diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
index ed8564c..5735350 100644
--- a/Lib/test/test_io.py
+++ b/Lib/test/test_io.py
@@ -802,6 +802,20 @@
             buf.raw = x
 
 
+class SizeofTest:
+
+    @support.cpython_only
+    def test_sizeof(self):
+        bufsize1 = 4096
+        bufsize2 = 8192
+        rawio = self.MockRawIO()
+        bufio = self.tp(rawio, buffer_size=bufsize1)
+        size = sys.getsizeof(bufio) - bufsize1
+        rawio = self.MockRawIO()
+        bufio = self.tp(rawio, buffer_size=bufsize2)
+        self.assertEqual(sys.getsizeof(bufio), size + bufsize2)
+
+
 class BufferedReaderTest(unittest.TestCase, CommonBufferedTests):
     read_mode = "rb"
 
@@ -999,7 +1013,7 @@
                              "failed for {}: {} != 0".format(n, rawio._extraneous_reads))
 
 
-class CBufferedReaderTest(BufferedReaderTest):
+class CBufferedReaderTest(BufferedReaderTest, SizeofTest):
     tp = io.BufferedReader
 
     def test_constructor(self):
@@ -1260,7 +1274,7 @@
             self.tp(self.MockRawIO(), 8, 12)
 
 
-class CBufferedWriterTest(BufferedWriterTest):
+class CBufferedWriterTest(BufferedWriterTest, SizeofTest):
     tp = io.BufferedWriter
 
     def test_constructor(self):
@@ -1650,7 +1664,7 @@
     # You can't construct a BufferedRandom over a non-seekable stream.
     test_unseekable = None
 
-class CBufferedRandomTest(BufferedRandomTest):
+class CBufferedRandomTest(BufferedRandomTest, SizeofTest):
     tp = io.BufferedRandom
 
     def test_constructor(self):
diff --git a/Misc/NEWS b/Misc/NEWS
index 185892f..8848e1a 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -334,6 +334,9 @@
 Library
 -------
 
+- Issue #15487: Add a __sizeof__ implementation for buffered I/O objects.
+  Patch by Serhiy Storchaka.
+
 - Issue #15187: Bugfix: remove temporary directories test_shutil was leaving
   behind.
 
diff --git a/Modules/_io/bufferedio.c b/Modules/_io/bufferedio.c
index f717817..67b7cc4 100644
--- a/Modules/_io/bufferedio.c
+++ b/Modules/_io/bufferedio.c
@@ -398,6 +398,17 @@
     Py_TYPE(self)->tp_free((PyObject *)self);
 }
 
+static PyObject *
+buffered_sizeof(buffered *self, void *unused)
+{
+    Py_ssize_t res;
+
+    res = sizeof(buffered);
+    if (self->buffer)
+        res += self->buffer_size;
+    return PyLong_FromSsize_t(res);
+}
+
 static int
 buffered_traverse(buffered *self, visitproc visit, void *arg)
 {
@@ -1699,6 +1710,7 @@
     {"seek", (PyCFunction)buffered_seek, METH_VARARGS},
     {"tell", (PyCFunction)buffered_tell, METH_NOARGS},
     {"truncate", (PyCFunction)buffered_truncate, METH_VARARGS},
+    {"__sizeof__", (PyCFunction)buffered_sizeof, METH_NOARGS},
     {NULL, NULL}
 };
 
@@ -2079,6 +2091,7 @@
     {"flush", (PyCFunction)buffered_flush, METH_NOARGS},
     {"seek", (PyCFunction)buffered_seek, METH_VARARGS},
     {"tell", (PyCFunction)buffered_tell, METH_NOARGS},
+    {"__sizeof__", (PyCFunction)buffered_sizeof, METH_NOARGS},
     {NULL, NULL}
 };
 
@@ -2470,6 +2483,7 @@
     {"readline", (PyCFunction)buffered_readline, METH_VARARGS},
     {"peek", (PyCFunction)buffered_peek, METH_VARARGS},
     {"write", (PyCFunction)bufferedwriter_write, METH_VARARGS},
+    {"__sizeof__", (PyCFunction)buffered_sizeof, METH_NOARGS},
     {NULL, NULL}
 };