diff --git a/CHANGES/11355.misc.rst b/CHANGES/11355.misc.rst
new file mode 100644
index 00000000000..b7d1b063a50
--- /dev/null
+++ b/CHANGES/11355.misc.rst
@@ -0,0 +1,2 @@
+Improved ``ContentLengthError`` exception messages to include both expected and received byte counts. This enhancement provides better diagnostics when debugging response body size mismatches
+-- by :user:`bdraco`.
diff --git a/aiohttp/_http_parser.pyx b/aiohttp/_http_parser.pyx
index f5015b297b0..00e65b82d8d 100644
--- a/aiohttp/_http_parser.pyx
+++ b/aiohttp/_http_parser.pyx
@@ -282,6 +282,9 @@ cdef class HttpParser:
         bytes _raw_value
         bint      _has_value
 
+        size_t _content_length_received  # Track bytes received for content-length responses
+        size_t _original_content_length  # Store original content length
+
         object _protocol
         object _loop
         object _timer
@@ -340,6 +343,8 @@ cdef class HttpParser:
         cparser.llhttp_init(self._cparser, mode, self._csettings)
         self._cparser.data = <void*>self
         self._cparser.content_length = 0
+        self._content_length_received = 0
+        self._original_content_length = 0
 
         self._protocol = protocol
         self._loop = loop
@@ -417,6 +422,10 @@ cdef class HttpParser:
         upgrade = self._cparser.upgrade
         chunked = self._cparser.flags & cparser.F_CHUNKED
 
+        # Store original content length for error reporting
+        if self._cparser.flags & cparser.F_CONTENT_LENGTH:
+            self._original_content_length = self._cparser.content_length
+
         raw_headers = tuple(self._raw_headers)
         headers = CIMultiDictProxy(CIMultiDict(self._headers))
 
@@ -508,8 +517,13 @@ cdef class HttpParser:
                 raise TransferEncodingError(
                     "Not enough data to satisfy transfer length header.")
             elif self._cparser.flags & cparser.F_CONTENT_LENGTH:
+                # Get expected content length and received bytes
+                expected = self._original_content_length
+                received = self._content_length_received
                 raise ContentLengthError(
-                    "Not enough data to satisfy content length header.")
+                    f"Not enough data to satisfy content length header. "
+                    f"Expected {expected} bytes, got {received} bytes."
+                )
             elif cparser.llhttp_get_errno(self._cparser) != cparser.HPE_OK:
                 desc = cparser.llhttp_get_error_reason(self._cparser)
                 raise PayloadEncodingError(desc.decode('latin-1'))
@@ -668,6 +682,8 @@ cdef int cb_on_message_begin(cparser.llhttp_t* parser) except -1:
     pyparser._started = True
     pyparser._headers = []
     pyparser._raw_headers = []
+    pyparser._content_length_received = 0  # Reset counter for new message
+    pyparser._original_content_length = 0  # Reset original content length
     PyByteArray_Resize(pyparser._buf, 0)
     pyparser._path = None
     pyparser._reason = None
@@ -759,6 +775,11 @@ cdef int cb_on_body(cparser.llhttp_t* parser,
                     const char *at, size_t length) except -1:
     cdef HttpParser pyparser = <HttpParser>parser.data
     cdef bytes body = at[:length]
+
+    # Track bytes received for content-length responses
+    if parser.flags & cparser.F_CONTENT_LENGTH:
+        pyparser._content_length_received += length
+
     try:
         pyparser._payload.feed_data(body)
     except BaseException as underlying_exc:
diff --git a/aiohttp/http_parser.py b/aiohttp/http_parser.py
index 84b59afc486..ae2e109c0a4 100644
--- a/aiohttp/http_parser.py
+++ b/aiohttp/http_parser.py
@@ -768,6 +768,7 @@ def __init__(
         headers_parser: HeadersParser,
     ) -> None:
         self._length = 0
+        self._original_length = length  # Store original expected length
         self._type = ParseState.PARSE_UNTIL_EOF
         self._chunk = ChunkState.PARSE_CHUNKED_SIZE
         self._chunk_size = 0
@@ -807,8 +808,13 @@ def feed_eof(self) -> None:
         if self._type == ParseState.PARSE_UNTIL_EOF:
             self.payload.feed_eof()
         elif self._type == ParseState.PARSE_LENGTH:
+            # Calculate how much we received vs expected
+            expected = self._original_length if self._original_length is not None else 0
+            # self._length is decremented as data is received, so received = expected - remaining
+            received = expected - self._length if self._length <= expected else 0
             raise ContentLengthError(
-                "Not enough data to satisfy content length header."
+                f"Not enough data to satisfy content length header. "
+                f"Expected {expected} bytes, got {received} bytes."
             )
         elif self._type == ParseState.PARSE_CHUNKED:
             raise TransferEncodingError(
diff --git a/tests/test_http_parser.py b/tests/test_http_parser.py
index 69e1fd7d4cc..968d122d581 100644
--- a/tests/test_http_parser.py
+++ b/tests/test_http_parser.py
@@ -1653,6 +1653,52 @@ async def test_parse_length_payload_eof(self, protocol: BaseProtocol) -> None:
         with pytest.raises(http_exceptions.ContentLengthError):
             p.feed_eof()
 
