diff --git a/src/lib/grades.ts b/src/lib/grades.ts
new file mode 100644
index 0000000..9cc31c9
--- /dev/null
+++ b/src/lib/grades.ts
@@ -0,0 +1,36 @@
+// Weighted-category grade calculation shared by the gradebook and the grade views.
+export type Cat = { id: string; name: string; weight: number };
+export type Scale = { letter: string; min_percent: number };
+export type Assn = { id: string; category_id: string | null; max_points: number };
+export type Grd = { assignment_id: string; student_id: string; points: number | null };
+
+// Overall percent for a student: weighted average across categories that have graded work.
+export function studentPercent(studentId: string, assignments: Assn[], grades: Grd[], categories: Cat[]): number | null {
+  const byCat: Record<string, { earned: number; possible: number }> = {};
+  for (const a of assignments) {
+    if (!a.category_id) continue;
+    const g = grades.find((x) => x.assignment_id === a.id && x.student_id === studentId && x.points != null);
+    if (!g) continue;
+    const b = byCat[a.category_id] ?? { earned: 0, possible: 0 };
+    b.earned += Number(g.points);
+    b.possible += Number(a.max_points);
+    byCat[a.category_id] = b;
+  }
+  let weighted = 0, totalW = 0;
+  for (const c of categories) {
+    const b = byCat[c.id];
+    if (!b || b.possible === 0) continue;
+    weighted += (b.earned / b.possible) * 100 * Number(c.weight);
+    totalW += Number(c.weight);
+  }
+  return totalW === 0 ? null : weighted / totalW;
+}
+
+export function letterFor(pct: number | null, scale: Scale[]): string {
+  if (pct == null) return "—";
+  const s = [...scale].sort((a, b) => Number(b.min_percent) - Number(a.min_percent));
+  for (const r of s) if (pct >= Number(r.min_percent)) return r.letter;
+  return s.length ? s[s.length - 1].letter : "—";
+}
+
+export const fmtPct = (pct: number | null) => (pct == null ? "—" : `${pct.toFixed(1)}%`);