diff --git a/Misc/NEWS b/Misc/NEWS
index c0e65c446a4..ed6ab78b6c8 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -10,6 +10,8 @@ What's New in Python 3.7.0 alpha 1
 Core and Builtins
 -----------------
 
+- Issue #28509: dict.update() no longer allocate unnecessary large memory.
+
 - Issue #28426: Fixed potential crash in PyUnicode_AsDecodedObject() in debug
   build.
 
diff --git a/Objects/dictobject.c b/Objects/dictobject.c
index 2cca2c678e6..3c54926256b 100644
--- a/Objects/dictobject.c
+++ b/Objects/dictobject.c
@@ -2407,9 +2407,11 @@ dict_merge(PyObject *a, PyObject *b, int override)
          * incrementally resizing as we insert new items.  Expect
          * that there will be no (or few) overlapping keys.
          */
-        if (mp->ma_keys->dk_usable * 3 < other->ma_used * 2)
-            if (dictresize(mp, (mp->ma_used + other->ma_used)*2) != 0)
+        if (USABLE_FRACTION(mp->ma_keys->dk_size) < other->ma_used) {
+            if (dictresize(mp, ESTIMATE_SIZE(mp->ma_used + other->ma_used))) {
                return -1;
+            }
+        }
         ep0 = DK_ENTRIES(other->ma_keys);
         for (i = 0, n = other->ma_keys->dk_nentries; i < n; i++) {
             PyObject *key, *value;