diff --git a/docs/examples/simple.c b/docs/examples/simple.c
index 8579b0ba54cc95..47dd698b35cc8f 100644
--- a/docs/examples/simple.c
+++ b/docs/examples/simple.c
@@ -33,6 +33,34 @@ int main(void)
   CURL *curl;
   CURLcode res;
 
+#if defined(CURLDEBUG) && defined(_CRTDBG_MAP_ALLOC)
+#error "CURLDEBUG and _CRTDBG_MAP_ALLOC both defined, pick one"
+#elif defined(CURLDEBUG)
+  /* libcurl heap memory tracking. run tests/memanalyze.pl <logfile> */
+  {
+    const char *env = getenv("CURL_MEMDEBUG");
+    if(env)
+      curl_dbg_memdebug(env);
+  }
+#elif defined(_CRTDBG_MAP_ALLOC)
+#ifndef _INC_CRTDBG
+#error "missing crtdbg.h forced include: cl /FIcrtdbg.h /D_CRTDBG_MAP_ALLOC"
+#endif
+  /* Windows CRT heap memory tracking. on exit it prints heap memory leaks.
+     if libcurl was not built with _CRTDBG_MAP_ALLOC as well then there are no
+     filename or line numbers for libcurl allocations in the leak report. */
+  _CrtSetReportMode(_CRT_WARN, _CRTDBG_MODE_DEBUG | _CRTDBG_MODE_FILE);
+  _CrtSetReportFile(_CRT_WARN, _CRTDBG_FILE_STDERR);
+  _CrtSetDbgFlag(_CRTDBG_ALLOC_MEM_DF | _CRTDBG_LEAK_CHECK_DF);
+#endif
+
+#ifdef CURL_FORCEMEMLEAK
+  /* MEMORY LEAK TEST. See winbuild\README_HEAP_DEBUG.md. */
+  malloc(5);
+#endif
+
+  curl_global_init(CURL_GLOBAL_DEFAULT);
+
   curl = curl_easy_init();
   if(curl) {
     curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");
diff --git a/lib/curl_memory.h b/lib/curl_memory.h
index b8c46d79395f50..629d700c02a483 100644
--- a/lib/curl_memory.h
+++ b/lib/curl_memory.h
@@ -138,7 +138,15 @@ extern curl_calloc_callback Curl_ccalloc;
 extern curl_wcsdup_callback Curl_cwcsdup;
 #endif
 
-#ifndef CURLDEBUG
+#if defined(_CRTDBG_MAP_ALLOC) && defined(CURLDEBUG)
+#error "CURLDEBUG and _CRTDBG_MAP_ALLOC both defined, pick one"
+#endif
+
+#if defined(_CRTDBG_MAP_ALLOC) && !defined(_INC_CRTDBG)
+#error "missing crtdbg.h forced include: cl /FIcrtdbg.h /D_CRTDBG_MAP_ALLOC"
+#endif
+
+#if !defined(_CRTDBG_MAP_ALLOC) && !defined(CURLDEBUG)
 
 /*
  * libcurl's 'memory tracking' system defines strdup, malloc, calloc,
@@ -174,5 +182,5 @@ extern curl_wcsdup_callback Curl_cwcsdup;
 #  endif
 #endif
 
-#endif /* CURLDEBUG */
+#endif /* !_CRTDBG_MAP_ALLOC && !CURLDEBUG */
 #endif /* HEADER_CURL_MEMORY_H */
diff --git a/lib/easy.c b/lib/easy.c
index cf254ee555d785..405d4aabf6d470 100644
--- a/lib/easy.c
+++ b/lib/easy.c
@@ -143,9 +143,32 @@ static char *leakpointer;
  */
 static CURLcode global_init(long flags, bool memoryfuncs)
 {
+  /* If _CRTDBG_MAP_ALLOC (Windows CRT heap debugging) is defined then assume
+     we're using malloc -> _malloc_dbg, etc, from CRT's crtdbg.h instead of
+     libcurl's Curl_cmalloc etc.
+     If the user has set memory functions already, they do nothing. Permafail
+     to make it impossible to ignore. */
+#ifdef _CRTDBG_MAP_ALLOC
+  static bool permafail;
+  if(!memoryfuncs)
+    permafail = true;
+  if(permafail) {
+    fputs("Error: Use curl_global_init instead of curl_global_init_mem to "
+          "initialize libcurl. libcurl was built with _CRTDBG_MAP_ALLOC which "
+          "allows the Windows CRT to debug the heap if crtdbg.h was also "
+          "force-included. Custom memory functions can't be set.\n", stderr);
+    return CURLE_FAILED_INIT;
+  }
+#endif /* _CRTDBG_MAP_ALLOC */
+
   if(initialized++)
     return CURLE_OK;
 
+#ifdef CURL_FORCEMEMLEAK
+  /* MEMORY LEAK TEST. See winbuild\README_HEAP_DEBUG.md. */
+  malloc(7);
+#endif
+
   if(memoryfuncs) {
     /* Setup the default memory functions here (again) */
     Curl_cmalloc = (curl_malloc_callback)malloc;
diff --git a/src/tool_main.c b/src/tool_main.c
index 2f132e2d29c524..59a9de96b14860 100644
--- a/src/tool_main.c
+++ b/src/tool_main.c
@@ -261,8 +261,27 @@ int main(int argc, char *argv[])
   (void)signal(SIGPIPE, SIG_IGN);
 #endif
 
