diff --git a/src/lib/supplier-ledger.test.ts b/src/lib/supplier-ledger.test.ts
new file mode 100644
index 0000000..43eb34a
--- /dev/null
+++ b/src/lib/supplier-ledger.test.ts
@@ -0,0 +1,86 @@
+// src/lib/supplier-ledger.test.ts
+import { describe, it, expect } from "vitest";
+import { computeSupplierLedger } from "./supplier-ledger";
+
+describe("computeSupplierLedger", () => {
+  it("full delivery on a single PO nets to zero outstanding", () => {
+    const result = computeSupplierLedger(
+      [{ id: "item1", purchase_order: "po1", qty: 10, rate: 100 }],
+      [{ purchase_order_item: "item1", qty_received: 10 }],
+    );
+    expect(result.lines).toEqual([
+      { purchaseOrderId: "po1", ordered: 1000, delivered: 1000, outstanding: 0 },
+    ]);
+    expect(result.totals).toEqual({ ordered: 1000, delivered: 1000, outstanding: 0 });
+  });
+
+  it("partial delivery reports the correct partial value", () => {
+    const result = computeSupplierLedger(
+      [{ id: "item1", purchase_order: "po1", qty: 10, rate: 100 }],
+      [{ purchase_order_item: "item1", qty_received: 4 }],
+    );
+    expect(result.lines[0]).toEqual({
+      purchaseOrderId: "po1", ordered: 1000, delivered: 400, outstanding: 600,
+    });
+  });
+
+  it("over-delivery is negative outstanding, not clamped", () => {
+    const result = computeSupplierLedger(
+      [{ id: "item1", purchase_order: "po1", qty: 10, rate: 100 }],
+      [{ purchase_order_item: "item1", qty_received: 12 }],
+    );
+    expect(result.lines[0].outstanding).toBe(-200);
+  });
+
+  it("an unmatched delivery line counts toward nothing", () => {
+    const result = computeSupplierLedger(
+      [{ id: "item1", purchase_order: "po1", qty: 10, rate: 100 }],
+      [
+        { purchase_order_item: "", qty_received: 5 },
+        { purchase_order_item: "item1", qty_received: 4 },
+      ],
+    );
+    expect(result.lines[0]).toEqual({
+      purchaseOrderId: "po1", ordered: 1000, delivered: 400, outstanding: 600,
+    });
+  });
+
+  it("a line with no deliveries at all is fully outstanding", () => {
+    const result = computeSupplierLedger(
+      [{ id: "item1", purchase_order: "po1", qty: 10, rate: 100 }],
+      [],
+    );
+    expect(result.lines[0]).toEqual({
+      purchaseOrderId: "po1", ordered: 1000, delivered: 0, outstanding: 1000,
+    });
+  });
+
+  it("multiple POs for one supplier sum correctly into totals", () => {
+    const result = computeSupplierLedger(
+      [
+        { id: "item1", purchase_order: "po1", qty: 10, rate: 100 },
+        { id: "item2", purchase_order: "po2", qty: 5, rate: 50 },
+      ],
+      [
+        { purchase_order_item: "item1", qty_received: 10 },
+        { purchase_order_item: "item2", qty_received: 2 },
+      ],
+    );
+    expect(result.lines).toHaveLength(2);
+    expect(result.totals).toEqual({ ordered: 1250, delivered: 1100, outstanding: 150 });
+  });
+
+  it("multiple line items on the same PO combine into one ledger line", () => {
+    const result = computeSupplierLedger(
+      [
+        { id: "item1", purchase_order: "po1", qty: 10, rate: 100 },
+        { id: "item2", purchase_order: "po1", qty: 2, rate: 500 },
+      ],
+      [{ purchase_order_item: "item1", qty_received: 10 }],
+    );
+    expect(result.lines).toHaveLength(1);
+    expect(result.lines[0]).toEqual({
+      purchaseOrderId: "po1", ordered: 2000, delivered: 1000, outstanding: 1000,
+    });
+  });
+});
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diff --git a/src/lib/supplier-ledger.ts b/src/lib/supplier-ledger.ts
new file mode 100644
index 0000000..8c078dd
--- /dev/null
+++ b/src/lib/supplier-ledger.ts
@@ -0,0 +1,102 @@
+/**
+ * Accounts-payable ledger for one supplier: ordered/delivered/outstanding
+ * value in Rand, across every PO the supplier is linked to.
