diff --git a/mdadm.h b/mdadm.h
index 989d563..f219c95 100644
--- a/mdadm.h
+++ b/mdadm.h
@@ -1157,6 +1157,7 @@ extern unsigned long long get_component_size(int fd);
 extern void remove_partitions(int fd);
 extern int test_partition(int fd);
 extern int test_partition_from_id(dev_t id);
+extern int get_data_disks(int level, int layout, int raid_disks);
 extern unsigned long long calc_array_size(int level, int raid_disks, int layout,
 				   int chunksize, unsigned long long devsize);
 extern int flush_metadata_updates(struct supertype *st);
diff --git a/util.c b/util.c
index 0ea7e0d..50c98c1 100644
--- a/util.c
+++ b/util.c
@@ -702,6 +702,12 @@ void print_r10_layout(int layout)
 
 unsigned long long calc_array_size(int level, int raid_disks, int layout,
 				   int chunksize, unsigned long long devsize)
+{
+	devsize &= ~(unsigned long long)((chunksize>>9)-1);
+	return get_data_disks(level, layout, raid_disks) * devsize;
+}
+
+int get_data_disks(int level, int layout, int raid_disks)
 {
 	int data_disks = 0;
 	switch (level) {
@@ -713,8 +719,8 @@ unsigned long long calc_array_size(int level, int raid_disks, int layout,
 	case 10: data_disks = raid_disks / (layout & 255) / ((layout>>8)&255);
 		break;
 	}
-	devsize &= ~(unsigned long long)((chunksize>>9)-1);
-	return data_disks * devsize;
+
+	return data_disks;
 }
 
 #if !defined(MDASSEMBLE) || defined(MDASSEMBLE) && defined(MDASSEMBLE_AUTO)