+#if defined(CURLDEBUG) && defined(_CRTDBG_MAP_ALLOC)
+#error "CURLDEBUG and _CRTDBG_MAP_ALLOC both defined, pick one"
+#elif defined(CURLDEBUG)
   /* Initialize memory tracking */
   memory_tracking_init();
+#elif defined(_CRTDBG_MAP_ALLOC)
+#ifndef _INC_CRTDBG
+#error "missing crtdbg.h forced include: cl /FIcrtdbg.h /D_CRTDBG_MAP_ALLOC"
+#endif
+  /* Windows CRT heap memory tracking. on exit it prints heap memory leaks.
+     if libcurl was not built with _CRTDBG_MAP_ALLOC as well then there are no
+     filename or line numbers for libcurl allocations in the leak report. */
+  _CrtSetReportMode(_CRT_WARN, _CRTDBG_MODE_DEBUG | _CRTDBG_MODE_FILE);
+  _CrtSetReportFile(_CRT_WARN, _CRTDBG_FILE_STDERR);
+  _CrtSetDbgFlag(_CRTDBG_ALLOC_MEM_DF | _CRTDBG_LEAK_CHECK_DF);
+#endif
+
+#ifdef CURL_FORCEMEMLEAK
+  /* MEMORY LEAK TEST. See winbuild\README_HEAP_DEBUG.md. */
+  malloc(5);
+#endif
 
   /* Initialize the curl library - do not call any libcurl functions before
      this point */
diff --git a/winbuild/README_HEAP_DEBUG.md b/winbuild/README_HEAP_DEBUG.md
new file mode 100644
index 00000000000000..d29fc826669c8e
--- /dev/null
+++ b/winbuild/README_HEAP_DEBUG.md
@@ -0,0 +1,73 @@
+Ref: https://github.com/curl/curl/issues/12327
+
+This document shows how to build libcurl with CURLDEBUG (libcurl heap memory
+tracking) or CRTDBG (Windows CRT heap memory tracking). The latter will track
+libcurl and the application.
+
+The example commands use `%CL_CRTDBG%` for CRTDBG, but can be changed to
+`%CL_CURLDEBUG%` for CURLDEBUG.
+
+To force a memory leak in both libcurl and the application (curl.exe or
+simple.exe) add `/DCURL_FORCEMEMLEAK` to the CL options. This is useful to see
+what the output of a memory leak looks like. It should cause 7 bytes to leak
+from libcurl and 5 bytes to leak from the application.
+
+Open a Visual Studio command prompt and build curl:
+~~~
+set "CURL_SRCPATH=x:\j\curl\curl"
+set "CL_CRTDBG=cl /DCURL_FORCEMEMLEAK /FC /FIcrtdbg.h /D_CRTDBG_MAP_ALLOC /D_CRT_SECURE_NO_WARNINGS /D_CRT_NONSTDC_NO_DEPRECATE"
+set "CL_CURLDEBUG=cl /DCURL_FORCEMEMLEAK /FC /DCURLDEBUG /DMEMDEBUG_LOG_SYNC"
+
+cd /d "%CURL_SRCPATH%\winbuild"
+if not exist ..\src\tool_hugehelp.c (..\buildconf.bat)
+nmake /f Makefile.vc mode=dll VC=10 DEBUG=yes clean
+nmake /f Makefile.vc mode=dll VC=10 DEBUG=yes CC="%CL_CRTDBG%"
+~~~
+
+The build output will show an object directory ending in -obj-curl. Remove that
+suffix and switch to the bin output subdir. For example if the object directory
+is `libcurl-vc10-x86-debug-dll-ipv6-sspi-schannel-obj-curl`:
+~~~
+cd ..\builds\libcurl-vc10-x86-debug-dll-ipv6-sspi-schannel\bin
+~~~
+
+If the curl tool (curl.exe) is not the application then build the application:
+~~~
+%CL_CRTDBG% /MDd /I..\include "%CURL_SRCPATH%\docs\examples\simple.c" /link /LIBPATH:..\lib libcurl_debug.lib
+~~~
+
+**(CRTDBG)** Run the application. Heap memory allocations in the CRT are logged
+internally. Heap memory leaks are printed to stderr on exit.
+~~~
+simple
+~~~
+
+**(CRTDBG)** Examine the output. If `CURL_FORCEMEMLEAK` was defined then the
+last lines of output should show a memory leak in libcurl and the application:
+~~~
+Detected memory leaks!
+Dumping objects ->
+x:\j\curl\curl\lib\easy.c(169) : {88} normal block at 0x006A8E38, 7 bytes long.
+ Data: <       > CD CD CD CD CD CD CD
+x:\j\curl\curl\docs\examples\simple.c(59) : {87} normal block at 0x006E8FB0, 5 bytes long.
+ Data: <     > CD CD CD CD CD
+Object dump complete.
+~~~
+
+**(CURLDEBUG)** Run the application. Heap memory allocations in libcurl are
+logged to curldbg.log. Heap memory leaks can be printed by running
+memanalyze.pl.
+~~~
+set "CURL_MEMDEBUG=curldbg.log"
+simple
+if not exist "%CURL_MEMDEBUG%" echo problem: missing "%CURL_MEMDEBUG%"
+..\..\..\tests\memanalyze.pl curldbg.log
+~~~
+
+**(CURLDEBUG)** Examine the output. If `CURL_FORCEMEMLEAK` was defined then
+memanalyze should show a memory leak in libcurl:
+~~~
+Leak detected: memory still allocated: 7 bytes
+At 4e7a58, there's 7 bytes.
+ allocated by x:\j\curl\curl\lib\easy.c:169
+~~~