diff --git a/api_docs/changelog.md b/api_docs/changelog.md
index d0a02c2092..4f49975805 100644
--- a/api_docs/changelog.md
+++ b/api_docs/changelog.md
@@ -20,6 +20,11 @@ format used by the Zulip server that they are interacting with.
 
 ## Changes in Zulip 9.0
 
+**Feature level 251**
+
+* [`POST /register`](/api/register-queue): Fixed `realm_upload_quota_mib`
+  value to actually be in MiB. Until now the value was in bytes.
+
 **Feature level 250**
 
 * [`GET /messages`](/api/get-messages),
diff --git a/version.py b/version.py
index 04675df832..af78fb5c16 100644
--- a/version.py
+++ b/version.py
@@ -33,7 +33,7 @@ DESKTOP_WARNING_VERSION = "5.9.3"
 # Changes should be accompanied by documentation explaining what the
 # new level means in api_docs/changelog.md, as well as "**Changes**"
 # entries in the endpoint's documentation in `zulip.yaml`.
-API_FEATURE_LEVEL = 250
+API_FEATURE_LEVEL = 251
 
 # Bump the minor PROVISION_VERSION to indicate that folks should provision
 # only when going from an old version of the code to a newer version. Bump
diff --git a/web/src/attachments_ui.ts b/web/src/attachments_ui.ts
index d29eef8c34..c318f0740f 100644
--- a/web/src/attachments_ui.ts
+++ b/web/src/attachments_ui.ts
@@ -70,11 +70,15 @@ export function bytes_to_size(bytes: number, kb_with_1024_bytes = false): string
     return size + " " + sizes[i];
 }
 
+export function mib_to_bytes(mib: number): number {
+    return mib * 1024 * 1024;
+}
+
 export function percentage_used_space(uploads_size: number): string | null {
     if (realm.realm_upload_quota_mib === null) {
         return null;
     }
-    return ((100 * uploads_size) / realm.realm_upload_quota_mib).toFixed(1);
+    return ((100 * uploads_size) / mib_to_bytes(realm.realm_upload_quota_mib)).toFixed(1);
 }
 
 function set_upload_space_stats(): void {
@@ -84,7 +88,7 @@ function set_upload_space_stats(): void {
     const args = {
         show_upgrade_message: realm.realm_plan_type === 2,
         percent_used: percentage_used_space(upload_space_used),
-        upload_quota: bytes_to_size(realm.realm_upload_quota_mib, true),
+        upload_quota: bytes_to_size(mib_to_bytes(realm.realm_upload_quota_mib), true),
     };
     const rendered_upload_stats_html = render_settings_upload_space_stats(args);
     $("#attachment-stats-holder").html(rendered_upload_stats_html);
diff --git a/zerver/lib/events.py b/zerver/lib/events.py
index efc1dbe386..d62aeb100b 100644
--- a/zerver/lib/events.py
+++ b/zerver/lib/events.py
@@ -69,6 +69,7 @@ from zerver.lib.users import (
     is_administrator_role,
     max_message_id_for_user,
 )
+from zerver.lib.utils import optional_bytes_to_mib
 from zerver.models import (
     Client,
     CustomProfileField,
@@ -301,7 +302,8 @@ def fetch_initial_state_data(
         state["max_avatar_file_size_mib"] = settings.MAX_AVATAR_FILE_SIZE_MIB
         state["max_file_upload_size_mib"] = settings.MAX_FILE_UPLOAD_SIZE
         state["max_icon_file_size_mib"] = settings.MAX_ICON_FILE_SIZE_MIB
-        state["realm_upload_quota_mib"] = realm.upload_quota_bytes()
+        upload_quota_bytes = realm.upload_quota_bytes()
+        state["realm_upload_quota_mib"] = optional_bytes_to_mib(upload_quota_bytes)
 
         state["realm_icon_url"] = realm_icon_url(realm)
         state["realm_icon_source"] = realm.icon_source
@@ -1157,7 +1159,9 @@ def apply_event(
             if event["property"] == "plan_type":
                 # Then there are some extra fields that also need to be set.
                 state["zulip_plan_is_not_limited"] = event["value"] != Realm.PLAN_TYPE_LIMITED
-                state["realm_upload_quota_mib"] = event["extra_data"]["upload_quota"]
+                # upload_quota is in bytes, so we need to convert it to MiB.
+                upload_quota_bytes = event["extra_data"]["upload_quota"]
+                state["realm_upload_quota_mib"] = optional_bytes_to_mib(upload_quota_bytes)
 
             if field == "realm_jitsi_server_url":
                 state["jitsi_server_url"] = (
diff --git a/zerver/lib/utils.py b/zerver/lib/utils.py
index a0fd067841..768414a2a8 100644
--- a/zerver/lib/utils.py
+++ b/zerver/lib/utils.py
@@ -33,3 +33,10 @@ def process_list_in_batches(
             break
         process_batch(items)
         offset += chunk_size
+
+
+def optional_bytes_to_mib(value: Optional[int]) -> Optional[int]:
+    if value is None:
+        return None
+    else:
+        return value >> 20
diff --git a/zerver/openapi/zulip.yaml b/zerver/openapi/zulip.yaml
index e85d061980..5c5dd02f45 100644
--- a/zerver/openapi/zulip.yaml
+++ b/zerver/openapi/zulip.yaml
@@ -15277,7 +15277,10 @@ paths:
 
                           If `null`, there is no limit.
 
-                          **Changes**: New in Zulip 5.0 (feature level 72). Previously,
+                          **Changes**: Before Zulip 9.0 (feature level 251), this field
+                          was incorrectly measured in bytes, not MiB.
+
+                          New in Zulip 5.0 (feature level 72). Previously,
                           this was called `realm_upload_quota`.
                       realm_org_type:
                         type: integer