Index: usr.bin/mkimg/mkimg.1
===================================================================
--- usr.bin/mkimg/mkimg.1
+++ usr.bin/mkimg/mkimg.1
@@ -348,6 +348,18 @@
 utility supports assigning labels to the partitions specified.
 In the following example the file system partition is labeled as 'backup':
 .Dl % mkimg -s gpt -p freebsd-ufs/backup:=file-system.ufs -o gpt.img
+.Pp
+The
+.Nm
+utility also support flexible size partition if a maximum disk capacity
+have been provided. Flexible sizes are given in percentage and the resulting
+size is calculated from the whole disk capacity.
+.Dl % mkimg --capacity 1G -s gpt -p freebsd-swap::10% -p freebsd-ufs/ping::45% \
+-p freebsd-ufs/pong::45% -o pingpong.img
+You can also use ~ as size for specifying that one partition should take all
+the available size.
+.Dl % mkimg --capacity 1G -s gpt -p freebsd-swap::10% -p freebsd-ufs::~ -o fill.img
+Note that you cannot use fill partition with cascaded ones.
 .Sh SEE ALSO
 .Xr dd 1 ,
 .Xr gpart 8 ,
Index: usr.bin/mkimg/mkimg.c
===================================================================
--- usr.bin/mkimg/mkimg.c
+++ usr.bin/mkimg/mkimg.c
@@ -171,7 +171,8 @@
 	fputc('\n', stderr);
 	fprintf(stderr, "    partition specification:\n");
 	fprintf(stderr, "\t<t>[/<l>]::<size>\t-  empty partition of given "
-	    "size\n");
+	    "size (see the\n"
+	    "\t\t\t\t   manpage for flexible size example)\n");
 	fprintf(stderr, "\t<t>[/<l>]:=<file>\t-  partition content and size "
 	    "are determined\n\t\t\t\t   by the named file\n");
 	fprintf(stderr, "\t<t>[/<l>]:-<cmd>\t-  partition content and size "
@@ -400,16 +401,75 @@
 mkimg(void)
 {
 	FILE *fp;
-	struct part *part;
-	lba_t block;
-	off_t bytesize;
-	int error, fd;
+	struct part *part, *fillpart = NULL;
+	struct stat sb;
+	lba_t block, metablock, diskblock, partblock = 0;
+	off_t bytesize, percent;
+	char *percntptr;
+	int num_parts = 0, error, fd, have_pipe_part = 0;
+
+	TAILQ_FOREACH(part, &partlist, link) {
+		num_parts++;
+	}
+
+	/* 
+	 * Calculate the number of usable blocks on the disk, without
+	 * the scheme metadata.
+	 * Since the block after a partition can be rounded up, we add
+	 * one block per partition that exists.
+	 * It may not be the best calculation but seems to work with any
+	 * sector size.
+	 */
+	if (max_capacity != 0) {
+		metablock = scheme_metadata(SCHEME_META_IMG_START, 0) +
+			scheme_metadata(SCHEME_META_IMG_END, 0) + num_parts + 1;
+		diskblock = max_capacity - (metablock * secsz);
+	}
 
 	/* First check partition information */
 	TAILQ_FOREACH(part, &partlist, link) {
 		error = scheme_check_part(part);
 		if (error)
 			errc(EX_DATAERR, error, "partition %d", part->index+1);
+		switch (part->kind) {
+		case PART_KIND_SIZE:
+			percntptr = strchr(part->contents, '%');
+			if (percntptr != NULL && max_capacity != 0) {
+				*percntptr = '\0';
+				if (expand_number(part->contents, &percent) == -1) {
+					error = errno;
+					break;
+				}
+				part->size = diskblock * percent / 100;
+			} else if (strcmp(part->contents, "~") == 0) {
+				if (fillpart)
+					usage("cannot have two fill partition");
+				fillpart = part;
+			} else {
+				if (expand_number(part->contents, &part->size) == -1)
+					error = errno;
+			}
+			break;
+		case PART_KIND_FILE:
+			if (stat(part->contents, &sb) == -1)
+				errc(EX_DATAERR, errno, "partition %d", part->index+1);
+			part->size = sb.st_size;
+			break;
+		case PART_KIND_PIPE:
+			have_pipe_part = 1;
+			break;
+		}
+		partblock += part->size;
+	}
+
+	if (fillpart) {
+		if (max_capacity == 0)
+			usage("cannot fill partition without a capacity");
+		if (have_pipe_part == 1)
+			usage("cannot fill partition with cascade partition");
+		fillpart->size = diskblock - partblock;
+		if (fillpart->size <= 0)
+			usage("no more room for fill partition");
 	}
 
 	block = scheme_metadata(SCHEME_META_IMG_START, 0);
@@ -421,8 +481,7 @@
 		part->block = block;
 		switch (part->kind) {
 		case PART_KIND_SIZE:
-			if (expand_number(part->contents, &bytesize) == -1)
-				error = errno;
+			bytesize = part->size;
 			break;
 		case PART_KIND_FILE:
 			fd = open(part->contents, O_RDONLY, 0);