diff --git a/aiohttp/multipart.py b/aiohttp/multipart.py
index e3ea335bf48..d87920634ab 100644
--- a/aiohttp/multipart.py
+++ b/aiohttp/multipart.py
@@ -247,7 +247,7 @@ def read_chunk(self, size=chunk_size):
             'Content-Length required for chunked read'
         chunk_size = min(size, self._length - self._read_bytes)
         chunk = yield from self._content.read(chunk_size)
-        self._read_bytes += chunk_size
+        self._read_bytes += len(chunk)
         if self._read_bytes == self._length:
             self._at_eof = True
             assert b'\r\n' == (yield from self._content.readline()), \
diff --git a/tests/test_multipart.py b/tests/test_multipart.py
index 2d0986d71b3..6860117c05a 100644
--- a/tests/test_multipart.py
+++ b/tests/test_multipart.py
@@ -74,6 +74,20 @@ def readline(self):
         return self.content.readline()
 
 
+class StreamWithShortenRead(Stream):
+
+    def __init__(self, content):
+        self._first = True
+        super().__init__(content)
+
+    @asyncio.coroutine
+    def read(self, size=None):
+        if size is not None and self._first:
+            self._first = False
+            size = size // 2
+        return super().read(size)
+
+
 class MultipartResponseWrapperTestCase(TestCase):
 
     def setUp(self):
@@ -148,6 +162,22 @@ def test_read_chunk_requires_content_length(self):
         with self.assertRaises(AssertionError):
             yield from obj.read_chunk()
 
+    def test_read_chunk_properly_counts_read_bytes(self):
+        expected = b'.' * 10
+        size = len(expected)
+        obj = aiohttp.multipart.BodyPartReader(
+            self.boundary, {'CONTENT-LENGTH': size},
+            StreamWithShortenRead(expected + b'\r\n--:--'))
+        result = bytearray()
+        while True:
+            chunk = yield from obj.read_chunk()
+            if not chunk:
+                break
+            result.extend(chunk)
+        self.assertEqual(size, len(result))
+        self.assertEqual(b'.' * size, result)
+        self.assertTrue(obj.at_eof())
+
     def test_read_does_reads_boundary(self):
         stream = Stream(b'Hello, world!\r\n--:')
         obj = aiohttp.multipart.BodyPartReader(