diff --git a/lib/dns/include/dns/rbt.h b/lib/dns/include/dns/rbt.h
index 3b62e12114..c144f83b49 100644
--- a/lib/dns/include/dns/rbt.h
+++ b/lib/dns/include/dns/rbt.h
@@ -992,4 +992,11 @@ dns__rbtnode_namelen(dns_rbtnode_t *node);
  * Returns the length of the full name of the node. Used only internally
  * and in unit tests.
  */
+
+unsigned int
+dns__rbtnode_getsize(dns_rbtnode_t *node);
+/*
+ * Return allocated size for a node.
+ */
+
 ISC_LANG_ENDDECLS
diff --git a/lib/dns/rbt.c b/lib/dns/rbt.c
index 57dcd54542..4d8c142fc2 100644
--- a/lib/dns/rbt.c
+++ b/lib/dns/rbt.c
@@ -302,6 +302,13 @@ dns__rbtnode_namelen(dns_rbtnode_t *node) {
 	return (len);
 }
 
+unsigned int
+dns__rbtnode_getsize(dns_rbtnode_t *node) {
+	REQUIRE(DNS_RBTNODE_VALID(node));
+
+	return (NODE_SIZE(node));
+}
+
 /*
  * Initialize a red/black tree of trees.
  */
diff --git a/lib/dns/rbtdb.c b/lib/dns/rbtdb.c
index 55776d5a74..864a159097 100644
--- a/lib/dns/rbtdb.c
+++ b/lib/dns/rbtdb.c
@@ -10201,7 +10201,9 @@ overmem_purge(dns_rbtdb_t *rbtdb, rdatasetheader_t *newheader,
 	      bool tree_locked) {
 	uint32_t locknum_start = rbtdb->lru_sweep++ % rbtdb->node_lock_count;
 	uint32_t locknum = locknum_start;
-	size_t purgesize = rdataset_size(newheader);
+	/* Size of added data, possible node and possible ENT node. */
+	size_t purgesize = rdataset_size(newheader) +
+			   2 * dns__rbtnode_getsize(newheader->node);
 	size_t purged = 0;
 	isc_stdtime_t min_last_used = 0;
 	size_t max_passes = 8;