diff --git a/octa/internal/hashtable.h b/octa/internal/hashtable.h
index 2d3cadc..a88864c 100644
--- a/octa/internal/hashtable.h
+++ b/octa/internal/hashtable.h
@@ -339,6 +339,7 @@ namespace detail {
 
         template<typename U>
         T *access_base(const U &key, octa::Size &h) const {
+            if (!p_size) return NULL;
             h = get_hash()(key) & (p_size - 1);
             for (Chain *c = p_data.first()[h]; c; c = c->next) {
                 if (get_eq()(key, B::get_key(c->value)))
@@ -363,6 +364,7 @@ namespace detail {
 
         template<typename ...Args>
         octa::Pair<Range, bool> emplace(Args &&...args) {
+            rehash_ahead(1);
             E elem(octa::forward<Args>(args)...);
             octa::Size h = get_hash()(B::get_key(elem)) & (p_size - 1);
             Chain *found = nullptr;
@@ -379,8 +381,10 @@ namespace detail {
                 B::swap_elem(found->value, elem);
             }
             Chain **hch = p_data.first();
-            return octa::make_pair(Range(hch + h + 1, hch + bucket_count(),
+            auto ret = octa::make_pair(Range(hch + h + 1, hch + bucket_count(),
                 found), ins);
+            rehash_up();
+            return octa::move(ret);
         }
 
         float load_factor() const { return float(p_len) / p_size; }
@@ -435,10 +439,28 @@ namespace detail {
             delete_chunks(chunks);
         }
 
+        void rehash_up() {
+            if (load_factor() <= max_load_factor()) return;
+            rehash(octa::Size(size() / max_load_factor()) * 2);
+        }
+
         void reserve(octa::Size count) {
             rehash(octa::Size(ceil(count / max_load_factor())));
         }
 
+        void reserve_at_least(octa::Size count) {
+            octa::Size nc = octa::Size(ceil(count / max_load_factor()));
+            if (p_size > nc) return;
+            rehash(nc);
+        }
+
+        void rehash_ahead(octa::Size n) {
+            if (!bucket_count())
+                reserve(n);
+            else if ((float(size() + n) / bucket_count()) > max_load_factor())
+                rehash(octa::Size((size() + 1) / max_load_factor()) * 2);
+        }
+
         Range each() {
             return Range(p_data.first(), p_data.first() + bucket_count());
         }
diff --git a/octa/map.h b/octa/map.h
index ba40cdd..5e8f877 100644
--- a/octa/map.h
+++ b/octa/map.h
@@ -50,6 +50,22 @@ private:
         octa::detail::MapBase<K, T, A>, octa::Pair<const K, T>, K, T, H, C, A
     >;
     Base p_table;
+
+    template<typename R>
+    octa::Size estimate_rsize(const R &range,
+        octa::EnableIf<octa::IsFiniteRandomAccessRange<R>::value, bool> = true
+    ) {
+        return range.size();
+    }
+
+    template<typename R>
+    octa::Size estimate_rsize(const R &,
+        octa::EnableIf<!octa::IsFiniteRandomAccessRange<R>::value, bool> = true
+    ) {
+        /* we have no idea how big the range actually is */
+        return 16;
+    }
+
 public:
 
     using Key = K;
@@ -72,8 +88,8 @@ public:
         const C &eqf = C(), const A &alloc = A()
     ): p_table(size, hf, eqf, alloc) {}
 
-    Map(): Map(octa::Size(1 << 10)) {}
-    explicit Map(const A &alloc): Map(octa::Size(1 << 10), H(), C(), alloc) {}
+    Map(): Map(0) {}
+    explicit Map(const A &alloc): Map(0, H(), C(), alloc) {}
 
     Map(octa::Size size, const A &alloc): Map(size, H(), C(), alloc) {}
     Map(octa::Size size, const H &hf, const A &alloc): Map(size, hf, C(), alloc) {}
@@ -87,16 +103,17 @@ public:
     Map(Map &&m, const A &alloc): p_table(octa::move(m.p_table), alloc) {}
 
     template<typename R>
-    Map(R range, octa::Size size = 1 << 10, const H &hf = H(),
+    Map(R range, octa::Size size = 0, const H &hf = H(),
         const C &eqf = C(), const A &alloc = A(),
         octa::EnableIf<
             octa::IsInputRange<R>::value &&
             octa::IsConvertible<RangeReference<R>, Value>::value,
             bool
         > = true
-    ): p_table(size, hf, eqf, alloc) {
+    ): p_table(size ? size : estimate_rsize(range), hf, eqf, alloc) {
         for (; !range.empty(); range.pop_front())
             emplace(range.front());
+        p_table.rehash_up();
     }
 
     template<typename R>
@@ -107,7 +124,7 @@ public:
     Map(R range, octa::Size size, const H &hf, const A &alloc)
     : Map(range, size, hf, C(), alloc) {}
 
-    Map(octa::InitializerList<Value> init, octa::Size size = 1 << 10,
+    Map(octa::InitializerList<Value> init, octa::Size size = 0,
         const H &hf = H(), const C &eqf = C(), const A &alloc = A()
     ): Map(octa::each(init), size, hf, eqf, alloc) {}
 
@@ -135,14 +152,17 @@ public:
         Map &
     > operator=(R range) {
         clear();
+        p_table.reserve_at_least(estimate_rsize(range));
         for (; !range.empty(); range.pop_front())
             emplace(range.front());
+        p_table.rehash_up();
         return *this;
     }
 
     Map &operator=(InitializerList<Value> il) {
-        Value *beg = il.begin(), *end = il.end();
+        const Value *beg = il.begin(), *end = il.end();
         clear();
+        p_table.reserve_at_least(end - beg);
         while (beg != end)
             emplace(*beg++);
         return *this;
@@ -175,12 +195,14 @@ public:
         octa::Size h;
         T *v = p_table.access_base(key, h);
         if (v) return *v;
+        p_table.rehash_ahead(1);
         return p_table.insert(h, key);
     }
     T &operator[](K &&key) {
         octa::Size h;
         T *v = p_table.access_base(key, h);
         if (v) return *v;
+        p_table.rehash_ahead(1);
         return p_table.insert(h, octa::move(key));
     }