diff --git a/src/libstd/collections/hash/map.rs b/src/libstd/collections/hash/map.rs
index 4eb2c8f064414..4c58394b5f1ab 100644
--- a/src/libstd/collections/hash/map.rs
+++ b/src/libstd/collections/hash/map.rs
@@ -1906,7 +1906,10 @@ impl<K, V, S> FromIterator<(K, V)> for HashMap<K, V, S>
 impl<K, V, S> Extend<(K, V)> for HashMap<K, V, S>
     where K: Eq + Hash, S: BuildHasher
 {
-    fn extend<T: IntoIterator<Item=(K, V)>>(&mut self, iter: T) {
+    fn extend<T: IntoIterator<Item=(K, V)>>(&mut self, iterable: T) {
+        let iter = iterable.into_iter();
+        let (lower_bound, _) = iter.size_hint();
+        self.reserve(lower_bound);
         for (k, v) in iter {
             self.insert(k, v);
         }
diff --git a/src/libstd/collections/hash/set.rs b/src/libstd/collections/hash/set.rs
index ca5137e957362..c94c9f3ad893c 100644
--- a/src/libstd/collections/hash/set.rs
+++ b/src/libstd/collections/hash/set.rs
@@ -643,7 +643,10 @@ impl<T, S> Extend<T> for HashSet<T, S>
     where T: Eq + Hash,
           S: BuildHasher,
 {
-    fn extend<I: IntoIterator<Item=T>>(&mut self, iter: I) {
+    fn extend<I: IntoIterator<Item=T>>(&mut self, iterable: I) {
+        let iter = iterable.into_iter();
+        let (lower_bound, _) = iter.size_hint();
+        self.reserve(lower_bound);
         for k in iter {
             self.insert(k);
         }