Schools set their own cutoffs
A percentage is a measurement; a letter is a decision about what that measurement means. College Board's table is one commonly used system: 90-100 is an A, 80-89 a B, 70-79 a C, 66-69 a D and below 65 an F. It states that some schools do not use pluses and minuses and others have different cutoffs.
Bands against a smooth line
A table groups many percentages into one value. On the College Board table, 80 and 89 both give 3.0. A proportional method, grade divided by 100 times 4, gives 3.20 for 80 and 3.56 for 89. Neither is correct in an absolute sense; they answer different questions.
Rounding and gaps
Tables leave edge cases open. The College Board table lists D as 66-69 and F as below 65, so a grade of exactly 65 is not listed. Decimals such as 89.5 also need a rule. This site rounds to the nearest whole percentage, with .5 rounding up, and says so in each result.
- Ask how your teacher rounds a 89.5.
- Ask whether your school rounds before or after assigning the letter.
What to do with your own grades
Use the cutoffs printed in your school's grading policy whenever you have them. A converter is for the cases where you do not, or where you are comparing across schools. The percentage page here shows the whole table so you can see where a grade sits. If your school uses plus and minus grades, its percentage cutoffs are not part of the College Board table, so take them from your school's own policy instead of guessing.
Sources and further reading
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