I just taught may / might and the book tells the students that, "you use 'may' with things that have around a 50% chance of happening and might with a 30% chance".

Is this true?

I, for one, use might much more than may, and I dare say it's the same for lots of others. I certainly don't go around thinking about what the percentage chance of me going to the cinema is.

I may have got this wrong, sorry, might have.

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    Have you looked the terms up in a dictionary? If so, please explain what you've found and what deductions you've arrived at. If not, please do so. Note that one reason for closing a question is "Please include the research you've done, .... Questions that can be answered using commonly-available references are off-topic." – TrevorD May 10 '16 at 15:29
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    Related questions: “May” & “Might”: What's the right context?; May, might confusion; and many more from merely entering "may might" in the search box in the top-right corner. – TrevorD May 10 '16 at 15:34
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    Nobody computes the probability of an event and then decides whether to use may or might. In general, I think you may safely say that when people use may for something, it's often more likely than if they use might. But people use may for things that are much less likely than 30%. See Ngram. – Peter Shor May 10 '16 at 15:57
  • Although the topic of may and might has been extensively discussed in the linked question of which this one has been marked a duplicate, the one interesting matter raised by OP here that needs clarification is whether may implies a greater possibility/probability than might -- I myself had this doubt last year (doubt = confusion/question in Indian English) and looked it up in online resources, most of which agreed that may and might imply the same degree of probability and can therefore be used interchangeably except where 'might' needs to be used as the past tense form of 'may'. – English Student Apr 26 '17 at 23:29

Since 'may/might' are modals of 'possibility', there is no need to separate them into actual/hypothetical; they are always hypothetical -- so we get some extra use from 'may' by using it as a modal of 'permission'. Since permission involves someone's choice, you might mistakenly assume that that means a 50% chance, but neither 'may' nor 'might' implies anything about the actual probability.

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