diff --git a/sys/sys/disk/gpt.h b/sys/sys/disk/gpt.h
--- a/sys/sys/disk/gpt.h
+++ b/sys/sys/disk/gpt.h
@@ -82,6 +82,13 @@
 CTASSERT(offsetof(struct gpt_hdr, padding) == 92);
 #endif
 
+/*
+ * The GPT standard (section 5.3 of UEFI standard version 2.10) requires
+ * we reserved at least 16k after the PMBR and the GPT header for the gpt
+ * array entries.
+ */
+#define GPT_MIN_RESERVED	16384
+
 struct gpt_ent {
 	gpt_uuid_t	ent_type;
 	gpt_uuid_t	ent_uuid;
diff --git a/usr.bin/mkimg/gpt.c b/usr.bin/mkimg/gpt.c
--- a/usr.bin/mkimg/gpt.c
+++ b/usr.bin/mkimg/gpt.c
@@ -26,6 +26,7 @@
 
 #include <sys/cdefs.h>
 #include <sys/errno.h>
+#include <sys/param.h>
 #include <stddef.h>
 #include <stdint.h>
 #include <stdlib.h>
@@ -129,8 +130,14 @@
 {
 	u_int ents;
 
+	/*
+	 * Round the number of sectors used by the existing partitions up to the
+	 * sector size. The GPT standard (see section 5.3 of UEFI standard
+	 * version 2.1) requires the reserved space be at last 16k (stated in an
+	 * odd way), so enforce that minimum here as well.
+	 */
 	ents = secsz / sizeof(struct gpt_ent);
-	return ((nparts + ents - 1) / ents);
+	return (MAX(GPT_MIN_RESERVED / secsz, (nparts + ents - 1) / ents));
 }
 
 static lba_t