diff --git a/apps/web/lib/work-effort-engine.test.ts b/apps/web/lib/work-effort-engine.test.ts
new file mode 100644
index 0000000..8d6c4a1
--- /dev/null
+++ b/apps/web/lib/work-effort-engine.test.ts
@@ -0,0 +1,31 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+
+import { aggregateWorkEffort, roundHalfHour } from './work-effort-engine';
+
+test('roundHalfHour rounds to nearest half hour', () => {
+  assert.equal(roundHalfHour(0.24), 0);
+  assert.equal(roundHalfHour(0.25), 0.5);
+  assert.equal(roundHalfHour(1.24), 1);
+  assert.equal(roundHalfHour(1.25), 1.5);
+});
+
+test('aggregateWorkEffort calculates weighted progress by estimate', () => {
+  const result = aggregateWorkEffort([
+    { estimateHours: 1, actualHours: 0, progress: 0 },
+    { estimateHours: 3, actualHours: 1.25, progress: 100 },
+  ]);
+
+  assert.equal(result.estimateHours, 4);
+  assert.equal(result.actualHours, 1.5);
+  assert.equal(result.progress, 75);
+});
+
+test('aggregateWorkEffort falls back to item average when estimates are zero', () => {
+  const result = aggregateWorkEffort([
+    { estimateHours: 0, actualHours: 0, progress: 50 },
+    { estimateHours: 0, actualHours: 0, progress: 100 },
+  ]);
+
+  assert.equal(result.progress, 75);
+});
diff --git a/apps/web/lib/work-effort-engine.ts b/apps/web/lib/work-effort-engine.ts
new file mode 100644
index 0000000..08e1e24
--- /dev/null
+++ b/apps/web/lib/work-effort-engine.ts
@@ -0,0 +1,42 @@
+export interface WorkEffortItem {
+  estimateHours: number;
+  actualHours: number;
+  progress: number;
+}
+
+export interface WorkEffortSummary {
+  estimateHours: number;
+  actualHours: number;
+  progress: number;
+}
+
+export function roundHalfHour(hours: number): number {
+  if (!Number.isFinite(hours) || hours <= 0) return 0;
+  return Math.round(hours * 2) / 2;
+}
+
+export function aggregateWorkEffort(items: WorkEffortItem[]): WorkEffortSummary {
+  if (items.length === 0) return { estimateHours: 0, actualHours: 0, progress: 0 };
+
+  const estimateHours = roundHalfHour(items.reduce((sum, item) => sum + Math.max(0, item.estimateHours || 0), 0));
+  const actualHours = roundHalfHour(items.reduce((sum, item) => sum + Math.max(0, item.actualHours || 0), 0));
+
+  if (estimateHours <= 0) {
+    return {
+      estimateHours,
+      actualHours,
+      progress: Math.round(items.reduce((sum, item) => sum + item.progress, 0) / items.length),
+    };
+  }
+
+  const weightedProgress = items.reduce((sum, item) => {
+    const estimate = Math.max(0, item.estimateHours || 0);
+    return sum + estimate * item.progress;
+  }, 0);
+
+  return {
+    estimateHours,
+    actualHours,
+    progress: Math.round(weightedProgress / estimateHours),
+  };
+}