To begin with, the TOEFL contains four sections: Reading, Listening, Speaking, and Writing. Each section’s score is 0-30, so your total TOEFL score will be 0-120.
ETS, the organization that designs and administers the TOEFL, collects data on how well test takers do. The top 25% of test takers get a total score of 98 or above. So, if you score 98, you have done better than 75% of students who take the TOEFL. The average score is 84, so if you beat that, you have done better than average. Finally, the bottom 25% of test takers score 69 or lower.
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If a "good" TOEFL score just means a better-than-average score, then 84 or higher would qualify. But for many students that is not the best definition. It should instead be one that helps you achieve your goals. Specifically, a "good" TOEFL score is the score that gets you into your schools of choice. Most schools have a minimum required score.
So, what score is "good" by that standard? Well, it depends on which school you want to apply to. The University of Michigan, for instance, has a minimum TOEFL requirement of 100. MIT requires a score of 90, but recommends 100. The University of Iowa has a minimum requirement of 80. And Iowa State University’s minimum is 71.
What is a good score for you?
In the end, a "good" score on the TOEFL is simply the score that meets your personal goals. Here is how to find out your personal TOEFL score requirement:
First, make a list of schools that you plan to apply to. Next, write down the minimum TOEFL score for each one. Finally, find the highest minimum score on that list. That is the TOEFL score you should be aiming at.