diff --git a/packages/backend/src/lib/seats.ts b/packages/backend/src/lib/seats.ts
new file mode 100644
index 0000000..0f4b563
--- /dev/null
+++ b/packages/backend/src/lib/seats.ts
@@ -0,0 +1,44 @@
+import { stripe } from "../stripe/client";
+import { getOrganisationsByUserId, getOrganisationMembers } from "../db/queries/organisations";
+import { getSubscriptionByUserId, updateSubscription } from "../db/queries/subscriptions";
+import { getUserById } from "../db/queries/users";
+
+export async function updateSeatCount(userId: number) {
+    const user = await getUserById(userId);
+
+    // only update if user has active pro subscription
+    if (!user || user.plan !== "pro") {
+        return;
+    }
+
+    const subscription = await getSubscriptionByUserId(userId);
+    if (!subscription || subscription.status !== "active") {
+        return;
+    }
+
+    // calculate total members across all owned organisations
+    const organisations = await getOrganisationsByUserId(userId);
+    const ownedOrgs = organisations.filter((o) => o.OrganisationMember.role === "owner");
+
+    let totalMembers = 0;
+    for (const org of ownedOrgs) {
+        const members = await getOrganisationMembers(org.Organisation.id);
+        totalMembers += members.length;
+    }
+
+    const newQuantity = Math.max(1, totalMembers - 4);
+
+    // skip if quantity hasn't changed
+    if (newQuantity === subscription.quantity) {
+        return;
+    }
+
+    // update stripe
+    await stripe.subscriptionItems.update(subscription.stripeSubscriptionItemId!, {
+        quantity: newQuantity,
+        proration_behavior: "always_invoice",
+    });
+
+    // update local record
+    await updateSubscription(subscription.id, { quantity: newQuantity });
+}