diff --git a/packages/backend/src/server/api/endpoints/i/update.ts b/packages/backend/src/server/api/endpoints/i/update.ts
index ad8f38703b..f35e395841 100644
--- a/packages/backend/src/server/api/endpoints/i/update.ts
+++ b/packages/backend/src/server/api/endpoints/i/update.ts
@@ -330,8 +330,13 @@ export default class extends Endpoint<typeof meta, typeof paramDef> { // eslint-
 			if (ps.chatScope !== undefined) updates.chatScope = ps.chatScope;
 
 			function checkMuteWordCount(mutedWords: (string[] | string)[], limit: number) {
-				// TODO: ちゃんと数える
-				const length = JSON.stringify(mutedWords).length;
+				const length = mutedWords.reduce((sum, word) => {
+					const wordLength = Array.isArray(word)
+						? word.reduce((l, w) => l + w.length, 0)
+						: word.length;
+					return sum + wordLength;
+				}, 0);
+
 				if (length > limit) {
 					throw new ApiError(meta.errors.tooManyMutedWords);
 				}