Reasoned Response in the writing paper; the Written Assignment; and the Interactive Oral Activities - and so this might have some effect on the overall pattern of marks (that's the overall percentage column in the chart below). However, since grade boundaries in the overall percentage are defined by applying the grade boundaries decided in the individual components, these boundaries will be rigorously justified by the decisions taken in relation to well-established techniques in Writing, Reading/Listening and Oral assessments. So, I repeat, I don't forsee any striking changes...
In the meantime, here is a rough chart to help you to convert marks into grades.
NOTE: This is not an official IB chart - just my personal estimation
In all cases here, you can assume that grade boundaries may vary by +1 or -1 ... but for the sake of providing grades during the course, these figures are, I believe, within the range of variation which will apply in the real marking.
And let us never forget that the boundaries are notoriously imprecise. The difference between a script which is at the very top of Grade 5 and a script which is at the very bottom of Grade will be very slight indeed ... and very debateable as well! But in the end, in order to have grades, we have to draw lines between grades - and very often those lines are placed on the basis of very tiny distinctions, so tiny that they are barely detectable!
Adjusting for overall percentages
You can use the chart above to calculate how to transform marks into grades for the various different components, thus:-
Paper 1 Writing - the 30 marks column
Paper 2 Receptive skills - overall, the 65 marks column
** Listening = 25 marks column
** Reading = 40 marks column
Oral Interview - the 30 marks column
But... the marks available for each component are not the same as the percentage weighting for each component, thus:-
Paper 1 Writing ... 30 marks > 25%
Paper 2 Receptive skills ... 65 marks > 50%
Oral Interview ... 30 marks > 25%
So... we have to scale the marks mathematically, as follows:-
Paper 1 Writing ... divide mark by 6, multiply by 5
Paper 2 Receptive skills ... divide mark by 13, multiply by 10
Oral Interview ... divide mark by 6, multiply by 5
Which is simple enough, if a bit of a fiddly bore! And remember to round up and round down: the IB only recognises whole numbers, not decimal points or fractions.