diff --git a/Tests/test_file_icns.py b/Tests/test_file_icns.py
index 47de38d06..3afbbeaac 100644
--- a/Tests/test_file_icns.py
+++ b/Tests/test_file_icns.py
@@ -1,8 +1,9 @@
 import io
+import os
 
 import pytest
 
-from PIL import IcnsImagePlugin, Image, features
+from PIL import IcnsImagePlugin, Image, _binary, features
 
 from .helper import assert_image_equal, assert_image_similar_tofile
 
@@ -38,6 +39,11 @@ def test_save(tmp_path):
         assert reread.size == (1024, 1024)
         assert reread.format == "ICNS"
 
+    file_length = os.path.getsize(temp_file)
+    with open(temp_file, "rb") as fp:
+        fp.seek(4)
+        assert _binary.i32be(fp.read(4)) == file_length
+
 
 def test_save_append_images(tmp_path):
     temp_file = str(tmp_path / "temp.icns")
diff --git a/src/PIL/IcnsImagePlugin.py b/src/PIL/IcnsImagePlugin.py
index d30eaf90f..6412d1cfb 100644
--- a/src/PIL/IcnsImagePlugin.py
+++ b/src/PIL/IcnsImagePlugin.py
@@ -337,23 +337,28 @@ def _save(im, fp, filename):
     entries = []
     for type, size in sizes.items():
         stream = size_streams[size]
-        entries.append({"type": type, "size": len(stream), "stream": stream})
+        entries.append(
+            {"type": type, "size": HEADERSIZE + len(stream), "stream": stream}
+        )
 
     # Header
     fp.write(MAGIC)
-    fp.write(struct.pack(">i", sum(entry["size"] for entry in entries)))
+    file_length = HEADERSIZE  # Header
+    file_length += HEADERSIZE + 8 * len(entries)  # TOC
+    file_length += sum(entry["size"] for entry in entries)
+    fp.write(struct.pack(">i", file_length))
 
     # TOC
     fp.write(b"TOC ")
     fp.write(struct.pack(">i", HEADERSIZE + len(entries) * HEADERSIZE))
     for entry in entries:
         fp.write(entry["type"])
-        fp.write(struct.pack(">i", HEADERSIZE + entry["size"]))
+        fp.write(struct.pack(">i", entry["size"]))
 
     # Data
     for entry in entries:
         fp.write(entry["type"])
-        fp.write(struct.pack(">i", HEADERSIZE + entry["size"]))
+        fp.write(struct.pack(">i", entry["size"]))
         fp.write(entry["stream"])
 
     if hasattr(fp, "flush"):