diff --git a/spectator/writer/line_buffer.py b/spectator/writer/line_buffer.py
index 35d1599..8f13e0a 100644
--- a/spectator/writer/line_buffer.py
+++ b/spectator/writer/line_buffer.py
@@ -2,7 +2,7 @@
 
 
 class LineBuffer:
-    """Line buffer for Writers."""
+    """Line buffer for Writers, sized in UTF-8 bytes including newline separators."""
 
     def __init__(self, max_size: int) -> None:
         self._size: int = 0
@@ -14,8 +14,11 @@ def __len__(self) -> int:
 
     def append(self, line: str) -> bool:
         """Return value indicates whether the buffer should be flushed."""
+        line_size = len(line) if line.isascii() else len(line.encode("utf-8"))
+        if self._lines:
+            self._size += 1  # Newline inserted between lines by flush().
         self._lines.append(line)
-        self._size += len(line)
+        self._size += line_size
         return self._size >= self._max_size
 
     def flush(self) -> str:
diff --git a/tests/writer/test_line_buffer.py b/tests/writer/test_line_buffer.py
index 98373c1..6e7228f 100644
--- a/tests/writer/test_line_buffer.py
+++ b/tests/writer/test_line_buffer.py
@@ -14,7 +14,7 @@ def test_all_methods(self):
         self.assertFalse(should_flush)
 
         should_flush = buffer.append("e")
-        self.assertEqual(5, len(buffer))
+        self.assertEqual(6, len(buffer))
         self.assertTrue(should_flush)
 
         self.assertEqual("abcd\ne", buffer.flush())
@@ -26,3 +26,39 @@ def test_all_methods(self):
 
         self.assertEqual("abcdefghij", buffer.flush())
         self.assertEqual(0, len(buffer))
+
+    def test_flush_threshold_includes_newline_separators(self):
+        buffer = LineBuffer(7)
+        self.assertFalse(buffer.append("abc"))
+        self.assertTrue(buffer.append("def"))
+        self.assertEqual(7, len(buffer))
+        self.assertEqual("abc\ndef", buffer.flush())
+
+    def test_size_matches_utf8_payload(self):
+        buffer = LineBuffer(6)
+        self.assertFalse(buffer.append("é"))
+        self.assertEqual(2, len(buffer))
+        self.assertTrue(buffer.append("€"))
+        self.assertEqual(6, len(buffer))
+        payload = buffer.flush()
+        self.assertEqual(6, len(payload.encode("utf-8")))
+        self.assertEqual(0, len(buffer))
+        self.assertFalse(buffer.append("abc"))
+        self.assertEqual(3, len(buffer))
+
+    def test_empty_lines_include_separators(self):
+        buffer = LineBuffer(1)
+        self.assertFalse(buffer.append(""))
+        self.assertTrue(buffer.append(""))
+        self.assertEqual(1, len(buffer))
+        self.assertEqual("\n", buffer.flush())
+
+    def test_invalid_utf8_leaves_buffer_unchanged(self):
+        buffer = LineBuffer(10)
+        self.assertFalse(buffer.append("foo"))
+        with self.assertRaises(UnicodeEncodeError):
+            buffer.append("\ud800")
+        self.assertEqual(3, len(buffer))
+        self.assertFalse(buffer.append("bar"))
+        self.assertEqual(7, len(buffer))
+        self.assertEqual("foo\nbar", buffer.flush())
diff --git a/tests/writer/test_socket_writer_udp.py b/tests/writer/test_socket_writer_udp.py
index 36523bd..278955f 100644
--- a/tests/writer/test_socket_writer_udp.py
+++ b/tests/writer/test_socket_writer_udp.py
@@ -1,6 +1,7 @@
 import threading
 import unittest
 from contextlib import closing
+from unittest.mock import patch
 
 from spectator import Config, SocketWriter
 from ..udp_server import UdpServer
@@ -34,10 +35,15 @@ def test_udp_with_buffer_flushes_on_close(self) -> None:
     def test_udp_close_closes_socket_on_encode_error(self) -> None:
         with closing(UdpServer()) as server:
             w = SocketWriter(Config(server.address(), buffer_size=1024))
-            w.write("\ud800")
+            w.write("foo")
             sock = w._sock  # pylint: disable=protected-access
             assert sock is not None
-            self.assertRaises(UnicodeEncodeError, w.close)
+            # Invalid text is now rejected by write(); simulate an encoding failure
+            # during close to retain coverage of the socket cleanup guarantee.
+            buffer = w._buffer  # pylint: disable=protected-access
+            assert buffer is not None
+            with patch.object(buffer, "flush", return_value="\ud800"):
+                self.assertRaises(UnicodeEncodeError, w.close)
             self.assertEqual(-1, sock.fileno())
             w.write("foo")
             self.assertIsNone(w._sock)  # pylint: disable=protected-access