Here are some common numbers floating around the language teaching world, their misinterpretations, and some clarifications to go with each one:

90%
This figure comes from an ACTFL position statement on target language use. The biggest misinterpretation I’ve seen is thinking something like students should be speaking the target language 90% of the time. The 90% figure is actually about percentage OF language during class that IS the target language. For example, if you’re reading a 100% Latin text during class and ask SOME questions about it 100% in English, it’s quite possible that target language use is still above 90% (e.g., students read 1,000 total words of Latin and hear 100 words of English questions = 90% target language use). Students need input. This 90% figure is an attempt to prioritize high quantities of language. The alternative to avoid would be something like doing a close reading on a short paragraph of Latin with the discussion entirely in English, perhaps resulting in the opposite 10% target language use, which was quite common in the teaching of languages prior to ACTFL’s statement. Somehow, you’ll still find such low levels of target language use today, despite ACTFL’s efforts back in 2010. N.B. if you do a general search, some ACTFL page might come up that contradicts their original 2010 statement, referring explicitly to class time. It also name-drops Krashen, pointlessly throws in the i+1 concept that itself became misunderstood, cites Vygotsky in some kind of weird sociocultural nod, and ends with an obligatory reference to Long & Swain’s work on output theory. This hodgepodge of ideas is startling, but probably represents how ACTFL tends to remain as NEUTRAL as possible, taking bits and pieces from way too many popular ideas and approaches as if it makes sense to combine them all. Don’t fall into that trap. Follow principles that you know align with each other and don’t conflict!

95%
See 98%, below.

96%
Perhaps more commonly known as “4%,” the figure of 96% represents the students who typically do NOT continue beyond the second year of language study, here in the US, with the dominant paradigm having been grammar-based teaching. Although programs have been moving to comprehension-based, and/or communicative approaches, it’s still possible that the 4% figure represents some programs today, though the origin dates back to a 1969 speech, detailed in this post. A common misinterpretation has been to claim that individual “4%ers” are the only students capable of learning a language via grammar-translation (GT). This is untrue. The “4%ers” are simply those who tend to stay enrolled. Recently, I’ve argued they represent the pedagogically immune.

98%
I’ve seen this (as well as 95%, and even 90% in some cases) shared as “minimum comprehension level” that students should have when reading. This figure actually represents recommended minimum text coverage while reading. It’s from various studies, detailed in this post. In short, a text coverage as high as 98% has been shown to produce comprehension scores as low as 70%. That’s not incredibly good. Also from the studies, a text coverage of 95% produced scores as low as 55%! You don’t want to know what the scores were for 80%!

100%
I’ve heard people say that knowing 100% of words means a text will be 100% understandable. That’s not true (see above). In an age of promoting “productive struggle” and “grit,” the accusation is that a 100% understandable text is undesirable because students will somehow not get their +1 level to their i (again, see this post on that whole concept demystified). This is most definitely untrue. Even when comprehension is 100% there are aspects of language (e.g., word order, forms, etc.) that can still be learned. Given what is known about text coverage, there’s very little justification for using target language students don’t know, like above-level texts. Therefore, the 100% figure should be a goal for text coverage or known vocab.