diff --git a/src/hb-ot-hdmx-table.hh b/src/hb-ot-hdmx-table.hh
index deedd3808..5f0d197ad 100644
--- a/src/hb-ot-hdmx-table.hh
+++ b/src/hb-ot-hdmx-table.hh
@@ -53,15 +53,6 @@ struct DeviceRecord
       return this->subset_plan->gids_to_retain_sorted.len;
     }
 
-    inline unsigned int get_size () const
-    {
-      unsigned int raw_size = min_size + len () * HBUINT8::static_size;
-      if (raw_size % 4)
-        /* Align to 32 bits */
-        return raw_size + (4 - (raw_size % 4));
-      return raw_size;
-    }
-
     inline const HBUINT8& operator [] (unsigned int i) const
     {
       if (unlikely (i >= len())) return Null(HBUINT8);
@@ -70,11 +61,20 @@ struct DeviceRecord
     }
   };
 
+  static inline unsigned int get_size (unsigned int count)
+  {
+    unsigned int raw_size = min_size + count * HBUINT8::static_size;
+    if (raw_size % 4)
+      /* Align to 32 bits */
+      return raw_size + (4 - (raw_size % 4));
+    return raw_size;
+  }
+
   inline bool serialize (hb_serialize_context_t *c, const SubsetView &subset_view)
   {
     TRACE_SERIALIZE (this);
 
-    if (unlikely (!c->allocate_size<DeviceRecord> (subset_view.get_size())))
+    if (unlikely (!c->allocate_size<DeviceRecord> (get_size (subset_view.len()))))
       return_trace (false);
 
     this->pixel_size.set (subset_view.source_device_record->pixel_size);
@@ -93,8 +93,6 @@ struct DeviceRecord
   DEFINE_SIZE_MIN (2);
 };
 
-
-
 struct hdmx
 {
 
@@ -117,7 +115,7 @@ struct hdmx
 
     this->version.set (source_hdmx->version);
     this->num_records.set (source_hdmx->num_records);
-    this->size_device_record.set (source_hdmx->size_device_record);
+    this->size_device_record.set (DeviceRecord::get_size (plan->gids_to_retain_sorted.len));
 
     for (unsigned int i = 0; i < source_hdmx->num_records; i++)
     {
@@ -130,6 +128,11 @@ struct hdmx
     return_trace (true);
   }
 
+  static inline unsigned int get_subsetted_size (hb_subset_plan_t *plan)
+  {
+    return min_size + DeviceRecord::get_size (plan->gids_to_retain_sorted.len);
+  }
+
   inline bool subset (hb_subset_plan_t *plan) const
   {
     // TODO(grieger)