diff --git a/library/std/src/sys_common/wtf8.rs b/library/std/src/sys_common/wtf8.rs
index 7d4b0d52831..1bd3cfd2200 100644
--- a/library/std/src/sys_common/wtf8.rs
+++ b/library/std/src/sys_common/wtf8.rs
@@ -853,10 +853,11 @@ fn next(&mut self) -> Option<u16> {
     #[inline]
     fn size_hint(&self) -> (usize, Option<usize>) {
         let (low, high) = self.code_points.size_hint();
+        let ext = (self.extra != 0) as usize;
         // every code point gets either one u16 or two u16,
         // so this iterator is between 1 or 2 times as
         // long as the underlying iterator.
-        (low, high.and_then(|n| n.checked_mul(2)))
+        (low + ext, high.and_then(|n| n.checked_mul(2)).and_then(|n| n.checked_add(ext)))
     }
 }
 
diff --git a/library/std/src/sys_common/wtf8/tests.rs b/library/std/src/sys_common/wtf8/tests.rs
index cd9c6ee3250..1bafbaa6939 100644
--- a/library/std/src/sys_common/wtf8/tests.rs
+++ b/library/std/src/sys_common/wtf8/tests.rs
@@ -395,3 +395,15 @@ fn wtf8_encode_wide() {
         vec![0x61, 0xE9, 0x20, 0xD83D, 0xD83D, 0xDCA9]
     );
 }
+
+#[test]
+fn wtf8_encode_wide_size_hint() {
+    let string = Wtf8Buf::from_str("\u{12345}");
+    let mut iter = string.encode_wide();
+    assert_eq!((1, Some(8)), iter.size_hint());
+    iter.next().unwrap();
+    assert_eq!((1, Some(1)), iter.size_hint());
+    iter.next().unwrap();
+    assert_eq!((0, Some(0)), iter.size_hint());
+    assert!(iter.next().is_none());
+}