Index: uspace/app/mkfat/fat.h
===================================================================
--- uspace/app/mkfat/fat.h	(revision 0be611b165e8500d2cab8cb84b16ef832e39bf4a)
+++ uspace/app/mkfat/fat.h	(revision 51d0ee96220005ad1f14f5eef909675e50f15541)
@@ -47,5 +47,5 @@
 #define FAT32	32
 
-#define FAT_SIZE(a) (((a) == FAT12) ? 1.5 : (((a) == FAT16) ? 2 : 4)) 
+#define FAT_CLUSTER_DOUBLE_SIZE(a) ((a) / 4)
 
 typedef struct fat_bs {
Index: uspace/app/mkfat/mkfat.c
===================================================================
--- uspace/app/mkfat/mkfat.c	(revision 0be611b165e8500d2cab8cb84b16ef832e39bf4a)
+++ uspace/app/mkfat/mkfat.c	(revision 51d0ee96220005ad1f14f5eef909675e50f15541)
@@ -253,5 +253,6 @@
 		return ENOSPC;
 
-	fat_bytes = (cfg->total_clusters + 2) * FAT_SIZE(cfg->fat_type);
+	fat_bytes = div_round_up((cfg->total_clusters + 2) *
+	    FAT_CLUSTER_DOUBLE_SIZE(cfg->fat_type), 2);
 	cfg->fat_sectors = div_round_up(fat_bytes, cfg->sector_size);