+    async def test_parse_length_payload_eof_error_message(
+        self, protocol: BaseProtocol
+    ) -> None:
+        """Test that ContentLengthError includes expected vs received bytes."""
+        out = aiohttp.StreamReader(protocol, 2**16, loop=asyncio.get_running_loop())
+
+        # Expect 10 bytes, but only send 3
+        p = HttpPayloadParser(out, length=10, headers_parser=HeadersParser())
+        p.feed_data(b"abc")
+
+        with pytest.raises(
+            http_exceptions.ContentLengthError, match=r"Expected 10 bytes, got 3 bytes"
+        ):
+            p.feed_eof()
+
+    async def test_parse_length_payload_eof_no_data(
+        self, protocol: BaseProtocol
+    ) -> None:
+        """Test ContentLengthError when no data is received."""
+        out = aiohttp.StreamReader(protocol, 2**16, loop=asyncio.get_running_loop())
+
+        # Expect 20 bytes, but send nothing
+        p = HttpPayloadParser(out, length=20, headers_parser=HeadersParser())
+
+        with pytest.raises(
+            http_exceptions.ContentLengthError, match=r"Expected 20 bytes, got 0 bytes"
+        ):
+            p.feed_eof()
+
+    async def test_parse_length_payload_partial_data(
+        self, protocol: BaseProtocol
+    ) -> None:
+        """Test ContentLengthError with various amounts of partial data."""
+        out = aiohttp.StreamReader(protocol, 2**16, loop=asyncio.get_running_loop())
+
+        # Expect 100 bytes, but only send 45
+        p = HttpPayloadParser(out, length=100, headers_parser=HeadersParser())
+        p.feed_data(b"a" * 25)
+        p.feed_data(b"b" * 20)
+
+        with pytest.raises(
+            http_exceptions.ContentLengthError,
+            match=r"Expected 100 bytes, got 45 bytes",
+        ):
+            p.feed_eof()
+
     async def test_parse_chunked_payload_size_error(
         self, protocol: BaseProtocol
     ) -> None:
@@ -1936,3 +1982,71 @@ async def test_empty_body(self, protocol: BaseProtocol) -> None:
         dbuf.feed_eof()
 
         assert buf.at_eof()
+
+
+def test_response_parser_incomplete_body_error_message(
+    response: HttpResponseParser,
+) -> None:
+    """Test response parser error message for incomplete body."""
+    # Response expects 50 bytes
+    response.feed_data(b"HTTP/1.1 200 OK\r\nContent-Length: 50\r\n\r\n")
+    # Send only 15 bytes
+    response.feed_data(b"partial content")
+
+    with pytest.raises(
+        http_exceptions.ContentLengthError, match=r"Expected 50 bytes, got 15 bytes"
+    ):
+        response.feed_eof()
+
+
+def test_response_parser_no_body_error_message(response: HttpResponseParser) -> None:
+    """Test response parser error when no body is received."""
+    # Response expects 25 bytes
+    response.feed_data(b"HTTP/1.1 200 OK\r\nContent-Length: 25\r\n\r\n")
+    # Send no body data
+
+    with pytest.raises(
+        http_exceptions.ContentLengthError, match=r"Expected 25 bytes, got 0 bytes"
+    ):
+        response.feed_eof()
+
+
+def test_response_parser_partial_chunks_error_message(
+    response: HttpResponseParser,
+) -> None:
+    """Test error message when body is sent in multiple chunks."""
+    # Response expects 100 bytes
+    response.feed_data(b"HTTP/1.1 200 OK\r\nContent-Length: 100\r\n\r\n")
+    # Send data in chunks totaling 60 bytes
+    response.feed_data(b"a" * 20)
+    response.feed_data(b"b" * 20)
+    response.feed_data(b"c" * 20)
+
+    with pytest.raises(
+        http_exceptions.ContentLengthError, match=r"Expected 100 bytes, got 60 bytes"
+    ):
+        response.feed_eof()
+
+
+def test_request_parser_incomplete_body_error_message(
+    parser: HttpRequestParser,
+) -> None:
+    """Test request parser error message for incomplete body."""
+    # Request with Content-Length but incomplete body
+    parser.feed_data(b"POST /test HTTP/1.1\r\nContent-Length: 30\r\n\r\n")
+    # Send only 10 bytes
+    parser.feed_data(b"incomplete")
+
+    with pytest.raises(
+        http_exceptions.ContentLengthError, match=r"Expected 30 bytes, got 10 bytes"
+    ):
+        parser.feed_eof()
+
+
+def test_response_content_length_zero_no_error(response: HttpResponseParser) -> None:
+    """Test that Content-Length: 0 does not raise error on feed_eof."""
+    # Response with Content-Length: 0
+    response.feed_data(b"HTTP/1.1 200 OK\r\nContent-Length: 0\r\n\r\n")
+
+    # This should NOT raise an error
+    response.feed_eof()  # Should complete without exception