diff --git a/app/javascript/mastodon/features/compose/components/compose_form.jsx b/app/javascript/mastodon/features/compose/components/compose_form.jsx
index 9222b2dc8..289730265 100644
--- a/app/javascript/mastodon/features/compose/components/compose_form.jsx
+++ b/app/javascript/mastodon/features/compose/components/compose_form.jsx
@@ -100,7 +100,7 @@ class ComposeForm extends ImmutablePureComponent {
     const fulltext = this.getFulltextForCharacterCounting();
     const isOnlyWhitespace = fulltext.length !== 0 && fulltext.trim().length === 0;
 
-    return !(isSubmitting || isUploading || isChangingUpload || length(fulltext) > 500 || (isOnlyWhitespace && !anyMedia));
+    return !(isSubmitting || isUploading || isChangingUpload || length(fulltext) > 640 || (isOnlyWhitespace && !anyMedia));
   };
 
   handleSubmit = (e) => {
@@ -297,7 +297,7 @@ class ComposeForm extends ImmutablePureComponent {
             </div>
 
             <div className='character-counter__wrapper'>
-              <CharacterCounter max={500} text={this.getFulltextForCharacterCounting()} />
+              <CharacterCounter max={640} text={this.getFulltextForCharacterCounting()} />
             </div>
           </div>
         </div>
diff --git a/app/serializers/rest/instance_serializer.rb b/app/serializers/rest/instance_serializer.rb
index 14aeda946..d6baa4908 100644
--- a/app/serializers/rest/instance_serializer.rb
+++ b/app/serializers/rest/instance_serializer.rb
@@ -11,7 +11,7 @@ class REST::InstanceSerializer < ActiveModel::Serializer
 
   attributes :domain, :title, :version, :source_url, :description,
              :usage, :thumbnail, :languages, :configuration,
-             :registrations
+             :registrations, :max_post_chars
 
   has_one :contact, serializer: ContactSerializer
   has_many :rules, serializer: REST::RuleSerializer
@@ -89,6 +89,10 @@ class REST::InstanceSerializer < ActiveModel::Serializer
     }
   end
 
+  def max_post_chars
+    640
+  end
+
   private
 
   def registrations_enabled?
diff --git a/spec/validators/note_length_validator_spec.rb b/spec/validators/note_length_validator_spec.rb
index 66fccad3e..d3626637c 100644
--- a/spec/validators/note_length_validator_spec.rb
+++ b/spec/validators/note_length_validator_spec.rb
@@ -3,11 +3,11 @@
 require 'rails_helper'
 
 describe NoteLengthValidator do
-  subject { described_class.new(attributes: { note: true }, maximum: 500) }
+  subject { described_class.new(attributes: { note: true }, maximum: 640) }
 
   describe '#validate' do
-    it 'adds an error when text is over 500 characters' do
-      text = 'a' * 520
+    it 'adds an error when text is over 640 characters' do
+      text = 'a' * 650
       account = instance_double(Account, note: text, errors: activemodel_errors)
 
       subject.validate_each(account, 'note', text)