diff --git a/example_usage/lib/library_source.g.dart b/example_usage/lib/library_source.g.dart
index 26bcdcad..482ec38e 100644
--- a/example_usage/lib/library_source.g.dart
+++ b/example_usage/lib/library_source.g.dart
@@ -1,5 +1,4 @@
 // GENERATED CODE - DO NOT MODIFY BY HAND
-// dart format width=80
 
 // ignore_for_file: lint_a, lint_b
 
diff --git a/example_usage/lib/library_source.info.dart b/example_usage/lib/library_source.info.dart
index 99b7457e..47ff6ea3 100644
--- a/example_usage/lib/library_source.info.dart
+++ b/example_usage/lib/library_source.info.dart
@@ -1,5 +1,5 @@
-// GENERATED CODE - DO NOT MODIFY BY HAND
 // dart format width=80
+// GENERATED CODE - DO NOT MODIFY BY HAND
 
 // **************************************************************************
 // MemberCountLibraryGenerator
diff --git a/source_gen/CHANGELOG.md b/source_gen/CHANGELOG.md
index f21265b3..25df1300 100644
--- a/source_gen/CHANGELOG.md
+++ b/source_gen/CHANGELOG.md
@@ -13,7 +13,8 @@
 - `LibraryBuilder`, `PartBuilder`, and `SharedPartBuilder` now take an optional
   `writeDescriptions` boolean. When set to `false`, headers and generator
   descriptions for the files will not be included in the builder output.
-- Include `//dart format width=80` comments in generated Dart unit files.
+- Include `//dart format width=80` comments in files generated by a
+  `LibraryBuilder` or `PartBuilder` and formatted with the default callback.
 
 ## 1.5.0
 
diff --git a/source_gen/lib/builder.dart b/source_gen/lib/builder.dart
index e33bd4c0..9daadb0a 100644
--- a/source_gen/lib/builder.dart
+++ b/source_gen/lib/builder.dart
@@ -143,7 +143,6 @@ class CombiningBuilder implements Builder {
 
     final output = '''
 $defaultFileHeader
-$dartFormatWidth
 ${languageOverrideForLibrary(inputLibrary)}$ignoreForFile$preamble
 part of '$partOfUri';
 
diff --git a/source_gen/lib/src/builder.dart b/source_gen/lib/src/builder.dart
index 8945e107..53b8c2f7 100644
--- a/source_gen/lib/src/builder.dart
+++ b/source_gen/lib/src/builder.dart
@@ -50,7 +50,7 @@ class _Builder extends Builder {
   /// [options] to allow output files to be generated into a different directory
   _Builder(
     this._generators, {
-    this.formatOutput = _defaultFormatOutput,
+    required this.formatOutput,
     String generatedExtension = '.g.dart',
     List<String> additionalOutputExtensions = const [],
     String? header,
@@ -134,7 +134,6 @@ class _Builder extends Builder {
 
       if (this is PartBuilder) {
         contentBuffer
-          ..writeln(dartFormatWidth)
           ..write(languageOverrideForLibrary(library))
           ..writeln('part of \'$partOfUri\';');
         final part = computePartUrl(buildStep.inputId, outputId);
@@ -156,8 +155,6 @@ class _Builder extends Builder {
         // For shared-part builders, `part` statements will be checked by the
         // combining build step.
       }
-    } else {
-      contentBuffer.writeln(dartFormatWidth);
     }
 
