diff --git a/tokio-util/src/io/reader_stream.rs b/tokio-util/src/io/reader_stream.rs
index 93bc594f22f..e0348dac7ce 100644
--- a/tokio-util/src/io/reader_stream.rs
+++ b/tokio-util/src/io/reader_stream.rs
@@ -58,6 +58,11 @@ impl<R: AsyncRead> ReaderStream<R> {
     /// Convert an [`AsyncRead`] into a [`Stream`] with item type
     /// `Result<Bytes, std::io::Error>`.
     ///
+    /// Currently, the default capacity 4096 bytes (4 KiB).
+    /// This capacity is not part of the semver contract
+    /// and may be tweaked in future releases without
+    /// requiring a major version bump.
+    ///
     /// [`AsyncRead`]: tokio::io::AsyncRead
     /// [`Stream`]: futures_core::Stream
     pub fn new(reader: R) -> Self {