diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7520f8e9..25d35a88 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,7 @@
 
 * Change implicit to default value in example
 * Tidy up some declarations
+* Fix basic_streambuf::capacity
 
 --------------------------------------------------------------------------------
 
diff --git a/include/beast/core/impl/basic_streambuf.ipp b/include/beast/core/impl/basic_streambuf.ipp
index 76f15c57..c002af00 100644
--- a/include/beast/core/impl/basic_streambuf.ipp
+++ b/include/beast/core/impl/basic_streambuf.ipp
@@ -533,7 +533,7 @@ basic_streambuf<Allocator>::capacity() const
 {
     auto pos = out_;
     if(pos == list_.end())
-        return 0;
+        return in_size_;
     auto n = pos->size() - out_pos_;
     while(++pos != list_.end())
         n += pos->size();