diff --git a/backend/id_set.go b/backend/id_set.go
index 6b36e0aba..253147605 100644
--- a/backend/id_set.go
+++ b/backend/id_set.go
@@ -66,3 +66,10 @@ func (s *IDSet) Find(id ID) error {
 
 	return nil
 }
+
+func (s *IDSet) Len() int {
+	s.m.Lock()
+	defer s.m.Unlock()
+
+	return len(s.list)
+}
diff --git a/map.go b/map.go
index 1a5b385c4..080e46f25 100644
--- a/map.go
+++ b/map.go
@@ -140,6 +140,14 @@ func (bl *Map) Equals(other *Map) bool {
 	return true
 }
 
+// Len returns the number of blobs in the map.
+func (bl *Map) Len() int {
+	bl.m.Lock()
+	defer bl.m.Unlock()
+
+	return len(bl.list)
+}
+
 // Prune deletes all IDs from the map except the ones listed in ids.
 func (m *Map) Prune(ids *backend.IDSet) {
 	m.m.Lock()