diff --git a/src/UndoManager.cpp b/src/UndoManager.cpp
index bfeb2f3f9..979a0ce7d 100644
--- a/src/UndoManager.cpp
+++ b/src/UndoManager.cpp
@@ -72,7 +72,7 @@ UndoManager::~UndoManager()
 
 namespace {
    SpaceArray::value_type
-   CalculateUsage(TrackList *tracks, Set *prev, Set *cur)
+   CalculateUsage(TrackList *tracks, Set *seen)
    {
       SpaceArray::value_type result = 0;
 
@@ -90,18 +90,17 @@ namespace {
             {
                const auto &file = block.f;
 
-               // Accumulate space used by the file if the file didn't exist
-               // in the previous level
-               if (!prev || !cur ||
-                   (prev->count( &*file ) == 0 && cur->count( &*file ) == 0))
+               // Accumulate space used by the file if the file was not
+               // yet seen
+               if ( !seen || (seen->count( &*file ) == 0 ) )
                {
                   unsigned long long usage{ file->GetSpaceUsage() };
                   result += usage;
                }
 
                // Add file to current set
-               if (cur)
-                  cur->insert( &*file );
+               if (seen)
+                  seen->insert( &*file );
             }
          }
 
@@ -117,25 +116,31 @@ void UndoManager::CalculateSpaceUsage()
    space.clear();
    space.resize(stack.size(), 0);
 
-   Set s1, s2;
-   Set *prev = &s1;
-   Set *cur = &s2;
+   Set seen;
 
-   for (size_t i = 0, cnt = stack.size(); i < cnt; i++)
+   // After copies and pastes, a block file may be used in more than
+   // one place in one undo history state, and it may be used in more than
+   // one undo history state.  It might even be used in two states, but not
+   // in another state that is between them -- as when you have state A,
+   // then make a cut to get state B, but then paste it back into state C.
+
+   // So be sure to count each block file once only, in the last undo item that
+   // contains it.
+
+   // Why the last and not the first? Because the user of the History dialog
+   // may delete undo states, oldest first.  To reclaim disk space you must
+   // delete all states containing the block file.  So the block file's
+   // contribution to space usage should be counted only in that latest state.
+
+   for (size_t nn = stack.size(); nn--;)
    {
-      // Swap map pointers
-      std::swap(cur, prev);
-
-      // And clean out the NEW current map
-      cur->clear();
-
       // Scan all tracks at current level
-      auto tracks = stack[i]->state.tracks.get();
-      space[i] = CalculateUsage(tracks, prev, cur);
+      auto tracks = stack[nn]->state.tracks.get();
+      space[nn] = CalculateUsage(tracks, &seen);
    }
 
    mClipboardSpaceUsage = CalculateUsage
-      (AudacityProject::GetClipboardTracks(), nullptr, nullptr);
+      (AudacityProject::GetClipboardTracks(), nullptr);
 
    //TIMER_STOP( space_calc );
 }