diff --git a/app/Actions/BookAddon.php b/app/Actions/BookAddon.php
index 6267d1b..8f09d43 100644
--- a/app/Actions/BookAddon.php
+++ b/app/Actions/BookAddon.php
@@ -151,6 +151,10 @@ class BookAddon
                 'currency' => Subscription::catalogueCurrency(),
                 'quantity' => $quantity,
                 'booked_at' => now(),
+                // What this pack weighs, at the moment of purchase. Without
+                // these two figures the size would be a view again.
+                'pack_gb' => $addonKey === AddonCatalogue::STORAGE ? $catalogue->packGb() : 0,
+                'pack_disk_gb' => $addonKey === AddonCatalogue::STORAGE ? $catalogue->packDiskGb() : 0,
             ], $overrides));
 
             ($this->record)(
diff --git a/app/Models/SubscriptionAddon.php b/app/Models/SubscriptionAddon.php
index 34eeeae..f66479b 100644
--- a/app/Models/SubscriptionAddon.php
+++ b/app/Models/SubscriptionAddon.php
@@ -23,7 +23,7 @@ class SubscriptionAddon extends Model
     use HasUuids;
 
     /** What the customer agreed to. */
-    public const FROZEN = ['subscription_id', 'addon_key', 'price_cents', 'currency', 'quantity', 'booked_at'];
+    public const FROZEN = ['subscription_id', 'addon_key', 'price_cents', 'currency', 'quantity', 'booked_at', 'pack_gb', 'pack_disk_gb'];
 
     protected $guarded = [];
 
@@ -46,6 +46,8 @@ class SubscriptionAddon extends Model
         return [
             'price_cents' => 'integer',
             'quantity' => 'integer',
+            'pack_gb' => 'integer',
+            'pack_disk_gb' => 'integer',
             'booked_at' => 'datetime',
             'cancelled_at' => 'datetime',
             // The end this booking has an appointment with. A cancellation is
diff --git a/app/Services/Billing/AddonCatalogue.php b/app/Services/Billing/AddonCatalogue.php
index 687c338..e423c5d 100644
--- a/app/Services/Billing/AddonCatalogue.php
+++ b/app/Services/Billing/AddonCatalogue.php
@@ -45,6 +45,27 @@ final class AddonCatalogue
         return $price === null ? null : (int) $price;
     }
 
+    /**
+     * How big a pack is that someone buys TODAY.
+     *
+     * Only for new bookings. What an already-bought pack weighs lives on the
+     * booking itself — see StorageAllowance.
+     */
+    public function packGb(): int
+    {
+        return (int) config('provisioning.storage_addon.gb', 0);
+    }
+
+    /**
+     * What a pack takes up on the host: the usable gigabytes plus the
+     * headroom a stacked pack needs too. Defaults to the usable figure when
+     * none is configured — that was the state before headroom existed.
+     */
+    public function packDiskGb(): int
+    {
+        return (int) config('provisioning.storage_addon.disk_gb', $this->packGb());
+    }
+
     public function knows(string $key): bool
     {
         return $this->priceCents($key) !== null;
diff --git a/app/Services/Billing/StorageAllowance.php b/app/Services/Billing/StorageAllowance.php
index a8bb049..dc7d350 100644
--- a/app/Services/Billing/StorageAllowance.php
+++ b/app/Services/Billing/StorageAllowance.php
@@ -35,12 +35,16 @@ use App\Models\Subscription;
  */
 final readonly class StorageAllowance
 {
-    private function __construct(
+    public function __construct(
         /** What the package itself includes. */
         public int $planGb,
-        /** How many extra-storage packs are booked and still running. */
+        /** How many packs are running — the figure the customer reads. */
         public int $packs,
-        /** How big one pack is today — see packSizeGb() for why "today". */
+        /** What these packs give the customer, at the sizes they were booked. */
+        public int $bookedGb,
+        /** What they take up on the host, at the sizes they were booked. */
+        public int $bookedDiskGb,
+        /** How big a pack would be TODAY — for quotes, not for what is already booked. */
         public int $packSizeGb,
     ) {}
 
@@ -75,13 +79,15 @@ final readonly class StorageAllowance
      */
     public static function forPlan(int $planGb, ?Subscription $subscription): self
     {
-        return new self(max(0, $planGb), self::bookedPacks($subscription), self::packSizeGb());
-    }
+        $booked = self::bookedPacks($subscription);
 
