A 66 percent grade is a D letter grade. A letter grade D means less than satisfactory or below average performance in a class or on a test. Any percentage score in the 63-66.9 range is a D on the grade scale table.
A 66 is a D and is one percentage point below being a D+. The cutoff for a D+ is a 67 minimum. However, teachers may sometimes give students the benefit of the doubt and round up to a D+. This is especially true if your percentage is a fraction above a 66, like a 66.5, 66.75 or 66.9. If the D is rounded up you would benefit from an additional 0.3 grade points in your GPA calculation.
Aside from 66 out of 100, there are numerous ways students can end up with 66% grades. Here are scales for common number of test questions that can lead to a 66 grade.
Reference table of the percentage grade scale with letter grade and unweighted GPA point equivalents. This grade scale is used my most U.S. middle schools, high schools and colleges.