diff --git a/reactos/dll/win32/aclui/aclui.c b/reactos/dll/win32/aclui/aclui.c
index 729ac7ca539..4613a9dd76c 100644
--- a/reactos/dll/win32/aclui/aclui.c
+++ b/reactos/dll/win32/aclui/aclui.c
@@ -1047,7 +1047,8 @@ BuildDefaultPrincipalAce(IN PSECURITY_PAGE sp,
     WORD AceSize;
 
     SidLen = GetLengthSid(pSid);
-    AceSize = sizeof(ACCESS_ALLOWED_ACE) + (WORD)SidLen - sizeof(DWORD);
+    AceSize = FIELD_OFFSET(ACCESS_ALLOWED_ACE,
+                           SidStart) + (WORD)SidLen;
     Ace = HeapAlloc(GetProcessHeap(),
                     0,
                     AceSize);
diff --git a/reactos/dll/win32/advapi32/sec/sid.c b/reactos/dll/win32/advapi32/sec/sid.c
index f53e8cbabe1..c4658957642 100644
--- a/reactos/dll/win32/advapi32/sec/sid.c
+++ b/reactos/dll/win32/advapi32/sec/sid.c
@@ -184,7 +184,7 @@ static __inline BOOL set_ntstatus( NTSTATUS status )
     return !status;
 }
 
-#define	WINE_SIZE_OF_WORLD_ACCESS_ACL	(sizeof(ACL) + sizeof(ACCESS_ALLOWED_ACE) + sizeof(sidWorld) - sizeof(DWORD))
+#define	WINE_SIZE_OF_WORLD_ACCESS_ACL	(sizeof(ACL) + FIELD_OFFSET(ACCESS_ALLOWED_ACE, SidStart) + sizeof(sidWorld))
 
 
 /* some helper functions - taken from winehq cvs 20050916 */
@@ -428,7 +428,7 @@ static BOOL ParseStringAclToAcl(LPCWSTR StringAcl, LPDWORD lpdwFlags,
             goto lerr;
         StringAcl++;
 
-	length += sizeof(ACCESS_ALLOWED_ACE) - sizeof(DWORD) + sidlen;
+	length += FIELD_OFFSET(ACCESS_ALLOWED_ACE, SidStart) + sidlen;
     }
 
     *cBytes = length;
diff --git a/reactos/lib/rtl/sd.c b/reactos/lib/rtl/sd.c
index e5a382ecc31..efad4afd134 100644
--- a/reactos/lib/rtl/sd.c
+++ b/reactos/lib/rtl/sd.c
@@ -812,8 +812,6 @@ RtlSelfRelativeToAbsoluteSD2(IN OUT PSECURITY_DESCRIPTOR SelfRelativeSD,
                                 &pSacl,
                                 &SaclLength);
 
-    ASSERT(sizeof(SECURITY_DESCRIPTOR) > sizeof(SECURITY_DESCRIPTOR_RELATIVE));
-
     /* calculate the start and end of the data area, we simply just move the
        data by the difference between the size of the relative and absolute
        security descriptor structure */
diff --git a/reactos/lib/rtl/sid.c b/reactos/lib/rtl/sid.c
index d461c45ab1a..28fce407d64 100644
--- a/reactos/lib/rtl/sid.c
+++ b/reactos/lib/rtl/sid.c
@@ -42,8 +42,8 @@ RtlLengthRequiredSid(IN ULONG SubAuthorityCount)
 {
   PAGED_CODE_RTL();
 
-  return (sizeof(SID) - (ANYSIZE_ARRAY * sizeof(ULONG)) +
-          (SubAuthorityCount * sizeof(ULONG)));
+  return (ULONG)FIELD_OFFSET(SID,
+                             SubAuthority[SubAuthorityCount]);
 }
 
 
@@ -132,8 +132,8 @@ RtlLengthSid(IN PSID Sid_)
 
   PAGED_CODE_RTL();
 
-  return (sizeof(SID) - sizeof(Sid->SubAuthority) +
-          (Sid->SubAuthorityCount * sizeof(ULONG)));
+  return (ULONG)FIELD_OFFSET(SID,
+                             SubAuthority[Sid->SubAuthorityCount]);
 }
 
 
diff --git a/reactos/ntoskrnl/se/sid.c b/reactos/ntoskrnl/se/sid.c
index ec506a75443..cae927f6dd3 100644
--- a/reactos/ntoskrnl/se/sid.c
+++ b/reactos/ntoskrnl/se/sid.c
@@ -246,7 +246,8 @@ SepCaptureSid(IN PSID InputSid,
     _SEH_TRY
     {
       ProbeForRead(Sid,
-                   sizeof(*Sid) - sizeof(Sid->SubAuthority),
+                   FIELD_OFFSET(SID,
+                                SubAuthority),
                    sizeof(UCHAR));
       SidSize = RtlLengthRequiredSid(Sid->SubAuthorityCount);
       ProbeForRead(Sid,