diff --git a/dlls/msvcrt/mbcs.c b/dlls/msvcrt/mbcs.c
index 58826304b87..024c9b56a93 100644
--- a/dlls/msvcrt/mbcs.c
+++ b/dlls/msvcrt/mbcs.c
@@ -356,7 +356,7 @@ unsigned char* CDECL _mbsninc(const unsigned char* str, MSVCRT_size_t num)
  */
 unsigned int CDECL _mbclen(const unsigned char* str)
 {
-  return MSVCRT_isleadbyte(*str) ? 2 : 1;
+  return _ismbblead(*str) ? 2 : 1;
 }
 
 /*********************************************************************
@@ -379,17 +379,19 @@ int CDECL MSVCRT_mblen(const char* str, MSVCRT_size_t size)
  */
 MSVCRT_size_t CDECL _mbslen(const unsigned char* str)
 {
-  if(MSVCRT___mb_cur_max > 1)
+  MSVCRT_size_t len = 0;
+  while(*str)
   {
-    MSVCRT_size_t len = 0;
-    while(*str)
+    if (_ismbblead(*str))
     {
-      str += MSVCRT_isleadbyte(*str) ? 2 : 1;
-      len++;
+      str++;
+      if (!*str)  /* count only full chars */
+        break;
     }
-    return len;
+    str++;
+    len++;
   }
-  return u_strlen(str); /* ASCII CP */
+  return len;
 }
 
 /*********************************************************************
diff --git a/dlls/msvcrt/tests/string.c b/dlls/msvcrt/tests/string.c
index 8b55cf3c89f..209f1fabbe5 100644
--- a/dlls/msvcrt/tests/string.c
+++ b/dlls/msvcrt/tests/string.c
@@ -166,6 +166,8 @@ static void test_mbcp(void)
     int mb_orig_max = __mb_cur_max;
     int curr_mbcp = _getmbcp();
     unsigned char *mbstring = (unsigned char *)"\xb0\xb1\xb2 \xb3\xb4 \xb5"; /* incorrect string */
+    unsigned char *mbstring2 = (unsigned char *)"\xb0\xb1\xb2\xb3Q\xb4\xb5"; /* correct string */
+    unsigned char *mbsonlylead = (unsigned char *)"\xb0";
 
     /* some two single-byte code pages*/
     test_codepage(1252);
@@ -186,12 +188,19 @@ static void test_mbcp(void)
     ok(_ismbbtrail('\xb0'), "\xa0 should be a trail byte\n");
     ok(_ismbbtrail(' ') == FALSE, "' ' should not be a trail byte\n");
 
-
     /* _mbsnextc */
     expect_eq(_mbsnextc(mbstring), 0xb0b1, int, "%x");
     expect_eq(_mbsnextc(&mbstring[2]), 0xb220, int, "%x");  /* lead + invalid tail */
     expect_eq(_mbsnextc(&mbstring[3]), 0x20, int, "%x");    /* single char */
 
+    /* _mbclen/_mbslen */
+    expect_eq(_mbclen(mbstring), 2, int, "%d");
+    expect_eq(_mbclen(&mbstring[2]), 2, int, "%d");
+    expect_eq(_mbclen(&mbstring[3]), 1, int, "%d");
+    expect_eq(_mbslen(mbstring2), 4, int, "%d");
+    expect_eq(_mbslen(mbsonlylead), 0, int, "%d");          /* lead + NUL not counted as character */
+    expect_eq(_mbslen(mbstring), 4, int, "%d");             /* lead + invalid trail counted */
+
     _setmbcp(curr_mbcp);
 }