diff --git a/delta.h b/delta.h
index 7b3f86d85f..40ccf5a1e9 100644
--- a/delta.h
+++ b/delta.h
@@ -23,6 +23,13 @@ create_delta_index(const void *buf, unsigned long bufsize);
  */
 extern void free_delta_index(struct delta_index *index);
 
+/*
+ * sizeof_delta_index: returns memory usage of delta index
+ *
+ * Given pointer must be what create_delta_index() returned, or NULL.
+ */
+extern unsigned long sizeof_delta_index(struct delta_index *index);
+
 /*
  * create_delta: create a delta from given index for the given buffer
  *
diff --git a/diff-delta.c b/diff-delta.c
index faf96e4713..3af583536f 100644
--- a/diff-delta.c
+++ b/diff-delta.c
@@ -119,6 +119,7 @@ struct index_entry {
 };
 
 struct delta_index {
+	unsigned long memsize;
 	const void *src_buf;
 	unsigned long src_size;
 	unsigned int hash_mask;
@@ -159,6 +160,7 @@ struct delta_index * create_delta_index(const void *buf, unsigned long bufsize)
 	mem = hash + hsize;
 	entry = mem;
 
+	index->memsize = memsize;
 	index->src_buf = buf;
 	index->src_size = bufsize;
 	index->hash_mask = hmask;
@@ -228,6 +230,14 @@ void free_delta_index(struct delta_index *index)
 	free(index);
 }
 
+unsigned long sizeof_delta_index(struct delta_index *index)
+{
+	if (index)
+		return index->memsize;
+	else
+		return 0;
+}
+
 /*
  * The maximum size for any opcode sequence, including the initial header
  * plus Rabin window plus biggest copy.