     for (var item in generatedOutputs) {
@@ -239,7 +236,7 @@ class SharedPartBuilder extends _Builder {
   SharedPartBuilder(
     super.generators,
     String partId, {
-    super.formatOutput,
+    super.formatOutput = _defaultFormatOutput,
     super.additionalOutputExtensions,
     super.allowSyntaxErrors,
     super.writeDescriptions,
@@ -301,7 +298,7 @@ class PartBuilder extends _Builder {
   PartBuilder(
     super.generators,
     String generatedExtension, {
-    super.formatOutput,
+    super.formatOutput = _defaultFormatUnit,
     super.additionalOutputExtensions,
     super.writeDescriptions,
     super.header,
@@ -328,7 +325,8 @@ class LibraryBuilder extends _Builder {
   /// should be indicated in [additionalOutputExtensions].
   ///
   /// [formatOutput] is called to format the generated code. Defaults to
-  /// using the standard [DartFormatter].
+  /// using the standard [DartFormatter] and writing a comment specifying the
+  /// default format width of 80..
   ///
   /// [writeDescriptions] adds comments to the output used to separate the
   /// sections of the file generated from different generators, and reveals
@@ -344,7 +342,7 @@ class LibraryBuilder extends _Builder {
   /// libraries.
   LibraryBuilder(
     Generator generator, {
-    super.formatOutput,
+    super.formatOutput = _defaultFormatUnit,
     super.generatedExtension,
     super.additionalOutputExtensions,
     super.writeDescriptions,
@@ -414,6 +412,12 @@ const defaultFileHeader = '// GENERATED CODE - DO NOT MODIFY BY HAND';
 String _defaultFormatOutput(String code, Version version) =>
     DartFormatter(languageVersion: version).format(code);
 
+/// Prefixes a dart format width and formats [code].
+String _defaultFormatUnit(String code, Version version) {
+  code = '$dartFormatWidth\n$code';
+  return _defaultFormatOutput(code, version);
+}
+
 final _headerLine = '// '.padRight(77, '*');
 
 const partIdRegExpLiteral = r'[A-Za-z_\d-]+';
diff --git a/source_gen/test/builder_test.dart b/source_gen/test/builder_test.dart
index 2d6bb8b9..87261476 100644
--- a/source_gen/test/builder_test.dart
+++ b/source_gen/test/builder_test.dart
@@ -67,7 +67,7 @@ void main() {
       generateFor: {'$_pkgName|lib/test_lib.dart'},
       outputs: {
         '$_pkgName|lib/test_lib.g.dart': decodedMatches(
-          startsWith('$_customHeader\n$dartFormatWidth\n\n// ***'),
+          startsWith('$dartFormatWidth\n$_customHeader\n\n// ***'),
         ),
       },
     );
@@ -317,7 +317,7 @@ $dartFormatWidth
         generateFor: {'$_pkgName|lib/a.dart'},
         outputs: {
           '$_pkgName|lib/a.foo.dart': decodedMatches(
-            startsWith('$_customHeader\n\n$dartFormatWidth\npart of'),
+            startsWith('$dartFormatWidth\n$_customHeader\n\npart of'),
           ),
         },
       );
@@ -334,7 +334,7 @@ $dartFormatWidth
         generateFor: {'$_pkgName|lib/a.dart'},
         outputs: {
           '$_pkgName|lib/a.foo.dart':
-              decodedMatches(startsWith('$dartFormatWidth\npart of')),
+              decodedMatches(startsWith('$dartFormatWidth\n\npart of')),
         },
       );
     });
@@ -348,14 +348,13 @@ $dartFormatWidth
         ),
         {
           '$_pkgName|lib/a.dart': '''
-$dartFormatWidth
 // @dart=2.12
 part "a.foo.dart";''',
         },
         generateFor: {'$_pkgName|lib/a.dart'},
         outputs: {
           '$_pkgName|lib/a.foo.dart': decodedMatches(
-            startsWith('$dartFormatWidth\n// @dart=2.12\n'),
+            startsWith('$dartFormatWidth\n\n// @dart=2.12\n'),
           ),
         },
       );
@@ -422,7 +421,7 @@ part "a.foo.dart";''',
           generateFor: {'$_pkgName|lib/a.dart'},
           outputs: {
             '$_pkgName|lib/generated/a.foo.dart': decodedMatches(
-              startsWith("$dartFormatWidth\npart of '../a.dart';"),
+              startsWith("$dartFormatWidth\n\npart of '../a.dart';"),
             ),
           },
         );
@@ -570,7 +569,6 @@ part "a.foo.dart";''',
         const CombiningBuilder(),
         {
           '$_pkgName|lib/a.dart': '''
-$dartFormatWidth
 // @dart=2.12
 library a;
 part "a.g.dart";
@@ -582,7 +580,6 @@ part "a.g.dart";
           '$_pkgName|lib/a.g.dart': decodedMatches(
             '''
 // GENERATED CODE - DO NOT MODIFY BY HAND
-$dartFormatWidth
 // @dart=2.12
 
 part of 'a.dart';
@@ -1069,10 +1066,10 @@ const _testLibContentSyntaxError = r'''
 final int foo = 42
 ''';
 
