diff --git a/dlls/gdi/font.c b/dlls/gdi/font.c
index f1a175f1268..ea0033a4498 100644
--- a/dlls/gdi/font.c
+++ b/dlls/gdi/font.c
@@ -2517,3 +2517,43 @@ done:
         memset(fs, 0, sizeof(FONTSIGNATURE));
     return ret;
 }
+
+/***********************************************************************
+ *           GdiGetCharDimensions    (GDI32.@)
+ *
+ * Gets the average width of the characters in the English alphabet.
+ *
+ * PARAMS
+ *  hdc    [I] Handle to the device context to measure on.
+ *  lptm   [O] Pointer to memory to store the text metrics into.
+ *  height [O] On exit, the maximum height of characters in the English alphabet.
+ *
+ * RETURNS
+ *  The average width of characters in the English alphabet.
+ *
+ * NOTES
+ *  This function is used by the dialog manager to get the size of a dialog
+ *  unit. It should also be used by other pieces of code that need to know
+ *  the size of a dialog unit in logical units without having access to the
+ *  window handle of the dialog.
+ *  Windows caches the font metrics from this function, but we don't and
+ *  there doesn't appear to be an immediate advantage to do so.
+ *
+ * SEE ALSO
+ *  GetTextExtentPointW, GetTextMetricsW, MapDialogRect.
+ */
+LONG WINAPI GdiGetCharDimensions(HDC hdc, LPTEXTMETRICW lptm, LONG *height)
+{
+    SIZE sz;
+    static const WCHAR alphabet[] = {
+        'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q',
+        'r','s','t','u','v','w','x','y','z','A','B','C','D','E','F','G','H',
+        'I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z',0};
+
+    if(lptm && !GetTextMetricsW(hdc, lptm)) return 0;
+
+    if(!GetTextExtentPointW(hdc, alphabet, 52, &sz)) return 0;
+
+    if (height) *height = sz.cy;
+    return (sz.cx / 26 + 1) / 2;
+}
diff --git a/dlls/gdi/gdi32.spec b/dlls/gdi/gdi32.spec
index a973892b50e..5c136e0129f 100644
--- a/dlls/gdi/gdi32.spec
+++ b/dlls/gdi/gdi32.spec
@@ -155,6 +155,7 @@
 @ stub GdiDllInitialize
 @ stdcall GdiFlush()
 @ stdcall GdiGetBatchLimit()
+@ stdcall GdiGetCharDimensions(ptr ptr ptr)
 @ stub GdiGetLocalBitmap
 @ stub GdiGetLocalBrush
 @ stub GdiGetLocalDC
diff --git a/dlls/gdi/tests/gdiobj.c b/dlls/gdi/tests/gdiobj.c
index 0b8e4788e9a..dc1f76d623f 100644
--- a/dlls/gdi/tests/gdiobj.c
+++ b/dlls/gdi/tests/gdiobj.c
@@ -266,9 +266,45 @@ static void test_gdi_objects(void)
     ReleaseDC(NULL, hdc);
 }
 
+static void test_GdiGetCharDimensions(void)
+{
+    HDC hdc;
+    TEXTMETRICW tm;
+    LONG ret;
+    SIZE size;
+    LONG avgwidth, height;
+    static const char szAlphabet[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
+    typedef LONG (WINAPI *fnGdiGetCharDimensions)(HDC hdc, LPTEXTMETRICW lptm, LONG *height);
+    fnGdiGetCharDimensions GdiGetCharDimensions = (fnGdiGetCharDimensions)GetProcAddress(LoadLibrary("gdi32"), "GdiGetCharDimensions");
+    if (!GdiGetCharDimensions) return;
+
+    hdc = CreateCompatibleDC(NULL);
+
+    GetTextExtentPoint(hdc, szAlphabet, strlen(szAlphabet), &size);
+    avgwidth = ((size.cx / 26) + 1) / 2;
+
+    ret = GdiGetCharDimensions(hdc, &tm, &height);
+    ok(ret == avgwidth, "GdiGetCharDimensions should have returned width of %ld instead of %ld\n", avgwidth, ret);
+    ok(height == tm.tmHeight, "GdiGetCharDimensions should have set height to %ld instead of %ld\n", tm.tmHeight, height);
+
+    ret = GdiGetCharDimensions(hdc, &tm, NULL);
+    ok(ret == avgwidth, "GdiGetCharDimensions should have returned width of %ld instead of %ld\n", avgwidth, ret);
+
+    ret = GdiGetCharDimensions(hdc, NULL, NULL);
+    ok(ret == avgwidth, "GdiGetCharDimensions should have returned width of %ld instead of %ld\n", avgwidth, ret);
+
+    height = 0;
+    ret = GdiGetCharDimensions(hdc, NULL, &height);
+    ok(ret == avgwidth, "GdiGetCharDimensions should have returned width of %ld instead of %ld\n", avgwidth, ret);
+    ok(height == size.cy, "GdiGetCharDimensions should have set height to %ld instead of %ld\n", size.cy, height);
+
+    DeleteDC(hdc);
+}
+
 START_TEST(gdiobj)
 {
     test_logfont();
     test_bitmap_font();
     test_gdi_objects();
+    test_GdiGetCharDimensions();
 }
diff --git a/include/wingdi.h b/include/wingdi.h
index cb429acbbb8..f4ae99d6f57 100644
--- a/include/wingdi.h
+++ b/include/wingdi.h
@@ -3335,6 +3335,7 @@ BOOL      WINAPI GdiAlphaBlend(HDC,int,int,int,int,HDC,int,int,int,int,BLENDFUNC
 BOOL      WINAPI GdiComment(HDC,UINT,const BYTE *);
 DEVMODEW * WINAPI GdiConvertToDevmodeW(const DEVMODEA *);
 BOOL      WINAPI GdiFlush(void);
+LONG      WINAPI GdiGetCharDimensions(HDC, LPTEXTMETRICW, LONG *);
 BOOL      WINAPI GdiGradientFill(HDC,PTRIVERTEX,ULONG,PVOID,ULONG,ULONG);
 BOOL      WINAPI GdiIsMetaFileDC(HDC);
 BOOL      WINAPI GdiIsMetaPrintDC(HDC);