diff --git a/sys/fs/msdosfs/msdosfs_fat.c b/sys/fs/msdosfs/msdosfs_fat.c
--- a/sys/fs/msdosfs/msdosfs_fat.c
+++ b/sys/fs/msdosfs/msdosfs_fat.c
@@ -89,11 +89,17 @@
 fatblock(struct msdosfsmount *pmp, u_long ofs, u_long *bnp, u_long *sizep,
     u_long *bop)
 {
-	u_long bn, size;
+	u_long bn, size, fatblocksec;
 
+	fatblocksec = pmp->pm_fatblocksec;
+	if (FAT12(pmp) && fatblocksec % 3 != 0) {
+		fatblocksec *= 3;
+		if (fatblocksec % 6 == 0)
+			fatblocksec /= 2;
+	}
 	bn = ofs / pmp->pm_fatblocksize * pmp->pm_fatblocksec;
-	size = min(pmp->pm_fatblocksec, pmp->pm_FATsecs - bn)
-	    * DEV_BSIZE;
+	size = roundup(min(fatblocksec, pmp->pm_FATsecs - bn) * DEV_BSIZE,
+	    pmp->pm_BlkPerSec * DEV_BSIZE);
 	bn += pmp->pm_fatblk + pmp->pm_curfat * pmp->pm_FATsecs;
 
 	if (bnp)
diff --git a/sys/fs/msdosfs/msdosfs_vfsops.c b/sys/fs/msdosfs/msdosfs_vfsops.c
--- a/sys/fs/msdosfs/msdosfs_vfsops.c
+++ b/sys/fs/msdosfs/msdosfs_vfsops.c
@@ -700,11 +700,14 @@
 	}
 	pmp->pm_maxcluster = (pmp->pm_HugeSectors - pmp->pm_firstcluster) /
 	    SecPerClust + 1;
-	pmp->pm_fatsize = pmp->pm_FATsecs * DEV_BSIZE;	/* XXX not used? */
+	pmp->pm_fatsize = pmp->pm_FATsecs * DEV_BSIZE;
 
 	if (pmp->pm_fatmask == 0) {
-		if (pmp->pm_maxcluster <= ((CLUST_RSRVD - CLUST_FIRST) &
-		    FAT12_MASK)) {
+		/*
+		 * The last 10 (or 16?) clusters are reserved and must not
+		 * be allocated for data.
+		 */
+		if (pmp->pm_maxcluster < (CLUST_RSRVD & FAT12_MASK)) {
 			/*
 			 * This will usually be a floppy disk. This size makes
 			 * sure that one FAT entry will not be split across
@@ -720,11 +723,11 @@
 		}
 	}
 
-	clusters = (pmp->pm_fatsize / pmp->pm_fatmult) * pmp->pm_fatdiv;
+	clusters = (pmp->pm_fatsize / pmp->pm_fatmult) * pmp->pm_fatdiv ;
 	if (pmp->pm_maxcluster >= clusters) {
 #ifdef MSDOSFS_DEBUG
 		printf("Warning: number of clusters (%ld) exceeds FAT "
-		    "capacity (%ld)\n", pmp->pm_maxcluster + 1, clusters);
+		    "capacity (%ld)\n", pmp->pm_maxcluster - 1, clusters);
 #endif
 		pmp->pm_maxcluster = clusters - 1;
 	}
@@ -1045,7 +1048,7 @@
 	pmp = VFSTOMSDOSFS(mp);
 	sbp->f_bsize = pmp->pm_bpcluster;
 	sbp->f_iosize = pmp->pm_bpcluster;
-	sbp->f_blocks = pmp->pm_maxcluster + 1;
+	sbp->f_blocks = pmp->pm_maxcluster - CLUST_FIRST + 1;
 	sbp->f_bfree = pmp->pm_freeclustercount;
 	sbp->f_bavail = pmp->pm_freeclustercount;
 	sbp->f_files =	howmany(pmp->pm_rootdirsize * DEV_BSIZE,
diff --git a/sys/fs/msdosfs/msdosfs_vnops.c b/sys/fs/msdosfs/msdosfs_vnops.c
--- a/sys/fs/msdosfs/msdosfs_vnops.c
+++ b/sys/fs/msdosfs/msdosfs_vnops.c
@@ -313,8 +313,11 @@
 		vap->va_flags |= UF_SYSTEM;
 	vap->va_gen = 0;
 	vap->va_blocksize = pmp->pm_bpcluster;
-	vap->va_bytes =
-	    (dep->de_FileSize + pmp->pm_crbomask) & ~pmp->pm_crbomask;
+	if (dep->de_StartCluster != MSDOSFSROOT)
+		vap->va_bytes =
+		    (dep->de_FileSize + pmp->pm_crbomask) & ~pmp->pm_crbomask;
+	else
+		vap->va_bytes = 0; /* FAT12/FAT16 root dir in reserved area */
 	vap->va_type = ap->a_vp->v_type;
 	vap->va_filerev = dep->de_modrev;
 	return (0);