diff --git a/fs/xfs/libxfs/xfs_rtbitmap.c b/fs/xfs/libxfs/xfs_rtbitmap.c
index b6a1d240c5545..2a453f0215ee0 100644
--- a/fs/xfs/libxfs/xfs_rtbitmap.c
+++ b/fs/xfs/libxfs/xfs_rtbitmap.c
@@ -1142,3 +1142,30 @@ xfs_rtalloc_extent_is_free(
 	*is_free = matches;
 	return 0;
 }
+
+/*
+ * Compute the number of rtbitmap blocks needed to track the given number of rt
+ * extents.
+ */
+xfs_filblks_t
+xfs_rtbitmap_blockcount(
+	struct xfs_mount	*mp,
+	xfs_rtbxlen_t		rtextents)
+{
+	return howmany_64(rtextents, NBBY * mp->m_sb.sb_blocksize);
+}
+
+/*
+ * Compute the number of rtbitmap words needed to populate every block of a
+ * bitmap that is large enough to track the given number of rt extents.
+ */
+unsigned long long
+xfs_rtbitmap_wordcount(
+	struct xfs_mount	*mp,
+	xfs_rtbxlen_t		rtextents)
+{
+	xfs_filblks_t		blocks;
+
+	blocks = xfs_rtbitmap_blockcount(mp, rtextents);
+	return XFS_FSB_TO_B(mp, blocks) >> XFS_WORDLOG;
+}
diff --git a/fs/xfs/libxfs/xfs_rtbitmap.h b/fs/xfs/libxfs/xfs_rtbitmap.h
index f616956b28911..308ce814a908a 100644
--- a/fs/xfs/libxfs/xfs_rtbitmap.h
+++ b/fs/xfs/libxfs/xfs_rtbitmap.h
@@ -261,6 +261,11 @@ xfs_rtfree_extent(
 /* Same as above, but in units of rt blocks. */
 int xfs_rtfree_blocks(struct xfs_trans *tp, xfs_fsblock_t rtbno,
 		xfs_filblks_t rtlen);
+
+xfs_filblks_t xfs_rtbitmap_blockcount(struct xfs_mount *mp, xfs_rtbxlen_t
+		rtextents);
+unsigned long long xfs_rtbitmap_wordcount(struct xfs_mount *mp,
+		xfs_rtbxlen_t rtextents);
 #else /* CONFIG_XFS_RT */
 # define xfs_rtfree_extent(t,b,l)			(-ENOSYS)
 # define xfs_rtfree_blocks(t,rb,rl)			(-ENOSYS)
@@ -268,6 +273,13 @@ int xfs_rtfree_blocks(struct xfs_trans *tp, xfs_fsblock_t rtbno,
 # define xfs_rtalloc_query_all(m,t,f,p)			(-ENOSYS)
 # define xfs_rtbuf_get(m,t,b,i,p)			(-ENOSYS)
 # define xfs_rtalloc_extent_is_free(m,t,s,l,i)		(-ENOSYS)
+static inline xfs_filblks_t
+xfs_rtbitmap_blockcount(struct xfs_mount *mp, xfs_rtbxlen_t rtextents)
+{
+	/* shut up gcc */
+	return 0;
+}
+# define xfs_rtbitmap_wordcount(mp, r)			(0)
 #endif /* CONFIG_XFS_RT */
 
 #endif /* __XFS_RTBITMAP_H__ */
diff --git a/fs/xfs/libxfs/xfs_trans_resv.c b/fs/xfs/libxfs/xfs_trans_resv.c
index 9c3dbcbe05358..01fccc4ade8a5 100644
--- a/fs/xfs/libxfs/xfs_trans_resv.c
+++ b/fs/xfs/libxfs/xfs_trans_resv.c
@@ -221,11 +221,12 @@ xfs_rtalloc_block_count(
 	struct xfs_mount	*mp,
 	unsigned int		num_ops)
 {
-	unsigned int		blksz = XFS_FSB_TO_B(mp, 1);
-	unsigned int		rtbmp_bytes;
+	unsigned int		rtbmp_blocks;
+	xfs_rtxlen_t		rtxlen;
 
-	rtbmp_bytes = xfs_extlen_to_rtxlen(mp, XFS_MAX_BMBT_EXTLEN) / NBBY;
-	return (howmany(rtbmp_bytes, blksz) + 1) * num_ops;
+	rtxlen = xfs_extlen_to_rtxlen(mp, XFS_MAX_BMBT_EXTLEN);
+	rtbmp_blocks = xfs_rtbitmap_blockcount(mp, rtxlen);
+	return (rtbmp_blocks + 1) * num_ops;
 }
 
 /*
diff --git a/fs/xfs/scrub/rtsummary.c b/fs/xfs/scrub/rtsummary.c
index 901696c593dc9..29cd26b7779d1 100644
--- a/fs/xfs/scrub/rtsummary.c
+++ b/fs/xfs/scrub/rtsummary.c
@@ -172,12 +172,11 @@ xchk_rtsum_compute(
 	struct xfs_scrub	*sc)
 {
 	struct xfs_mount	*mp = sc->mp;
-	unsigned long long	rtbmp_bytes;
+	unsigned long long	rtbmp_blocks;
 
 	/* If the bitmap size doesn't match the computed size, bail. */
-	rtbmp_bytes = howmany_64(mp->m_sb.sb_rextents, NBBY);
-	if (roundup_64(rtbmp_bytes, mp->m_sb.sb_blocksize) !=
-			mp->m_rbmip->i_disk_size)
+	rtbmp_blocks = xfs_rtbitmap_blockcount(mp, mp->m_sb.sb_rextents);
+	if (XFS_FSB_TO_B(mp, rtbmp_blocks) != mp->m_rbmip->i_disk_size)
 		return -EFSCORRUPTED;
 
 	return xfs_rtalloc_query_all(sc->mp, sc->tp, xchk_rtsum_record_free,
diff --git a/fs/xfs/xfs_rtalloc.c b/fs/xfs/xfs_rtalloc.c
index 14d01b97b892d..32e3ac40e28f7 100644
--- a/fs/xfs/xfs_rtalloc.c
+++ b/fs/xfs/xfs_rtalloc.c
@@ -996,7 +996,7 @@ xfs_growfs_rt(
 	 */
 	nrextents = nrblocks;
 	do_div(nrextents, in->extsize);
-	nrbmblocks = howmany_64(nrextents, NBBY * sbp->sb_blocksize);
+	nrbmblocks = xfs_rtbitmap_blockcount(mp, nrextents);
 	nrextslog = xfs_highbit32(nrextents);
 	nrsumlevels = nrextslog + 1;
 	nrsumsize = (uint)sizeof(xfs_suminfo_t) * nrsumlevels * nrbmblocks;
