diff --git a/src/libstd/collections/hash/map.rs b/src/libstd/collections/hash/map.rs
index fbb69ca974930..96af227257824 100644
--- a/src/libstd/collections/hash/map.rs
+++ b/src/libstd/collections/hash/map.rs
@@ -588,6 +588,9 @@ impl<K, V, S> HashMap<K, V, S>
 impl<K: Hash + Eq, V> HashMap<K, V, RandomState> {
     /// Creates an empty `HashMap`.
     ///
+    /// The hash map is initially created with a capacity of 0, so it will not allocate until it
+    /// is first inserted into.
+    ///
     /// # Examples
     ///
     /// ```
diff --git a/src/libstd/collections/hash/set.rs b/src/libstd/collections/hash/set.rs
index 9771363d545cd..51698ce7c17ca 100644
--- a/src/libstd/collections/hash/set.rs
+++ b/src/libstd/collections/hash/set.rs
@@ -125,6 +125,9 @@ pub struct HashSet<T, S = RandomState> {
 impl<T: Hash + Eq> HashSet<T, RandomState> {
     /// Creates an empty `HashSet`.
     ///
+    /// The hash set is initially created with a capacity of 0, so it will not allocate until it
+    /// is first inserted into.
+    ///
     /// # Examples
     ///
     /// ```