diff --git a/source/ff.c b/source/ff.c
index 7f56871..7b04676 100644
--- a/source/ff.c
+++ b/source/ff.c
@@ -5642,7 +5642,7 @@ FRESULT f_mkfs (
 		b_fat = b_vol + 32;										/* FAT start at offset 32 */
 		sz_fat = ((sz_vol / au + 2) * 4 + ss - 1) / ss;			/* Number of FAT sectors */
 		b_data = (b_fat + sz_fat + sz_blk - 1) & ~(sz_blk - 1);	/* Align data area to the erase block boundary */
-		if (b_data >= sz_vol / 2) LEAVE_MKFS(FR_MKFS_ABORTED);	/* Too small volume? */
+		if (b_data - b_vol >= sz_vol / 2) LEAVE_MKFS(FR_MKFS_ABORTED);	/* Too small volume? */
 		n_clst = (sz_vol - (b_data - b_vol)) / au;				/* Number of clusters */
 		if (n_clst <16) LEAVE_MKFS(FR_MKFS_ABORTED);			/* Too few clusters? */
 		if (n_clst > MAX_EXFAT) LEAVE_MKFS(FR_MKFS_ABORTED);	/* Too many clusters? */