The whole subject is one line of algebra

A weighted course grade is a sum of products:

course grade = Σ (score in each part × that part’s weight)

with the weights expressed as fractions of the whole course and summing to 1 (or 100%, same thing). Homework 30%, midterm 30%, final 40%: a student carrying 92 in homework, 78 on the midterm and 85 on the final finishes at 92 × 0.30 + 78 × 0.30 + 85 × 0.40 = 27.6 + 23.4 + 34.0 = 85.0.

Every “what do I need on the final” calculation rearranges that line. Let c be your current grade on the work already graded, w the final’s weight as a fraction, and t the target. The completed work contributes c × (1 − w) no matter what happens next, so:

required final score = (t − c × (1 − w)) ÷ w

Concrete: you’re at 85, the final is worth 20%, you want a 90. Required = (90 − 85 × 0.80) ÷ 0.20 = (90 − 68) ÷ 0.2 = 110. Not a typo — the formula is telling you the target is unreachable, which is the single most useful thing a calculator can say.

Why the answer can be impossible — or free

The division by w is the whole story. When the remaining weight is small, it amplifies the gap between where you are and where you want to be:

• With 50% still open, a 10-point gap needs 20 points of over-target performance.
• With 10% left, the same gap needs 100 points — i.e., impossible twice over.

Two degenerate cases fall out of the same line:

• Required > 100 → unreachable. The exam simply can’t pay that many points. The honest companion number is the ceiling: c × (1 − w) + w × 100, the grade a perfect score produces. If that ceiling is 87, then 87 — not 90 — is the real top of the range.
• Required ≤ 0 → already secured. Even a literal 0 on the final keeps you at or above the target. A required score of −40 isn’t “negative forty percent”; it’s a 40-point cushion you can’t burn through.

Between those poles the answer is a real number with a real meaning: score below it and you miss, at or above it and you make the target. There’s no rounding mercy — if you need 93.4, a 93 misses.

The four mistakes that wreck the math

1. Using the wrong “current grade.” The current grade must be your average on graded work only — points earned ÷ points graded so far, not ÷ total course points. If 700 of 1000 points are graded and you have 595, your current grade is 595/700 = 85%, not 59.5%. Entering 59.5 makes every required score look catastrophically high.

2. Confusing the final’s weight with its point value. “The final is worth 200 points” means nothing until you divide by total course points. In a 1000-point course it’s 20%; in a 600-point course it’s 33%. Same exam, different leverage.

3. Forgetting ungraded work besides the final. If a project (10%) is also still open, the final isn’t the only remaining variable — the honest setup is “what average do I need across everything left,” which is the multi-assessment mode on the calculator. Solving for the final alone while ignoring the project overstates what the final can do.

4. Trusting the syllabus over the gradebook. Dropped lowest quizzes, curves, extra credit and “the final can replace a midterm” clauses all change the weights in practice. The formula is only as good as the numbers fed into it — when the syllabus and the gradebook disagree, the gradebook is right.

Category weights vs. a single exam weight

Two syllabus styles dominate. Single-item weighting says “everything so far is 75%, the final is 25%” — one division and you’re done. Category weighting splits the course into homework/quizzes/exams/final, each with its own weight and its own running average. The math is identical, just more terms: sum score × weight over the graded categories for your standing, then solve for the average the ungraded categories must produce. The weighted-categories panel on the home page does exactly this — leave averages blank for whatever isn’t graded yet.

One subtlety that bites people: in a category scheme, “your current grade” is not the plain average of your category averages — it’s the weight-adjusted sum over graded categories only. A 95 in a 10% homework category contributes less than an 80 in a 40% exams category. The calculator handles that normalization automatically, including the case where your listed weights don’t sum to exactly 100.

Reading the result like a strategist

The required score is a planning input, not a prophecy. Three follow-ups matter more than the number itself:

• How far above a boundary are you? Needing 60 to pass and sitting at 85 are different situations even if both “pass.” The letter-boundary table shows the required score for every letter at once, so you can see whether the next grade up is a stretch goal or a fantasy.
• What’s the downside? Also check what a bad exam does — the same formula run with score = 0 gives your floor. Sometimes the smart play is defending the B you already have rather than chasing an A that needs 98.
• Which course deserves the hours? Run the numbers for every course you’re taking. The cheapest grade improvements live where a modest score moves the letter — covered in study triage.

The formula doesn’t care about your hopes, which is precisely its value. It converts a week of vague anxiety into a specific number you can plan around — and when it says the target is gone, it’s giving you back the hours you’d have spent chasing it.

Frequently asked questions

What is the formula for a weighted course grade?

Course grade = Σ(earned score × weight) across every category, where weights are fractions that sum to 1. A 92 in homework (30%), 78 on the midterm (30%) and 85 on the final (40%) gives 92×0.3 + 78×0.3 + 85×0.4 = 27.6 + 23.4 + 34 = 85.0.

Why does 'what do I need' sometimes come out above 100 or below 0?

Because the formula divides by the remaining weight. Small weights amplify everything: with only 10% of the course left, each target point needs ten points of score. A required 130 means impossible; a required −20 means you'd keep the target even with a zero — the sign is information, not an error.

Is a 20% final really worth only 20 points?

It contributes at most 20 points to the course grade, but its leverage depends on what's already banked. If you're at 90 entering a 20%-weight final, the final can only move you ±20 points — from 72 (score 0) to 92 (score 100). You can't gain more than 2 points of upside; that's the asymmetry students misjudge.

How do points-based syllabi map to weights?

Weight = item's points ÷ total course points. A 200-point final in a 1000-point course is 20%. If some work isn't graded yet, the 'current grade' is points earned ÷ points graded so far — not ÷ total points.