diff --git a/src/modules/stock-count.ts b/src/modules/stock-count.ts
new file mode 100644
index 0000000..a5f7444
--- /dev/null
+++ b/src/modules/stock-count.ts
@@ -0,0 +1,20 @@
+/**
+ * 盤點差異模組（沙盤推演示意）
+ */
+
+/** 單一物料的盤點差異 */
+export interface StockCountDiff {
+  /** 物料識別碼 */
+  materialId: string;
+  /** 帳面數量 */
+  bookQty: number;
+  /** 實盤數量 */
+  countedQty: number;
+}
+
+/**
+ * 計算盤點差異數量
+ * @param diff - 單一物料的盤點資料
+ * @returns 實盤減帳面的差異值
+ */
+export const diffQty = (diff: StockCountDiff): number => diff.countedQty - diff.bookQty;