diff --git a/supabase/functions/update-subscription-quantity/index.ts b/supabase/functions/update-subscription-quantity/index.ts
new file mode 100644
index 0000000..839e86e
--- /dev/null
+++ b/supabase/functions/update-subscription-quantity/index.ts
@@ -0,0 +1,132 @@
+import { createClient } from "https://esm.sh/@supabase/supabase-js@2.45.0";
+import { z } from "npm:zod@3.23.8";
+import { type StripeEnv, createStripeClient } from "../_shared/stripe.ts";
+
+const corsHeaders = {
+  "Access-Control-Allow-Origin": "*",
+  "Access-Control-Allow-Headers":
+    "authorization, x-client-info, x-supabase-client-platform, x-supabase-api-version, apikey, content-type",
+  "Access-Control-Allow-Methods": "POST, OPTIONS",
+};
+
+const PRICE_PER_KG_ORE = 32500; // 325 kr exkl. moms
+const WEEKS_PER_MONTH = 4;
+
+const supabase = createClient(
+  Deno.env.get("SUPABASE_URL")!,
+  Deno.env.get("SUPABASE_SERVICE_ROLE_KEY")!,
+);
+
+const BodySchema = z.object({
+  new_kg_per_week: z.number().min(1).max(200),
+  environment: z.enum(["sandbox", "live"]),
+});
+
+Deno.serve(async (req) => {
+  if (req.method === "OPTIONS") return new Response("ok", { headers: corsHeaders });
+  if (req.method !== "POST") {
+    return new Response(JSON.stringify({ error: "Method not allowed" }), {
+      status: 405,
+      headers: { ...corsHeaders, "Content-Type": "application/json" },
+    });
+  }
+
+  try {
+    const token = req.headers.get("Authorization")?.replace("Bearer ", "") ?? "";
+    const { data: userData, error: authErr } = await supabase.auth.getUser(token);
+    if (authErr || !userData?.user) {
+      return new Response(JSON.stringify({ error: "Ej inloggad" }), {
+        status: 401,
+        headers: { ...corsHeaders, "Content-Type": "application/json" },
+      });
+    }
+
+    const parsed = BodySchema.safeParse(await req.json());
+    if (!parsed.success) {
+      return new Response(JSON.stringify({ error: "Ogiltiga fält" }), {
+        status: 400,
+        headers: { ...corsHeaders, "Content-Type": "application/json" },
+      });
+    }
+
+    const env: StripeEnv = parsed.data.environment;
+    const newKg = parsed.data.new_kg_per_week;
+
+    const { data: sub } = await supabase
+      .from("subscriptions")
+      .select("id, stripe_subscription_id, kg_per_week")
+      .eq("user_id", userData.user.id)
+      .eq("environment", env)
+      .in("status", ["active", "trialing", "past_due"])
+      .order("created_at", { ascending: false })
+      .limit(1)
+      .maybeSingle();
+
+    if (!sub?.stripe_subscription_id) {
+      return new Response(
+        JSON.stringify({ error: "Inget aktivt abonnemang hittades." }),
+        { status: 404, headers: { ...corsHeaders, "Content-Type": "application/json" } },
+      );
+    }
+
+    const stripe = createStripeClient(env);
+    const stripeSub = await stripe.subscriptions.retrieve(sub.stripe_subscription_id);
+    const item = stripeSub.items?.data?.[0];
+    if (!item) {
+      return new Response(JSON.stringify({ error: "Kunde inte hitta prisrad" }), {
+        status: 500,
+        headers: { ...corsHeaders, "Content-Type": "application/json" },
+      });
+    }
+
+    const productId =
+      typeof item.price.product === "string" ? item.price.product : item.price.product.id;
+    const newUnitAmount = Math.round(newKg * WEEKS_PER_MONTH * PRICE_PER_KG_ORE);
+
+    await stripe.subscriptions.update(sub.stripe_subscription_id, {
+      items: [
+        {
+          id: item.id,
+          price_data: {
+            currency: "sek",
+            product: productId,
+            recurring: { interval: "month" },
+            unit_amount: newUnitAmount,
+          } as any,
+          quantity: 1,
+        },
+      ],
+      proration_behavior: "create_prorations",
+      metadata: {
+        ...(stripeSub.metadata ?? {}),
+        kg_per_week: String(newKg),
+      },
+      description: `Kakabonnemang – ${newKg} kg/vecka`,
+    } as any);
+
+    // Uppdatera vår tabell direkt (webhook uppdaterar också, detta ger snabb UI-återkoppling)
+    await supabase
+      .from("subscriptions")
+      .update({
+        kg_per_week: newKg,
+        monthly_amount: newUnitAmount / 100,
+        updated_at: new Date().toISOString(),
+      })
+      .eq("id", sub.id);
+
+    return new Response(
+      JSON.stringify({
+        success: true,
+        new_kg_per_week: newKg,
+        new_monthly_amount_exkl_moms: newUnitAmount / 100,
+      }),
+      { status: 200, headers: { ...corsHeaders, "Content-Type": "application/json" } },
+    );
+  } catch (e: any) {
+    console.error("update-subscription-quantity error:", e);
+    return new Response(JSON.stringify({ error: e?.message ?? "Kunde inte uppdatera abonnemang" }), {
+      status: 500,
+      headers: { ...corsHeaders, "Content-Type": "application/json" },
+    });
+  }
+});