diff --git a/charset/internal.h b/charset/internal.h
index 32d0f6d5..7fd6628f 100644
--- a/charset/internal.h
+++ b/charset/internal.h
@@ -6,7 +6,13 @@
 #define charset_internal_h
 
 /* This invariably comes in handy */
-#define lenof(x) ( sizeof((x)) / sizeof(*(x)) )
+#ifndef lenof
+#if HAVE_COUNTOF
+#define lenof(x) _Countof(x)
+#else
+#define lenof(x) ( (sizeof((x))) / (sizeof(*(x))))
+#endif
+#endif
 
 /* This is an invalid Unicode value used to indicate an error. */
 #define ERROR 0xFFFFL                  /* Unicode value representing error */
diff --git a/cmake/cmake.h.in b/cmake/cmake.h.in
index 3d36b3d0..3b64d03f 100644
--- a/cmake/cmake.h.in
+++ b/cmake/cmake.h.in
@@ -5,6 +5,8 @@
 
 #cmakedefine NO_MULTIMON
 
+#cmakedefine01 HAVE_COUNTOF
+
 #cmakedefine01 HAVE_WINRESRC_H
 #cmakedefine01 HAVE_WINRES_H
 #cmakedefine01 HAVE_WIN_H
diff --git a/cmake/setup.cmake b/cmake/setup.cmake
index 1be448df..75af0eac 100644
--- a/cmake/setup.cmake
+++ b/cmake/setup.cmake
@@ -116,6 +116,12 @@ int main(int argc, char **argv) {
     free(p);
 }" HAVE_ALIGNED_ALLOC)
 
+check_c_source_compiles("
+int main(int argc, char **argv) {
+    int a[3];
+    return _Countof(a);
+}" HAVE_COUNTOF)
+
 if(PUTTY_DEBUG)
   add_compile_definitions(DEBUG)
 endif()
diff --git a/misc.h b/misc.h
index c64eb01b..67a1833b 100644
--- a/misc.h
+++ b/misc.h
@@ -353,8 +353,12 @@ const char *conf_id(int key);
 #endif
 
 #ifndef lenof
+#if HAVE_COUNTOF
+#define lenof(x) _Countof(x)
+#else
 #define lenof(x) ( (sizeof((x))) / (sizeof(*(x))))
 #endif
+#endif
 
 #ifndef min
 #define min(x,y) ( (x) < (y) ? (x) : (y) )
diff --git a/utils/tree234.c b/utils/tree234.c
index 004cfb8d..e7b3d4fc 100644
--- a/utils/tree234.c
+++ b/utils/tree234.c
@@ -1361,7 +1361,13 @@ int mycmp(void *av, void *bv)
     return strcmp(a, b);
 }
 
-#define lenof(x) ( sizeof((x)) / sizeof(*(x)) )
+#ifndef lenof
+#if HAVE_COUNTOF
+#define lenof(x) _Countof(x)
+#else
+#define lenof(x) ( (sizeof((x))) / (sizeof(*(x))))
+#endif
+#endif
 
 char *strings[] = {
     "a", "ab", "absque", "coram", "de",