-const _testGenPartContent = r'''
+const _testGenPartContent = '''
+$dartFormatWidth
 // GENERATED CODE - DO NOT MODIFY BY HAND
 
-// dart format width=80
 part of 'test_lib.dart';
 
 // **************************************************************************
@@ -1083,10 +1080,10 @@ part of 'test_lib.dart';
 // Code for "class Customer"
 ''';
 
-const _testGenPartContentForLibrary = r'''
+const _testGenPartContentForLibrary = '''
+$dartFormatWidth
 // GENERATED CODE - DO NOT MODIFY BY HAND
 
-// dart format width=80
 part of 'test_lib.dart';
 
 // **************************************************************************
@@ -1096,9 +1093,9 @@ part of 'test_lib.dart';
 // Code for "test_lib"
 ''';
 
-const _testGenStandaloneContent = r'''
+const _testGenStandaloneContent = '''
+$dartFormatWidth
 // GENERATED CODE - DO NOT MODIFY BY HAND
-// dart format width=80
 
 // **************************************************************************
 // CommentGenerator
@@ -1108,10 +1105,10 @@ const _testGenStandaloneContent = r'''
 // Code for "class Customer"
 ''';
 
-const _testGenPartContentForClassesAndLibrary = r'''
+const _testGenPartContentForClassesAndLibrary = '''
+$dartFormatWidth
 // GENERATED CODE - DO NOT MODIFY BY HAND
 
-// dart format width=80
 part of 'test_lib.dart';
 
 // **************************************************************************
@@ -1123,10 +1120,10 @@ part of 'test_lib.dart';
 // Code for "class Customer"
 ''';
 
-const _testGenNoLibrary = r'''
+const _testGenNoLibrary = '''
+$dartFormatWidth
 // GENERATED CODE - DO NOT MODIFY BY HAND
 
-// dart format width=80
 part of 'test_lib.dart';
 
 // **************************************************************************
@@ -1140,7 +1137,6 @@ part of 'test_lib.dart';
 const _whitespaceTrimmed = r'''
 // GENERATED CODE - DO NOT MODIFY BY HAND
 
-// dart format width=80
 part of 'test_lib.dart';
 
 // **************************************************************************
diff --git a/source_gen/test/generator_for_annotation_test.dart b/source_gen/test/generator_for_annotation_test.dart
index abd426a1..e78f6ad4 100644
--- a/source_gen/test/generator_for_annotation_test.dart
+++ b/source_gen/test/generator_for_annotation_test.dart
@@ -43,9 +43,9 @@ void main() {
       builder,
       _inputMap,
       outputs: {
-        'a|lib/file.g.dart': r'''
+        'a|lib/file.g.dart': '''
+$dartFormatWidth
 // GENERATED CODE - DO NOT MODIFY BY HAND
-// dart format width=80
 
 // **************************************************************************
 // Generator: Repeating
@@ -129,8 +129,8 @@ void main() {
       },
       outputs: {
         'a|lib/file.g.dart': '''
-// GENERATED CODE - DO NOT MODIFY BY HAND
 $dartFormatWidth
+// GENERATED CODE - DO NOT MODIFY BY HAND
 
 // **************************************************************************
 // Generator: Deprecated
@@ -166,8 +166,8 @@ $dartFormatWidth
       },
       outputs: {
         'a|lib/file.g.dart': '''
-// GENERATED CODE - DO NOT MODIFY BY HAND
 $dartFormatWidth
+// GENERATED CODE - DO NOT MODIFY BY HAND
 
 // **************************************************************************
 // Generator: Deprecated