diff --git a/era_copy.c b/era_copy.c
index 26a6e06..5ca625a 100644
--- a/era_copy.c
+++ b/era_copy.c
@@ -45,7 +45,7 @@ void read_era_invalidate_and_copy(FILE *fp, int src, int era_block_size)
 	// read input XML
 	char buf[XML_BUFSIZE] = { 0 };
 	char c = 0;
-	long long start = 0, length = 0;
+	long long start = 0, length = 0, total = 0;
 	if (fgets(buf, XML_BUFSIZE, fp) == NULL)
 	{
 		fprintf(stderr, "Input block list is empty\n");
@@ -70,47 +70,67 @@ void read_era_invalidate_and_copy(FILE *fp, int src, int era_block_size)
 		{
 			break;
 		}
-		start = start*era_block_size;
-		length = length*era_block_size;
-		// write a very simple binary format: SIGNATURE, start, length, data, ...
-		write(1, SIGNATURE, 8);
-		write(1, &start, 8);
-		write(1, &length, 8);
-		copy_blocks(src, 1, start, length);
+		start = start*era_block_size*512;
+		length = length*era_block_size*512;
+		if (src >= 0)
+		{
+			// write a very simple binary format: SIGNATURE, start, length, data, ...
+			write(1, SIGNATURE, 8);
+			write(1, &start, 8);
+			write(1, &length, 8);
+			copy_blocks(src, 1, start, length);
+		}
+		else
+		{
+			// calculate diff size
+			total += length;
+		}
 	}
 	if (sscanf(buf, " </blocks > %c", &c) == 0 || c != 0)
 	{
 		fprintf(stderr, "</blocks> expected, but \"%s\" found\n", buf);
 		exit(1);
 	}
+	if (src < 0)
+	{
+		printf("%lld bytes\n", total);
+	}
 }
 
 void era_copy(char *src_path, int era_block_size)
 {
-	int src;
-	src = open(src_path, O_RDONLY|O_LARGEFILE);
-	if (src < 0)
+	int src = -1;
+	if (src_path != NULL)
 	{
-		fprintf(stderr, "Failed to open %s for reading: %s\n", src_path, strerror(errno));
-		exit(1);
+		src = open(src_path, O_RDONLY|O_LARGEFILE);
+		if (src < 0)
+		{
+			fprintf(stderr, "Failed to open %s for reading: %s\n", src_path, strerror(errno));
+			exit(1);
+		}
 	}
 	read_era_invalidate_and_copy(stdin, src, era_block_size);
-	close(src);
+	if (src_path != NULL)
+	{
+		close(src);
+	}
 }
 
 int main(int narg, char *args[])
 {
-	if (narg < 3)
+	if (narg < 2)
 	{
 		fprintf(stderr,
 			"era_copy - parses era_invalidate output and saves changed blocks to a stream\n"
 			"(c) Vitaliy Filippov, 2019+, distributed under the terms of GNU GPLv3.0 or later license\n"
 			"\n"
 			"USAGE:\nera_invalidate --metadata-snapshot --written-since <ERA> <META_DEVICE> |\\\n"
-			"    era_copy <ERA_BLOCK_SIZE> <DATA_DEVICE> > era_copy.bin\n"
+			"    era_copy <ERA_BLOCK_SIZE> [<DATA_DEVICE>] > era_copy.bin\n"
 			"\n"
-			"ERA_BLOCK_SIZE is the block size you used when creating dm-era device\n"
-			"You can take it from `dmsetup table <DATA_DEVICE>`: it's the last number in the table\n"
+			"When invoked without <DATA_DEVICE>, era_copy only calculates the future diff size and prints it\n"
+			"\n"
+			"ERA_BLOCK_SIZE is dm-era's granularity in 512-byte sectors you used when creating dm-era device\n"
+			"You can take it from `dmsetup table <DATA_DEVICE>`: it's the last number in DM table\n"
 			"For example, in `0 3625546496 era 259:3 259:2 65536` it's 65536\n"
 		);
 		exit(1);
@@ -121,6 +141,6 @@ int main(int narg, char *args[])
 		fprintf(stderr, "Incorrect era_block_size = %d\n", bs);
 		exit(1);
 	}
-	era_copy(args[2], bs);
+	era_copy(narg == 2 ? NULL : args[2], bs);
 	return 0;
 }