diff --git a/src/PhoneNumbers/Formatters/PhoneNumberFormatter.cs b/src/PhoneNumbers/Formatters/PhoneNumberFormatter.cs
index b9ee9f42..66656a8a 100644
--- a/src/PhoneNumbers/Formatters/PhoneNumberFormatter.cs
+++ b/src/PhoneNumbers/Formatters/PhoneNumberFormatter.cs
@@ -44,7 +44,7 @@ protected static string FormatInternational(
             + 1 // add one for the + appended to the calling code
             + phoneNumber.Country.CallingCode.Length
             + (charBetweenCallingCodeAndNsn != Chars.Null ? 1 : 0)
-            + (wrapNdc ? 2 : 0)
+            + (wrapNdc ? 2 : 0) // add 2 for the parenthesis if wrapping the NDC
             + nsnMask.Length;
 
         Span<char> ar = stackalloc char[arSize];