+ *
+ * Computed, never stored -- same reasoning as src/lib/po-fulfilment.ts.
+ * A stored running balance would have to be kept correct on every PO,
+ * every delivery, every OCR re-run and correction, and PocketBase has no
+ * transactions to guarantee that. A sum computed fresh from the actual
+ * PO items and delivery lines every time cannot drift.
+ *
+ * Delivered value uses the PO LINE's rate, not a rate from the delivery
+ * note -- delivery_note_items has no rate field (qty_received,
+ * condition_notes, and two relations only; confirmed against the live
+ * schema). A partial delivery is priced at the same per-unit rate as the
+ * full order, because that is the only rate that exists to use.
+ *
+ * See spec section 5.
+ */
+
+export type SupplierPoItemLike = {
+  id: string;
+  purchase_order: string;
+  qty: number;
+  rate: number;
+};
+
+export type SupplierDeliveryLineLike = {
+  /** Empty when OCR could not match the line to a PO item. */
+  purchase_order_item: string;
+  qty_received: number;
+};
+
+export type SupplierLedgerLine = {
+  purchaseOrderId: string;
+  ordered: number;
+  delivered: number;
+  /** Negative means over-delivered. Deliberately not clamped. */
+  outstanding: number;
+};
+
+export type SupplierLedger = {
+  lines: SupplierLedgerLine[];
+  totals: { ordered: number; delivered: number; outstanding: number };
+};
+
+export function computeSupplierLedger(
+  items: SupplierPoItemLike[],
+  deliveryLines: SupplierDeliveryLineLike[],
+): SupplierLedger {
+  const deliveredQtyByItem = new Map<string, number>();
+  for (const line of deliveryLines) {
+    const key = line.purchase_order_item;
+    // An unmatched line counts toward nothing -- attaching it to the
+    // nearest item would corrupt a line nobody touched.
+    if (!key) continue;
+    deliveredQtyByItem.set(
+      key,
+      (deliveredQtyByItem.get(key) ?? 0) + (line.qty_received ?? 0),
+    );
+  }
+
+  const byPo = new Map<string, { ordered: number; delivered: number }>();
+  for (const item of items) {
+    const lineOrdered = item.qty * item.rate;
+    const deliveredQty = deliveredQtyByItem.get(item.id) ?? 0;
+    const lineDelivered = deliveredQty * item.rate;
+
+    const bucket = byPo.get(item.purchase_order) ?? { ordered: 0, delivered: 0 };
+    bucket.ordered += lineOrdered;
+    bucket.delivered += lineDelivered;
+    byPo.set(item.purchase_order, bucket);
+  }
+
+  const round2 = (n: number) => Math.round(n * 100) / 100;
+
+  const lines: SupplierLedgerLine[] = Array.from(byPo.entries()).map(
+    ([purchaseOrderId, { ordered, delivered }]) => ({
+      purchaseOrderId,
+      ordered: round2(ordered),
+      delivered: round2(delivered),
+      outstanding: round2(ordered - delivered),
+    }),
+  );
+
+  const totals = lines.reduce(
+    (acc, l) => ({
+      ordered: acc.ordered + l.ordered,
+      delivered: acc.delivered + l.delivered,
+      outstanding: acc.outstanding + l.outstanding,
+    }),
+    { ordered: 0, delivered: 0, outstanding: 0 },
+  );
+
+  return {
+    lines,
+    totals: {
+      ordered: round2(totals.ordered),
+      delivered: round2(totals.delivered),
+      outstanding: round2(totals.outstanding),
+    },
+  };
+}
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