diff --git a/include/ntfs-3g/acls.h b/include/ntfs-3g/acls.h
index a91eda7c..6506f78e 100644
--- a/include/ntfs-3g/acls.h
+++ b/include/ntfs-3g/acls.h
@@ -102,10 +102,11 @@
 #define ROOT_GROUP_UNMARK FILE_READ_EA	/* ACL granted to root as group */
 
 /*
- *		A type large enough to hold any SID
+ *		Maximum SID size and a type large enough to hold it
  */
 
-typedef char BIGSID[40];
+#define MAX_SID_SIZE (8 + SID_MAX_SUB_AUTHORITIES*4)
+typedef char BIGSID[MAX_SID_SIZE];
 
 /*
  *		Struct to hold the input mapping file
diff --git a/libntfs-3g/acls.c b/libntfs-3g/acls.c
index 1df88aa6..524cc83c 100644
--- a/libntfs-3g/acls.c
+++ b/libntfs-3g/acls.c
@@ -2790,10 +2790,10 @@ char *ntfs_build_descr_posix(struct MAPPING* const mapping[],
 	for (k=0; k<pxdesc->acccnt; k++) {
 		if ((pxdesc->acl.ace[k].tag == POSIX_ACL_USER)
 		    || (pxdesc->acl.ace[k].tag == POSIX_ACL_GROUP))
-			newattrsz += 3*40; /* fixme : maximum size */
+			newattrsz += 3*MAX_SID_SIZE;
 	}
 				/* account for default ACE's */
-	newattrsz += 2*40*pxdesc->defcnt;  /* fixme : maximum size */
+	newattrsz += 2*MAX_SID_SIZE*pxdesc->defcnt;
 	newattr = (char*)ntfs_malloc(newattrsz);
 	if (newattr) {
 		/* build the main header part */