The one line of algebra
Your course grade is your pre-final work and your final exam, mixed in the proportion the syllabus sets. Write C for your current grade, w for the final's weight as a decimal, and T for the grade you want:
C × (1 − w) + required × w = T, therefore required = (T − C × (1 − w)) ÷ w
Nothing else is going on. The whole page — the calculator above, both tables below, every warning — is that expression evaluated and then read out loud. Two consequences follow immediately, and they are the ones students get wrong. First, dividing by w means a small final exam magnifies the gap: with a 10% final, every point you want to move your course grade costs ten points on the exam. Second, the expression can return anything at all, including numbers above 100 and below 0, and both of those are meaningful answers, which is why the calculator prints them instead of an error.
Reference table: what you need for an 80%
Both tables below are computed from that formula for every combination of a common current grade and a common final-exam weight. Read down to your current grade, across to your final's weight. Red cells are impossible — they need more than a perfect paper. Amber cells need 95% or better, which is possible and not comfortable. Green cells are already secured before you sit down.
| Current grade | Final = 10% | Final = 15% | Final = 20% | Final = 25% | Final = 30% | Final = 40% | Final = 50% |
|---|---|---|---|---|---|---|---|
| 95% | secured | secured | 20.0% | 35.0% | 45.0% | 57.5% | 65.0% |
| 90% | secured | 23.3% | 40.0% | 50.0% | 56.7% | 65.0% | 70.0% |
| 85% | 35.0% | 51.7% | 60.0% | 65.0% | 68.3% | 72.5% | 75.0% |
| 80% | 80.0% | 80.0% | 80.0% | 80.0% | 80.0% | 80.0% | 80.0% |
| 75% | impossible (125.0%) | impossible (108.3%) | 100.0% | 95.0% | 91.7% | 87.5% | 85.0% |
| 70% | impossible (170.0%) | impossible (136.7%) | impossible (120.0%) | impossible (110.0%) | impossible (103.3%) | 95.0% | 90.0% |
| 65% | impossible (215.0%) | impossible (165.0%) | impossible (140.0%) | impossible (125.0%) | impossible (115.0%) | impossible (102.5%) | 95.0% |
Reference table: what you need for a 90%
| Current grade | Final = 10% | Final = 15% | Final = 20% | Final = 25% | Final = 30% | Final = 40% | Final = 50% |
|---|---|---|---|---|---|---|---|
| 95% | 45.0% | 61.7% | 70.0% | 75.0% | 78.3% | 82.5% | 85.0% |
| 90% | 90.0% | 90.0% | 90.0% | 90.0% | 90.0% | 90.0% | 90.0% |
| 85% | impossible (135.0%) | impossible (118.3%) | impossible (110.0%) | impossible (105.0%) | impossible (101.7%) | 97.5% | 95.0% |
| 80% | impossible (180.0%) | impossible (146.7%) | impossible (130.0%) | impossible (120.0%) | impossible (113.3%) | impossible (105.0%) | 100.0% |
| 75% | impossible (225.0%) | impossible (175.0%) | impossible (150.0%) | impossible (135.0%) | impossible (125.0%) | impossible (112.5%) | impossible (105.0%) |
| 70% | impossible (270.0%) | impossible (203.3%) | impossible (170.0%) | impossible (150.0%) | impossible (136.7%) | impossible (120.0%) | impossible (110.0%) |
| 65% | impossible (315.0%) | impossible (231.7%) | impossible (190.0%) | impossible (165.0%) | impossible (148.3%) | impossible (127.5%) | impossible (115.0%) |
Put the two tables side by side and the shape of the problem appears. Chasing a 90% from an 85% is impossible with a 20% final and only just possible with a 40% one — 97.5%, printed in amber. The same 85% student reaching for an 80% needs 60% on that 20% final, which is a different kind of afternoon entirely. The heavier the final, the more it can rescue you and the more it can ruin you, and that symmetry is worth internalising in September rather than in the exam hall.
Reading an impossible cell
Take a student on 85% with a 20% final aiming at 90%. The formula returns 110%, which does not exist. What that student can actually finish with is 85 × 0.8 + 100 × 0.2 = 88.00%, and that is the ceiling no matter how the exam goes. The useful move is not to revise harder; it is to check whether anything else is still open — an outstanding assignment, an extra-credit option, a misrecorded score — because those change C, and C is the only lever left. When a target is unreachable the calculator says so and prints your true ceiling, so you can spend the last week on the courses where effort still converts into grade.
Reading a secured cell
The green cells are the opposite situation: a student on 95% needing 80% overall, with a 10% final, comes out of the formula at −55%. The negative number is arithmetic being literal — the target was passed 55 points of exam-score ago. Neither the table nor the calculator asks you for it: the table prints secured, and the calculator reports the score you actually need, which is 0.00%. Two cautions. It assumes every other grade is final and correct, and a single late zero or a regraded paper moves C. And a blank exam is still a blank exam: attendance and completion policies are not part of this formula.
Two students in the last week of term
The reachable one
You are on 88% in a course whose final is worth 20%, and you want the 90% that keeps your A on a ten-point scale. Required = (90 − 88 × 0.8) ÷ 0.2 = (90 − 70.4) ÷ 0.2 = 98%. That is a real target and a hard one. Before committing, check the other direction with the table under the calculator: an 80% on that final leaves you at 86.40%, still comfortably a B, and a 60% leaves you at 82.40%. Knowing the downside is as useful as knowing the requirement.
The one that changes your plan
You are on 75% with a 20% final, and you want to finish on 80%. Required = (80 − 60) ÷ 0.2 = 100% exactly — the single cell in the first table that sits precisely on the edge. Arithmetically possible, practically a coin flip on a perfect paper. Now compare the 30% column in the same row: 91.7%. The identical student, in a course that weights its final more heavily, has a much easier job. That comparison is the argument for reading the weight off your syllabus in week one rather than in week fifteen.
What this assumes
- Your current grade already excludes the final exam entirely — not as a zero, not as full marks.
- The final's weight is the number on your syllabus, and the rest of the course accounts for the other 100 − w percent.
- Nothing is curved, dropped, replaced or rounded after the fact. All four are common and all four are local policy.
- Percentage targets are not letters. An 80% is a B on a ten-point scale and a C on a seven-point scale, where B does not start until 85 — see the scale comparison table on the grade calculator.
Related calculators
- Grade calculator — build your current grade from scratch if your portal's number is not trustworthy.
- Weighted grade calculator — see how much each category, not just the final, is worth.
- Semester grade calculator — for schools that combine two quarter grades with a separate final exam.