diff --git a/lib/dns/qp.c b/lib/dns/qp.c
index 60acf13ed5..fec71033b1 100644
--- a/lib/dns/qp.c
+++ b/lib/dns/qp.c
@@ -426,13 +426,13 @@ static void
 realloc_chunk_arrays(dns_qp_t *qp, qp_chunk_t newmax) {
 	size_t oldptrs = sizeof(qp->base->ptr[0]) * qp->chunk_max;
 	size_t newptrs = sizeof(qp->base->ptr[0]) * newmax;
-	size_t allbytes = sizeof(dns_qpbase_t) + newptrs;
+	size_t size = STRUCT_FLEX_SIZE(qp->base, ptr, newmax);
 
 	if (qp->base == NULL || qpbase_unref(qp)) {
-		qp->base = isc_mem_reallocate(qp->mctx, qp->base, allbytes);
+		qp->base = isc_mem_reallocate(qp->mctx, qp->base, size);
 	} else {
 		dns_qpbase_t *oldbase = qp->base;
-		qp->base = isc_mem_allocate(qp->mctx, allbytes);
+		qp->base = isc_mem_allocate(qp->mctx, size);
 		memmove(&qp->base->ptr[0], &oldbase->ptr[0], oldptrs);
 	}
 	memset(&qp->base->ptr[qp->chunk_max], 0, newptrs - oldptrs);
diff --git a/lib/isc/histo.c b/lib/isc/histo.c
index 395b27b6b6..870c3c0152 100644
--- a/lib/isc/histo.c
+++ b/lib/isc/histo.c
@@ -27,13 +27,6 @@
 #include <isc/mem.h>
 #include <isc/tid.h>
 
-/*
- * XXXFANF to be added to <isc/util.h> by a commmit in a qp-trie
- * feature branch
- */
-#define STRUCT_FLEX_SIZE(pointer, member, count) \
-	(sizeof(*(pointer)) + sizeof(*(pointer)->member) * (count))
-
 /*
  * XXXFANF this should probably be in <isc/util.h> too
  */
diff --git a/lib/isc/ht.c b/lib/isc/ht.c
index d9de112e54..4d771882dd 100644
--- a/lib/isc/ht.c
+++ b/lib/isc/ht.c
@@ -287,7 +287,7 @@ isc__ht_add(isc_ht_t *ht, const unsigned char *key, const uint32_t keysize,
 
 	hash = hash_32(hashval, ht->hashbits[idx]);
 
-	node = isc_mem_get(ht->mctx, sizeof(*node) + keysize);
+	node = isc_mem_get(ht->mctx, STRUCT_FLEX_SIZE(node, key, keysize));
 	*node = (isc_ht_node_t){
 		.keysize = keysize,
 		.hashval = hashval,
@@ -396,7 +396,7 @@ isc__ht_delete(isc_ht_t *ht, const unsigned char *key, const uint32_t keysize,
 				prev->next = node->next;
 			}
 			isc_mem_put(ht->mctx, node,
-				    sizeof(*node) + node->keysize);
+				    STRUCT_FLEX_SIZE(node, key, node->keysize));
 			ht->count--;
 
 			return (ISC_R_SUCCESS);
diff --git a/lib/isc/include/isc/util.h b/lib/isc/include/isc/util.h
index 7fe6715a4e..c8b6e1c99f 100644
--- a/lib/isc/include/isc/util.h
+++ b/lib/isc/include/isc/util.h
@@ -96,6 +96,14 @@
 
 #define ARRAY_SIZE(x) (sizeof(x) / sizeof(x[0]))
 
+/*%
+ * Get the allocation size for a struct with a flexible array member
+ * containing `count` elements. The struct is identified by a pointer,
+ * typically the one that points to (or will point to) the allocation.
+ */
+#define STRUCT_FLEX_SIZE(pointer, member, count) \
+	(sizeof(*(pointer)) + sizeof(*(pointer)->member) * (count))
+
 /*%
  * Use this in translation units that would otherwise be empty, to
  * suppress compiler warnings.