diff --git a/src/lib.rs b/src/lib.rs
index be68365..e6c16a3 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1678,6 +1678,10 @@ impl Bump {
     /// on it only counting the sum of the sizes of the things
     /// you've allocated in the arena.
     ///
+    /// The allocated bytes do not include the sizes of bumpalo chunk headers,
+    /// so the amount of memory requested from the Rust allocator is higher
+    /// than the returned value.
+    ///
     /// ## Example
     ///
     /// ```