diff --git a/Lib/test/test_sys.py b/Lib/test/test_sys.py
index de13f22..e6c17a2 100644
--- a/Lib/test/test_sys.py
+++ b/Lib/test/test_sys.py
@@ -1,9 +1,12 @@
 # -*- coding: iso-8859-1 -*-
 import unittest, test.test_support
 from test.script_helper import assert_python_ok, assert_python_failure
-import sys, os, cStringIO
-import struct
+import cStringIO
+import gc
 import operator
+import os
+import struct
+import sys
 
 class SysModuleTest(unittest.TestCase):
 
@@ -754,6 +757,32 @@
         check(xrange(1), size('3l'))
         check(xrange(66000), size('3l'))
 
+    def check_slots(self, obj, base, extra):
+        expected = sys.getsizeof(base) + struct.calcsize(extra)
+        if gc.is_tracked(obj) and not gc.is_tracked(base):
+            expected += struct.calcsize('3P')  # PyGC_Head
+        self.assertEqual(sys.getsizeof(obj), expected)
+
+    def test_slots(self):
+        # check all subclassable types defined in Objects/ that allow
+        # non-empty __slots__
+        check = self.check_slots
+        class BA(bytearray):
+            __slots__ = 'a', 'b', 'c'
+        check(BA(), bytearray(), '3P')
+        class D(dict):
+            __slots__ = 'a', 'b', 'c'
+        check(D(x=[]), {'x': []}, '3P')
+        class L(list):
+            __slots__ = 'a', 'b', 'c'
+        check(L(), [], '3P')
+        class S(set):
+            __slots__ = 'a', 'b', 'c'
+        check(S(), set(), '3P')
+        class FS(frozenset):
+            __slots__ = 'a', 'b', 'c'
+        check(FS(), frozenset(), '3P')
+
     def test_pythontypes(self):
         # check all types defined in Python/
         size = test.test_support.calcobjsize
diff --git a/Misc/NEWS b/Misc/NEWS
index adb4a15..315ebf0 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -10,6 +10,10 @@
 Core and Builtins
 -----------------
 
+- Issue #25421: __sizeof__ methods of builtin types now use dynamic basic size.
+  This allows sys.getsize() to work correctly with their subclasses with
+  __slots__ defined.
+
 - Issue #19543: Added Py3k warning for decoding unicode.
 
 - Issue #24097: Fixed crash in object.__reduce__() if slot name is freed inside
diff --git a/Modules/_collectionsmodule.c b/Modules/_collectionsmodule.c
index aaac660..d4cc960 100644
--- a/Modules/_collectionsmodule.c
+++ b/Modules/_collectionsmodule.c
@@ -1103,7 +1103,7 @@
     Py_ssize_t res;
     Py_ssize_t blocks;
 
-    res = sizeof(dequeobject);
+    res = _PyObject_SIZE(Py_TYPE(deque));
     blocks = (deque->leftindex + deque->len + BLOCKLEN - 1) / BLOCKLEN;
     assert(deque->leftindex + deque->len - 1 ==
            (blocks - 1) * BLOCKLEN + deque->rightindex);
diff --git a/Modules/_io/bufferedio.c b/Modules/_io/bufferedio.c
index b4632ed..dd518d7 100644
--- a/Modules/_io/bufferedio.c
+++ b/Modules/_io/bufferedio.c
@@ -391,7 +391,7 @@
 {
     Py_ssize_t res;
 
-    res = sizeof(buffered);
+    res = _PyObject_SIZE(Py_TYPE(self));
     if (self->buffer)
         res += self->buffer_size;
     return PyLong_FromSsize_t(res);
diff --git a/Modules/_io/bytesio.c b/Modules/_io/bytesio.c
index f643e91..82f3851 100644
--- a/Modules/_io/bytesio.c
+++ b/Modules/_io/bytesio.c
@@ -809,7 +809,7 @@
 {
     Py_ssize_t res;
 
-    res = sizeof(bytesio);
+    res = _PyObject_SIZE(Py_TYPE(self));
     if (self->buf)
         res += self->buf_size;
     return PyLong_FromSsize_t(res);
diff --git a/Modules/_struct.c b/Modules/_struct.c
index bca7a2e..02dd7d3 100644
--- a/Modules/_struct.c
+++ b/Modules/_struct.c
@@ -1737,7 +1737,7 @@
 {
     Py_ssize_t size;
 
