diff --git a/frontend/src/lib/person/genealogy/panZoom.test.ts b/frontend/src/lib/person/genealogy/panZoom.test.ts
new file mode 100644
index 00000000..509acdd8
--- /dev/null
+++ b/frontend/src/lib/person/genealogy/panZoom.test.ts
@@ -0,0 +1,26 @@
+import { describe, it, expect } from 'vitest';
+import { clampZoom, MIN_ZOOM, MAX_ZOOM } from './panZoom';
+
+describe('clampZoom', () => {
+	it('returns the value unchanged when within range', () => {
+		expect(clampZoom(1)).toBe(1);
+		expect(clampZoom(0.5)).toBe(0.5);
+		expect(clampZoom(2.75)).toBe(2.75);
+	});
+
+	it('clamps below MIN_ZOOM up to MIN_ZOOM', () => {
+		expect(clampZoom(0.1)).toBe(MIN_ZOOM);
+		expect(clampZoom(0)).toBe(MIN_ZOOM);
+		expect(clampZoom(-5)).toBe(MIN_ZOOM);
+	});
+
+	it('clamps above MAX_ZOOM down to MAX_ZOOM', () => {
+		expect(clampZoom(5)).toBe(MAX_ZOOM);
+		expect(clampZoom(3.0001)).toBe(MAX_ZOOM);
+	});
+
+	it('exposes the resolved zoom bounds', () => {
+		expect(MIN_ZOOM).toBe(0.25);
+		expect(MAX_ZOOM).toBe(3.0);
+	});
+});
diff --git a/frontend/src/lib/person/genealogy/panZoom.ts b/frontend/src/lib/person/genealogy/panZoom.ts
new file mode 100644
index 00000000..7b107440
--- /dev/null
+++ b/frontend/src/lib/person/genealogy/panZoom.ts
@@ -0,0 +1,19 @@
+/**
+ * Pan/zoom geometry for the Stammbaum canvas (#692).
+ *
+ * The Stammbaum renders zoom by deriving the SVG `viewBox` rather than applying
+ * a CSS transform (see `StammbaumTree.svelte`). This module is the single source
+ * of truth for the zoom bounds, the view-state shape, and every pure geometry
+ * helper used by the gesture action, the URL serialiser, and the page. Keeping
+ * the math here (and free of DOM access) makes it unit-testable in the node
+ * project. See ADR-026 for why this is custom rather than a third-party library.
+ */
+
+/** Resolved zoom bounds (OQ-001). */
+export const MIN_ZOOM = 0.25;
+export const MAX_ZOOM = 3.0;
+
+/** Clamp a zoom factor into the supported range. */
+export function clampZoom(z: number): number {
+	return Math.min(MAX_ZOOM, Math.max(MIN_ZOOM, z));
+}