diff --git a/src/utils/stats.ts b/src/utils/stats.ts
new file mode 100644
index 0000000..77f8c23
--- /dev/null
+++ b/src/utils/stats.ts
@@ -0,0 +1,154 @@
+import { NostrEvent, NStore } from '@nostrify/nostrify';
+import { Kysely, UpdateObject } from 'kysely';
+
+import { DittoTables } from '@/db/DittoTables.ts';
+import { getTagSet } from '@/utils/tags.ts';
+
+interface UpdateStatsOpts {
+  kysely: Kysely<DittoTables>;
+  store: NStore;
+  event: NostrEvent;
+}
+
+/** Handle one event at a time and update relevant stats for it. */
+// deno-lint-ignore require-await
+export async function updateStats({ event, kysely, store }: UpdateStatsOpts): Promise<void> {
+  switch (event.kind) {
+    case 1:
+      return handleEvent1(kysely, event);
+    case 3:
+      return handleEvent3(kysely, store, event);
+    case 6:
+      return handleEvent6(kysely, event);
+    case 7:
+      return handleEvent7(kysely, event);
+  }
+}
+
+/** Update stats for kind 1 event. */
+async function handleEvent1(kysely: Kysely<DittoTables>, event: NostrEvent): Promise<void> {
+  await updateAuthorStats(kysely, event.pubkey, ({ notes_count }) => ({ notes_count: notes_count + 1 }));
+}
+
+/** Update stats for kind 3 event. */
+async function handleEvent3(kysely: Kysely<DittoTables>, store: NStore, event: NostrEvent): Promise<void> {
+  const following = getTagSet(event.tags, 'p');
+
+  await updateAuthorStats(kysely, event.pubkey, () => ({ following_count: following.size }));
+
+  const [prev] = await store.query([
+    { kinds: [3], authors: [event.pubkey], limit: 1 },
+  ]);
+
+  const { added, removed } = getFollowDiff(event.tags, prev?.tags);
+
+  for (const pubkey of added) {
+    await updateAuthorStats(kysely, pubkey, ({ followers_count }) => ({ followers_count: followers_count + 1 }));
+  }
+
+  for (const pubkey of removed) {
+    await updateAuthorStats(kysely, pubkey, ({ followers_count }) => ({ followers_count: followers_count - 1 }));
+  }
+}
+
+/** Update stats for kind 6 event. */
+async function handleEvent6(kysely: Kysely<DittoTables>, event: NostrEvent): Promise<void> {
+  const id = event.tags.find(([name]) => name === 'e')?.[1];
+  if (id) {
+    await updateEventStats(kysely, id, ({ reposts_count }) => ({ reposts_count: reposts_count + 1 }));
+  }
+}
+
+/** Update stats for kind 7 event. */
+async function handleEvent7(kysely: Kysely<DittoTables>, event: NostrEvent): Promise<void> {
+  const id = event.tags.find(([name]) => name === 'e')?.[1];
+  if (id) {
+    await updateEventStats(kysely, id, ({ reactions_count }) => ({ reactions_count: reactions_count + 1 }));
+  }
+}
+
+/** Get the pubkeys that were added and removed from a follow event. */
+export function getFollowDiff(
+  tags: string[][],
+  prevTags: string[][] = [],
+): { added: Set<string>; removed: Set<string> } {
+  const pubkeys = getTagSet(tags, 'p');
+  const prevPubkeys = getTagSet(prevTags, 'p');
+
+  return {
+    added: pubkeys.difference(prevPubkeys),
+    removed: prevPubkeys.difference(pubkeys),
+  };
+}
+
+/** Retrieve the author stats by the pubkey, then call the callback to update it. */
+export async function updateAuthorStats(
+  kysely: Kysely<DittoTables>,
+  pubkey: string,
+  fn: (prev: DittoTables['author_stats']) => UpdateObject<DittoTables, 'author_stats'>,
+): Promise<void> {
+  const empty = {
+    pubkey,
+    followers_count: 0,
+    following_count: 0,
+    notes_count: 0,
+  };
+
+  const prev = await kysely
+    .selectFrom('author_stats')
+    .selectAll()
+    .where('pubkey', '=', pubkey)
+    .executeTakeFirst();
+
+  const stats = fn(prev ?? empty);
+
+  if (prev) {
+    await kysely.updateTable('author_stats')
+      .set(stats)
+      .where('pubkey', '=', pubkey)
+      .execute();
+  } else {
+    await kysely.insertInto('author_stats')
+      .values({
+        ...empty,
+        ...stats,
+      })
+      .execute();
+  }
+}
+
+/** Retrieve the event stats by the event ID, then call the callback to update it. */
+export async function updateEventStats(
+  kysely: Kysely<DittoTables>,
+  eventId: string,
+  fn: (prev: DittoTables['event_stats']) => UpdateObject<DittoTables, 'event_stats'>,
+): Promise<void> {
+  const empty = {
+    event_id: eventId,
+    replies_count: 0,
+    reposts_count: 0,
+    reactions_count: 0,
+  };
+
+  const prev = await kysely
+    .selectFrom('event_stats')
+    .selectAll()
+    .where('event_id', '=', eventId)
+    .executeTakeFirst();
+
+  const stats = fn(prev ?? empty);
+
+  if (prev) {
+    await kysely.updateTable('event_stats')
+      .set(stats)
+      .where('event_id', '=', eventId)
+      .execute();
+  } else {
+    await kysely.insertInto('event_stats')
+      .values({
+        ...empty,
+        ...stats,
+      })
+      .execute();
+  }
+}