-    size = sizeof(PyStructObject) + sizeof(formatcode) * (self->s_len + 1);
+    size = _PyObject_SIZE(Py_TYPE(self)) + sizeof(formatcode) * (self->s_len + 1);
     return PyLong_FromSsize_t(size);
 }
 
diff --git a/Modules/arraymodule.c b/Modules/arraymodule.c
index 1d1f0d3..f147115 100644
--- a/Modules/arraymodule.c
+++ b/Modules/arraymodule.c
@@ -1541,7 +1541,7 @@
 array_sizeof(arrayobject *self, PyObject *unused)
 {
     Py_ssize_t res;
-    res = sizeof(arrayobject) + self->allocated * self->ob_descr->itemsize;
+    res = _PyObject_SIZE(Py_TYPE(self)) + self->allocated * self->ob_descr->itemsize;
     return PyLong_FromSsize_t(res);
 }
 
diff --git a/Modules/mmapmodule.c b/Modules/mmapmodule.c
index 8fdf7f7..1ebccdf 100644
--- a/Modules/mmapmodule.c
+++ b/Modules/mmapmodule.c
@@ -655,7 +655,7 @@
 {
     Py_ssize_t res;
 
-    res = sizeof(mmap_object);
+    res = _PyObject_SIZE(Py_TYPE(self));
     if (self->tagname)
         res += strlen(self->tagname) + 1;
     return PyLong_FromSsize_t(res);
diff --git a/Modules/parsermodule.c b/Modules/parsermodule.c
index eb2d600..2434c67 100644
--- a/Modules/parsermodule.c
+++ b/Modules/parsermodule.c
@@ -720,7 +720,7 @@
 {
     Py_ssize_t res;
 
-    res = sizeof(PyST_Object) + _PyNode_SizeOf(st->st_node);
+    res = _PyObject_SIZE(Py_TYPE(st)) + _PyNode_SizeOf(st->st_node);
     return PyLong_FromSsize_t(res);
 }
 
diff --git a/Objects/bytearrayobject.c b/Objects/bytearrayobject.c
index 5276da5..1fdd916 100644
--- a/Objects/bytearrayobject.c
+++ b/Objects/bytearrayobject.c
@@ -2780,7 +2780,7 @@
 {
     Py_ssize_t res;
 
-    res = sizeof(PyByteArrayObject) + self->ob_alloc * sizeof(char);
+    res = _PyObject_SIZE(Py_TYPE(self)) + self->ob_alloc * sizeof(char);
     return PyInt_FromSsize_t(res);
 }
 
diff --git a/Objects/dictobject.c b/Objects/dictobject.c
index 50afa3f..b281948 100644
--- a/Objects/dictobject.c
+++ b/Objects/dictobject.c
@@ -2152,7 +2152,7 @@
 {
     Py_ssize_t res;
 
-    res = sizeof(PyDictObject);
+    res = _PyObject_SIZE(Py_TYPE(mp));
     if (mp->ma_table != mp->ma_smalltable)
         res = res + (mp->ma_mask + 1) * sizeof(PyDictEntry);
     return PyInt_FromSsize_t(res);
diff --git a/Objects/listobject.c b/Objects/listobject.c
index 1f43ba2..27365b6 100644
--- a/Objects/listobject.c
+++ b/Objects/listobject.c
@@ -2469,7 +2469,7 @@
 {
     Py_ssize_t res;
 
-    res = sizeof(PyListObject) + self->allocated * sizeof(void*);
+    res = _PyObject_SIZE(Py_TYPE(self)) + self->allocated * sizeof(void*);
     return PyInt_FromSsize_t(res);
 }
 
diff --git a/Objects/setobject.c b/Objects/setobject.c
index 8a58d65..a9ebbec 100644
--- a/Objects/setobject.c
+++ b/Objects/setobject.c
@@ -1985,7 +1985,7 @@
 {
     Py_ssize_t res;
 
-    res = sizeof(PySetObject);
+    res = _PyObject_SIZE(Py_TYPE(so));
     if (so->table != so->smalltable)
         res = res + (so->mask + 1) * sizeof(setentry);
     return PyInt_FromSsize_t(res);