diff --git a/core/include/fxcrt/fx_string.h b/core/include/fxcrt/fx_string.h
index a7cf2e1..c98a77c 100644
--- a/core/include/fxcrt/fx_string.h
+++ b/core/include/fxcrt/fx_string.h
@@ -7,6 +7,7 @@
 #ifndef _FX_STRING_H_
 #define _FX_STRING_H_
 
+#include <stdint.h>  // For intptr_t.
 #include <algorithm>
 
 #include "fx_memory.h"
@@ -168,14 +169,17 @@
 typedef const CFX_ByteStringC& FX_BSTR;
 #define FX_BSTRC(str) CFX_ByteStringC(str, sizeof str-1)
 #define FXBSTR_ID(c1, c2, c3, c4) ((c1 << 24) | (c2 << 16) | (c3 << 8) | (c4))
+
+// To ensure ref counts do not overflow, consider the worst possible case:
+// the entire address space contains nothing but pointers to this object.
+// Since the count increments with each new pointer, the largest value is
+// the number of pointers that can fit into the address space. The size of
+// the address space itself is a good upper bound on it; we need not go
+// larger.
 struct CFX_StringData {
-
-    long		m_nRefs;
-
+    intptr_t	m_nRefs;  // Would prefer ssize_t, but no windows support.
     FX_STRSIZE	m_nDataLength;
-
     FX_STRSIZE	m_nAllocLength;
-
     FX_CHAR		m_String[1];
 };
 class CFX_ByteString 
@@ -586,13 +590,9 @@
 typedef const CFX_WideStringC&	FX_WSTR;
 #define FX_WSTRC(wstr) CFX_WideStringC(wstr, FX_ArraySize(wstr) - 1)
 struct CFX_StringDataW {
-
-    long		m_nRefs;
-
+    intptr_t	m_nRefs;  // Would prefer ssize_t, but no windows support.
     FX_STRSIZE	m_nDataLength;
-
     FX_STRSIZE	m_nAllocLength;
-
     FX_WCHAR	m_String[1];
 };
 class CFX_WideString 
diff --git a/core/src/fxcrt/fx_basic_bstring.cpp b/core/src/fxcrt/fx_basic_bstring.cpp
index 2c8f7a7..9cf084c 100644
--- a/core/src/fxcrt/fx_basic_bstring.cpp
+++ b/core/src/fxcrt/fx_basic_bstring.cpp
@@ -4,6 +4,8 @@
 
 // Original code copyright 2014 Foxit Software Inc. http://www.foxitsoftware.com
 
+#include <stddef.h>  // For offsetof().
+
 #include "../../include/fxcrt/fx_basic.h"
 #include "../../../third_party/base/numerics/safe_math.h"
 
@@ -53,7 +55,9 @@
         return NULL;
     }
 
-    int overhead = sizeof(long) * 3 + 1;  // 3 longs in header plus 1 for NUL.
+    // Fixed portion of header plus a NUL char not included in m_nAllocLength.
+    // sizeof(FX_CHAR) is always 1, used for consistency with CFX_Widestring.
+    int overhead = offsetof(CFX_StringData, m_String) + sizeof(FX_CHAR);
     pdfium::base::CheckedNumeric<int> nSize = nLen;
     nSize += overhead;
 
diff --git a/core/src/fxcrt/fx_basic_wstring.cpp b/core/src/fxcrt/fx_basic_wstring.cpp
index 42a7ad7..742f249 100644
--- a/core/src/fxcrt/fx_basic_wstring.cpp
+++ b/core/src/fxcrt/fx_basic_wstring.cpp
@@ -4,6 +4,8 @@
 
 // Original code copyright 2014 Foxit Software Inc. http://www.foxitsoftware.com
 
+#include <stddef.h>  // For offsetof().
+
 #include "../../include/fxcrt/fx_basic.h"
 #include "../../../third_party/base/numerics/safe_math.h"
 
@@ -15,7 +17,8 @@
         return NULL;
     }
 
-    int overhead = 3 * sizeof(long) + sizeof(FX_WCHAR);  // +WCHAR is for NUL.
+    // Fixed portion of header plus a NUL wide char not in m_nAllocLength.
+    int overhead = offsetof(CFX_StringDataW, m_String) + sizeof(FX_WCHAR);
     pdfium::base::CheckedNumeric<int> iSize = nLen;
     iSize *= sizeof(FX_WCHAR);
     iSize += overhead;