diff --git a/example/asset/demo_extended.webp b/example/asset/demo_extended.webp
new file mode 100644
index 0000000..f8a83a1
Binary files /dev/null and b/example/asset/demo_extended.webp differ
diff --git a/library/lib/src/decoder/async/decoder.dart b/library/lib/src/decoder/async/decoder.dart
index 2719524..bb81ef6 100644
--- a/library/lib/src/decoder/async/decoder.dart
+++ b/library/lib/src/decoder/async/decoder.dart
@@ -1,3 +1,4 @@
+import 'package:collection/collection.dart';
 import 'package:image_size_getter/src/core/size.dart';
 
 import 'package:image_size_getter/src/entity/block_entity.dart';
diff --git a/library/lib/src/decoder/async/webp_decoder.dart b/library/lib/src/decoder/async/webp_decoder.dart
index e94a676..10b5099 100644
--- a/library/lib/src/decoder/async/webp_decoder.dart
+++ b/library/lib/src/decoder/async/webp_decoder.dart
@@ -5,15 +5,28 @@ class WebpDecoder extends AsyncImageDecoder {
 
   @override
   Future<Size> get size async {
-    final widthList = await input.getRange(0x1a, 0x1c);
-    final heightList = await input.getRange(0x1c, 0x1e);
-    final width = convertRadix16ToInt(widthList, reverse: true);
-    final height = convertRadix16ToInt(heightList, reverse: true);
-    return Size(width, height);
+    if (await _isExtendedFormat()) {
+      final widthList = await input.getRange(0x18, 0x1b);
+      final heightList = await input.getRange(0x1b, 0x1d);
+      final width = convertRadix16ToInt(widthList, reverse: true) + 1;
+      final height = convertRadix16ToInt(heightList, reverse: true) + 1;
+      return Size(width, height);
+    } else {
+      final widthList = await input.getRange(0x1a, 0x1c);
+      final heightList = await input.getRange(0x1c, 0x1e);
+      final width = convertRadix16ToInt(widthList, reverse: true);
+      final height = convertRadix16ToInt(heightList, reverse: true);
+      return Size(width, height);
+    }
   }
 
   Future<bool> isLossy() async {
     final v8Tag = await input.getRange(12, 16);
     return v8Tag[3] == 0x20;
   }
+
+  Future<bool> _isExtendedFormat() async {
+    final chunkHeader = await input.getRange(12, 16);
+    return ListEquality().equals(chunkHeader, "VP8X".codeUnits);
+  }
 }
diff --git a/library/lib/src/decoder/sync/decoder.dart b/library/lib/src/decoder/sync/decoder.dart
index caba1b3..617ba27 100644
--- a/library/lib/src/decoder/sync/decoder.dart
+++ b/library/lib/src/decoder/sync/decoder.dart
@@ -1,3 +1,4 @@
+import 'package:collection/collection.dart';
 import 'package:image_size_getter/src/core/size.dart';
 
 import 'package:image_size_getter/src/entity/block_entity.dart';
diff --git a/library/lib/src/decoder/sync/webp_decoder.dart b/library/lib/src/decoder/sync/webp_decoder.dart
index 8a7cbce..311b619 100644
--- a/library/lib/src/decoder/sync/webp_decoder.dart
+++ b/library/lib/src/decoder/sync/webp_decoder.dart
@@ -7,15 +7,28 @@ class WebpDecoder extends ImageDecoder {
 
   @override
   Size get size {
-    final widthList = input.getRange(0x1a, 0x1c);
-    final heightList = input.getRange(0x1c, 0x1e);
-    final width = convertRadix16ToInt(widthList, reverse: true);
-    final height = convertRadix16ToInt(heightList, reverse: true);
-    return Size(width, height);
+    if (_isExtendedFormat()) {
+      final widthList = input.getRange(0x18, 0x1b);
+      final heightList = input.getRange(0x1b, 0x1d);
+      final width = convertRadix16ToInt(widthList, reverse: true) + 1;
+      final height = convertRadix16ToInt(heightList, reverse: true) + 1;
+      return Size(width, height);
+    } else {
+      final widthList = input.getRange(0x1a, 0x1c);
+      final heightList = input.getRange(0x1c, 0x1e);
+      final width = convertRadix16ToInt(widthList, reverse: true);
+      final height = convertRadix16ToInt(heightList, reverse: true);
+      return Size(width, height);
+    }
   }
 
   Future<bool> isLossy() async {
     final v8Tag = input.getRange(12, 16);
     return v8Tag[3] == 0x20;
   }
+
+  bool _isExtendedFormat() {
+    final chunkHeader = input.getRange(12, 16);
+    return ListEquality().equals(chunkHeader, "VP8X".codeUnits);
+  }
 }
diff --git a/library/test/image_size_getter_test.dart b/library/test/image_size_getter_test.dart
index 00fe771..e53f8b1 100644
--- a/library/test/image_size_getter_test.dart
+++ b/library/test/image_size_getter_test.dart
@@ -12,6 +12,13 @@ void main() {
     await expectLater(size, Size(988, 466));
   });
 
+  test('Test webp extended format size', () async {
+    final file = File('../example/asset/demo_extended.webp');
+    final size = ImageSizeGetter.getSize(FileInput(file));
+    print('size = $size');
+    await expectLater(size, Size(988, 466));
+  });
+
   test('Test jpeg size', () async {
     final file = File('../example/asset/IMG_20180908_080245.jpg');
     final size = ImageSizeGetter.getSize(FileInput(file));