-    /** Everything the booked packs add. */
-    public function packGb(): int
-    {
-        return $this->packs * $this->packSizeGb;
+        return new self(
+            max(0, $planGb),
+            $booked['packs'],
+            $booked['gb'],
+            $booked['disk_gb'],
+            app(AddonCatalogue::class)->packGb(),
+        );
     }
 
     /** The figure Nextcloud is told, the disk is sized for, and the portal shows. */
@@ -114,35 +120,41 @@ final readonly class StorageAllowance
     }
 
     /**
-     * Every pack still running on this contract, counted by quantity.
+     * The packs still running on this contract, with their own sizes.
      *
-     * Queried rather than read off a loaded relation: this is asked immediately
-     * after a pack is booked or cancelled, and a relation loaded before that
-     * would answer with the state the caller has just changed.
+     * Queried rather than read off a loaded relation, for the same reason as
+     * before: this is asked immediately after a pack is booked or cancelled,
+     * and a relation loaded before that would answer with the state before
+     * the change.
+     *
+     * @return array{packs: int, gb: int, disk_gb: int}
      */
-    private static function bookedPacks(?Subscription $subscription): int
+    private static function bookedPacks(?Subscription $subscription): array
     {
         if ($subscription === null) {
-            return 0;
+            return ['packs' => 0, 'gb' => 0, 'disk_gb' => 0];
         }
 
-        return (int) $subscription->addons()->active()
+        $rows = $subscription->addons()->active()
             ->where('addon_key', AddonCatalogue::STORAGE)
-            ->sum('quantity');
+            ->get(['quantity', 'pack_gb', 'pack_disk_gb']);
+
+        return [
+            'packs' => (int) $rows->sum('quantity'),
+            'gb' => (int) $rows->sum(fn ($row) => (int) $row->quantity * (int) $row->pack_gb),
+            'disk_gb' => (int) $rows->sum(fn ($row) => (int) $row->quantity * (int) $row->pack_disk_gb),
+        ];
     }
 
