diff --git a/AK/Base64.cpp b/AK/Base64.cpp
index 5df2eebac99..906a608569a 100644
--- a/AK/Base64.cpp
+++ b/AK/Base64.cpp
@@ -96,7 +96,7 @@ ByteBuffer decode_base64(const StringView& input)
 String encode_base64(ReadonlyBytes input)
 {
     constexpr auto alphabet = make_alphabet();
-    StringBuilder output(calculate_base64_decoded_length(input));
+    StringBuilder output(calculate_base64_encoded_length(input));
 
     auto get = [&](const size_t offset, bool* need_padding = nullptr) -> u8 {
         if (offset >= input.size()) {