At my high school, 90+ was 4.0, 80-90 was 3.0, etc. There was no +/-. You need to figure out how your school calculates it.
On frosted's scale, an 85 is a B (3.0), and 87 is a B+ (3.3), a 90 is an A- (3.7), and 93 is A (4.0)
Also figure out how honors/AP/IB change GPA at your school