diff --git a/db/repl.h b/db/repl.h
index f1a48a6eeeb..942e8e1f894 100644
--- a/db/repl.h
+++ b/db/repl.h
@@ -158,6 +158,15 @@ namespace mongo {
             else
                 imp_[ ns ].erase( id );
         }
+        long long roughSize() const {
+            long long size = 0;
+            for( map< string, BSONObjSetDefaultOrder >::const_iterator i = imp_.begin();
+                i != imp_.end(); ++i )
+                for( BSONObjSetDefaultOrder::const_iterator j = i->second.begin();
+                    j != i->second.end(); ++j )
+                    size += sizeof( BSONObj );
+            return size;
+        }
     private:
         typedef map< string, BSONObjSetDefaultOrder > IdSets;
         IdSets imp_;
diff --git a/dbtests/repltests.cpp b/dbtests/repltests.cpp
index 277093546b3..86756bbc373 100644
--- a/dbtests/repltests.cpp
+++ b/dbtests/repltests.cpp
@@ -735,14 +735,25 @@ namespace ReplTests {
     public:
         MemIdsTest() : s_( "MemIdsTest" ) {}
         void run() {
+            int n = sizeof( BSONObj );
+            
             s_.reset();
+            ASSERT_EQUALS( 0, s_.roughSize() );
             ASSERT( !s_.get( "a", BSON( "_id" << 4 ) ) );
             ASSERT( !s_.get( "b", BSON( "_id" << 4 ) ) );
             s_.set( "a", BSON( "_id" << 4 ), true );
+            ASSERT_EQUALS( n, s_.roughSize() );
             ASSERT( s_.get( "a", BSON( "_id" << 4 ) ) );
             ASSERT( !s_.get( "b", BSON( "_id" << 4 ) ) );
             s_.set( "a", BSON( "_id" << 4 ), false );
+            ASSERT_EQUALS( 0, s_.roughSize() );
             ASSERT( !s_.get( "a", BSON( "_id" << 4 ) ) );
+
+            s_.set( "a", BSON( "_id" << 4 ), true );
+            s_.set( "b", BSON( "_id" << 4 ), true );
+            s_.set( "b", BSON( "_id" << 100 ), true );
+            s_.set( "b", BSON( "_id" << 101 ), true );
+            ASSERT_EQUALS( n * 4, s_.roughSize() );
         }
     private:
         MemIds s_;
@@ -782,6 +793,7 @@ namespace ReplTests {
             add< Idempotence::EmptyPush >();
             add< DeleteOpIsIdBased >();
             add< DbIdsTest >();
+            add< MemIdsTest >();
         }
     };