diff --git a/usr.sbin/makefs/ffs/buf.c b/usr.sbin/makefs/ffs/buf.c
--- a/usr.sbin/makefs/ffs/buf.c
+++ b/usr.sbin/makefs/ffs/buf.c
@@ -70,7 +70,7 @@
 		printf("%s: blkno %lld size %d\n", __func__, (long long)blkno,
 		    size);
 	*bpp = getblk(vp, blkno, size, 0, 0, 0);
-	offset = (*bpp)->b_blkno * fs->sectorsize + fs->offset;
+	offset = (off_t)(*bpp)->b_blkno * fs->sectorsize + fs->offset;
 	if (debug & DEBUG_BUF_BREAD)
 		printf("%s: blkno %lld offset %lld bcount %ld\n", __func__,
 		    (long long)(*bpp)->b_blkno, (long long) offset,
@@ -130,7 +130,7 @@
 	fsinfo_t *fs = bp->b_fs;
 
 	assert (bp != NULL);
-	offset = bp->b_blkno * fs->sectorsize + fs->offset;
+	offset = (off_t)bp->b_blkno * fs->sectorsize + fs->offset;
 	if (debug & DEBUG_BUF_BWRITE)
 		printf("bwrite: blkno %lld offset %lld bcount %ld\n",
 		    (long long)bp->b_blkno, (long long) offset,
diff --git a/usr.sbin/makefs/ffs/mkfs.c b/usr.sbin/makefs/ffs/mkfs.c
--- a/usr.sbin/makefs/ffs/mkfs.c
+++ b/usr.sbin/makefs/ffs/mkfs.c
@@ -794,7 +794,7 @@
 	int n;
 	off_t offset;
 
-	offset = bno * fsopts->sectorsize + fsopts->offset;
+	offset = (off_t)bno * fsopts->sectorsize + fsopts->offset;
 	if (lseek(fsopts->fd, offset, SEEK_SET) < 0)
 		err(1, "%s: seek error for sector %lld", __func__,
 		    (long long)bno);
@@ -818,7 +818,7 @@
 	int n;
 	off_t offset;
 
-	offset = bno * fsopts->sectorsize + fsopts->offset;
+	offset = (off_t)bno * fsopts->sectorsize + fsopts->offset;
 	if (lseek(fsopts->fd, offset, SEEK_SET) < 0)
 		err(1, "%s: seek error for sector %lld", __func__,
 		    (long long)bno);