diff --git a/dlls/iphlpapi/ifenum.c b/dlls/iphlpapi/ifenum.c
index aff88cd059e..d8eebca860d 100644
--- a/dlls/iphlpapi/ifenum.c
+++ b/dlls/iphlpapi/ifenum.c
@@ -211,13 +211,10 @@ InterfaceIndexTable *getInterfaceIndexTable(void)
 
   if (indexes) {
     struct if_nameindex *p;
-    DWORD size = sizeof(InterfaceIndexTable);
 
     for (p = indexes, numInterfaces = 0; p && p->if_name; p++)
       numInterfaces++;
-    if (numInterfaces > 1)
-      size += (numInterfaces - 1) * sizeof(DWORD);
-    ret = HeapAlloc(GetProcessHeap(), 0, size);
+    ret = HeapAlloc(GetProcessHeap(), 0, FIELD_OFFSET(InterfaceIndexTable, indexes[numInterfaces]));
     if (ret) {
       ret->numIndexes = 0;
       for (p = indexes; p && p->if_name; p++)
@@ -241,14 +238,11 @@ InterfaceIndexTable *getNonLoopbackInterfaceIndexTable(void)
 
     if (indexes) {
       struct if_nameindex *p;
-      DWORD size = sizeof(InterfaceIndexTable);
 
       for (p = indexes, numInterfaces = 0; p && p->if_name; p++)
         if (!isLoopbackInterface(fd, p->if_name))
           numInterfaces++;
-      if (numInterfaces > 1)
-        size += (numInterfaces - 1) * sizeof(DWORD);
-      ret = HeapAlloc(GetProcessHeap(), 0, size);
+      ret = HeapAlloc(GetProcessHeap(), 0, FIELD_OFFSET(InterfaceIndexTable, indexes[numInterfaces]));
       if (ret) {
         ret->numIndexes = 0;
         for (p = indexes; p && p->if_name; p++)
diff --git a/dlls/iphlpapi/iphlpapi_main.c b/dlls/iphlpapi/iphlpapi_main.c
index 16f138c18bf..9cffe063842 100644
--- a/dlls/iphlpapi/iphlpapi_main.c
+++ b/dlls/iphlpapi/iphlpapi_main.c
@@ -1505,10 +1505,8 @@ DWORD WINAPI GetIpAddrTable(PMIB_IPADDRTABLE pIpAddrTable, PULONG pdwSize, BOOL
     ret = getIPAddrTable(&table, GetProcessHeap(), 0);
     if (ret == NO_ERROR)
     {
-      ULONG size = sizeof(MIB_IPADDRTABLE);
+      ULONG size = FIELD_OFFSET(MIB_IPADDRTABLE, table[table->dwNumEntries]);
 
-      if (table->dwNumEntries > 1)
-        size += (table->dwNumEntries - 1) * sizeof(MIB_IPADDRROW);
       if (!pIpAddrTable || *pdwSize < size) {
         *pdwSize = size;
         ret = ERROR_INSUFFICIENT_BUFFER;