-    /**
-     * What one pack is worth, from the catalogue.
-     *
-     * Read live rather than frozen onto the booking, unlike the PRICE. That is
-     * not an oversight but it is a limit worth naming: `subscription_addons`
-     * freezes what the customer agreed to PAY, and if the owner ever re-cuts the
-     * pack from 100 GB to 200 GB, existing bookings would grow with it. Today
-     * there is one pack size and it has never moved; the day it does, the size
-     * belongs on the booking beside the price.
-     */
-    private static function packSizeGb(): int
+    /** Everything the booked packs give the customer. */
+    public function packGb(): int
     {
-        return max(0, (int) config('provisioning.storage_addon.gb', 0));
+        return $this->bookedGb;
+    }
+
+    /** Everything the booked packs take up on the host. */
+    public function packDiskGb(): int
+    {
+        return $this->bookedDiskGb;
     }
 }
diff --git a/database/migrations/2026_08_01_000001_add_pack_size_to_subscription_addons.php b/database/migrations/2026_08_01_000001_add_pack_size_to_subscription_addons.php
new file mode 100644
index 0000000..f844ef5
--- /dev/null
+++ b/database/migrations/2026_08_01_000001_add_pack_size_to_subscription_addons.php
@@ -0,0 +1,44 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\DB;
+use Illuminate\Support\Facades\Schema;
+
+/**
+ * Wie groß ein gekaufter Block ist — auf der Buchung, nicht in der Konfiguration.
+ *
+ * `subscription_addons` friert seit jeher ein, was der Kunde zu ZAHLEN
+ * zugesagt hat. Was er dafür BEKOMMT, stand weiter in der Konfiguration und
+ * wurde bei jeder Anzeige neu gelesen. Solange es eine einzige Packungsgröße
+ * gab, war das folgenlos; mit dem Zuschnitt von 100 GB auf 20 GB wäre es der
+ * stille Verlust von 80 GB je gekauftem Block.
+ *
+ * Die 100 steht hier als Zahl und nicht als config()-Aufruf. Was verkauft
+ * wurde, ist Geschichte: eine Migration, die den Nachtrag aus der
+ * Konfiguration zöge, schriebe je nach Reihenfolge des Deployments 20 hinein
+ * und wäre damit genau der Fehler, den sie behebt.
+ */
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::table('subscription_addons', function (Blueprint $table) {
+            // 0 für alles, was kein Speicher ist: ein Support-Tarif hat keine
+            // Größe, und NULL wäre eine dritte Antwort auf eine Frage mit zwei.
+            $table->unsignedInteger('pack_gb')->default(0)->after('quantity');
+            $table->unsignedInteger('pack_disk_gb')->default(0)->after('pack_gb');
+        });
+
+        DB::table('subscription_addons')
+            ->where('addon_key', 'storage')
+            ->update(['pack_gb' => 100, 'pack_disk_gb' => 100]);
+    }
+
+    public function down(): void
+    {
+        Schema::table('subscription_addons', function (Blueprint $table) {
+            $table->dropColumn(['pack_gb', 'pack_disk_gb']);
+        });
+    }
+};
diff --git a/tests/Feature/Billing/StoragePackSizeTest.php b/tests/Feature/Billing/StoragePackSizeTest.php
new file mode 100644
index 0000000..af35770
--- /dev/null
+++ b/tests/Feature/Billing/StoragePackSizeTest.php
@@ -0,0 +1,65 @@
+<?php
+
+use App\Actions\BookAddon;
+use App\Actions\OpenSubscription;
+use App\Models\Order;
+use App\Models\Subscription;
+use App\Models\SubscriptionAddon;
+use App\Services\Billing\AddonCatalogue;
+use App\Services\Billing\StorageAllowance;
+
+/**
+ * Wie groß ein gekaufter Block ist.
+ *
+ * Die Größe stand in der Konfiguration und wurde bei jeder Anzeige neu gelesen.
+ * Solange sie sich nie bewegte, fiel das nicht auf; sie bewegt sich jetzt von
+ * 100 GB auf 20 GB. Ein Block, dessen Größe erst zur Anzeigezeit entsteht, ist
+ * kein gekauftes Gut, sondern eine Ansicht.
+ */
+function packContract(string $plan = 'team'): Subscription
+{
+    $order = Order::factory()->create(['plan' => $plan, 'datacenter' => 'fsn', 'status' => 'paid']);
+
+    return app(OpenSubscription::class)($order);
+}
+
+it('friert die Packungsgröße auf der Buchung ein', function () {
+    config(['provisioning.storage_addon.gb' => 100, 'provisioning.storage_addon.disk_gb' => 100]);
+
+    $subscription = packContract();
+    app(BookAddon::class)($subscription, AddonCatalogue::STORAGE, 1);
+
+    // Der Betreiber schneidet das Paket neu zu.
+    config(['provisioning.storage_addon.gb' => 20, 'provisioning.storage_addon.disk_gb' => 22]);
+
+    $allowance = StorageAllowance::forPlan(85, $subscription->refresh());
+
+    expect($allowance->packGb())->toBe(100)
+        ->and($allowance->packDiskGb())->toBe(100)
+        ->and($allowance->totalGb())->toBe(185);
+});
+
+it('bucht neue Blöcke zur heutigen Größe', function () {
+    config(['provisioning.storage_addon.gb' => 20, 'provisioning.storage_addon.disk_gb' => 22]);
+
+    $subscription = packContract();
+    app(BookAddon::class)($subscription, AddonCatalogue::STORAGE, 2);
+
+    $allowance = StorageAllowance::forPlan(30, $subscription->refresh());
+
+    expect($allowance->packs)->toBe(2)
+        ->and($allowance->packGb())->toBe(40)
+        ->and($allowance->packDiskGb())->toBe(44);
+});
+
+it('lässt die eingefrorene Größe nicht nachträglich ändern', function () {
+    config(['provisioning.storage_addon.gb' => 100, 'provisioning.storage_addon.disk_gb' => 100]);
+
+    $subscription = packContract();
+    $addon = app(BookAddon::class)($subscription, AddonCatalogue::STORAGE, 1);
+
+    expect(fn () => $addon->update(['pack_gb' => 20]))
+        ->toThrow(RuntimeException::class);
+
+    expect(SubscriptionAddon::query()->whereKey($addon->id)->value('pack_gb'))->toBe(100);
+});