The short answer: 21 out of 30 is 70%, a C- on the standard 10-point scale with plus/minus grades. But "what grade is X out of Y" questions are really three questions wearing one coat: what's the percentage, what letter is that on my school's scale, and how many points would I have needed for the letter above? This guide answers all three, with the arithmetic shown, plus the quick-grade trick teachers use to mark a whole stack of tests without a calculator.
One division and one multiplication: percentage = points earned ÷ points possible × 100. For 21 out of 30, that's 21 ÷ 30 = 0.70, times 100 gives 70%. A few more, worked the same way:
| Raw score | Arithmetic | Percentage | Letter (10-pt, +/−) |
|---|---|---|---|
| 21 / 30 | 21 ÷ 30 = 0.70 | 70% | C− |
| 18 / 25 | 18 ÷ 25 = 0.72 | 72% | C− |
| 17 / 20 | 17 ÷ 20 = 0.85 | 85% | B |
| 45 / 50 | 45 ÷ 50 = 0.90 | 90% | A− |
| 19 / 20 | 19 ÷ 20 = 0.95 | 95% | A |
| 24 / 30 | 24 ÷ 30 = 0.80 | 80% | B− |
Notice how the same letter shows up at different raw scores depending on the total. That's why "I got a 21" means nothing without the "out of 30" attached — a 21 out of 25 is 84%, a whole letter and a half higher.
It depends on the scale, and the scale is set by your school, not by math. The three you'll run into:
When in doubt, the syllabus wins. The conversion from letter to GPA is also school-specific, but the standard mapping runs 4.0 for an A down to 0.7 for a D−, with F at 0.
Flip the formula: points needed = cutoff percentage × points possible, rounded up. On a 30-point test with the 10-point scale, the letter boundaries land on clean numbers:
| Letter goal | Cutoff | Points on a 30-point test | Resulting % |
|---|---|---|---|
| D− | 60% | 18 points | 60% |
| C− | 70% | 21 points | 70% |
| C | 73% | 22 points | 73.3% |
| B− | 80% | 24 points | 80% |
| A− | 90% | 27 points | 90% |
So a student sitting at 21 out of 30 is exactly one point from a C, three from a B−, and six from an A−. On odd totals the rounding matters more: for a C at 73% on a 45-point test you need 73% × 45 = 32.85, rounded up to 33 points, which scores 73.3%.
Enter points earned and points possible, pick your scale, and get the percentage, letter, GPA points, and the exact points needed for the next letter up — plus a full quick-grade table for the whole test.
Open the Test Grade Calculator →Less than you'd think, more than you'd hope. GPA is credit-weighted, so a C− (1.7) on a test usually isn't the whole course grade — it's one score inside a category. If tests are 60% of the course and this is one of three, that single 70% drags the course grade down by a couple of points before other work pulls it back. If you want the reverse math — what you need on the final to land a target course grade — that's a different formula, and the final grade calculator runs it. To see what a course grade does to a semester GPA, stack terms in the GPA calculator.
They pre-compute the division table and never touch a calculator again. Points possible in, percentages out, one row per raw score — a 30-point test has 31 possible scores and you only need the rows near the letter boundaries marked. Post-test, grading becomes lookup: 27? A−. 24? B−. The test grade calculator builds this quick-grade table for any point total up to 200, in any of the three scales, which beats rebuilding it in a spreadsheet every time a quiz has 23 questions.
Two marking habits worth stealing from experienced graders: always double-check the points possible before you divide (the classic error is dividing by the last question number when one question was worth 2 points), and grade the same question across all papers rather than one paper at a time — it keeps partial-credit judgments consistent.
21 out of 30 is 70%, a C- on the 10-point scale with plus/minus grading (70-72.9% is the C- band). On a plain letter scale it's a C. One more point, 22 out of 30, is 73.3% and a solid C.
Divide points earned by points possible, then multiply by 100. For 18 out of 25: 18 ÷ 25 = 0.72, so 72%. The division works for any point total, including decimals on partial credit.
On the 10-point scale, 70% is the floor of the C range: a C- with plus/minus grading (1.7 GPA points) or a plain C without it. On a 7-point scale where a C- starts at 70, it's also a C-, but a 73% would be a C there instead of a plain-C territory letter.
On the 10-point scale a B- starts at 80%, which is 24 out of 30 exactly. A solid B at 83% needs 24.9 rounded up to 25 points, and a B+ at 87% needs 27 (26.1 rounded up).