diff --git a/src/sandbox/external-pointer-table.cc b/src/sandbox/external-pointer-table.cc
index 03e08a7ad2..9e86734400 100644
--- a/src/sandbox/external-pointer-table.cc
+++ b/src/sandbox/external-pointer-table.cc
@@ -175,18 +175,20 @@ void ExternalPointerTable::StartCompactingIfNeeded() {
   uint32_t current_capacity = capacity();
 
   // Current (somewhat arbitrary) heuristic: need compacting if the table is
-  // more than 1MB in size and is at least 10% empty.
+  // more than 1MB in size, is at least 10% empty, and if at least one block
+  // can be decommitted after successful compaction.
   uint32_t table_size = current_capacity * kSystemPointerSize;
   double free_ratio = static_cast<double>(freelist_size) /
                       static_cast<double>(current_capacity);
-  bool should_compact = (table_size >= 1 * MB) && (free_ratio >= 0.10);
+  uint32_t num_blocks_to_evacuate = (freelist_size / 2) / kEntriesPerBlock;
+  bool should_compact = (table_size >= 1 * MB) && (free_ratio >= 0.10) &&
+                        (num_blocks_to_evacuate >= 1);
 
   if (should_compact) {
-    uint32_t num_entries_to_free = freelist_size / 2;
-    num_entries_to_free = RoundDown(num_entries_to_free, kBlockSize);
-    DCHECK_GT(num_entries_to_free, 0);
+    uint32_t num_entries_to_evacuate =
+        num_blocks_to_evacuate * kEntriesPerBlock;
     // A non-zero value for this member indicates that compaction is running.
-    start_of_evacuation_area_ = current_capacity - num_entries_to_free;
+    start_of_evacuation_area_ = current_capacity - num_entries_to_evacuate;
   }
 }
 
diff --git a/src/sandbox/external-pointer-table.h b/src/sandbox/external-pointer-table.h
index 0f9c1756fa..529fe3f0de 100644
--- a/src/sandbox/external-pointer-table.h
+++ b/src/sandbox/external-pointer-table.h
@@ -109,10 +109,13 @@ class V8_EXPORT_PRIVATE ExternalPointerTable {
   // This method is atomic and can be called from background threads.
   inline ExternalPointerHandle AllocateEntry();
 
-  // Determines the size of the freelist.
+  // Determines the number of entries currently on the freelist.
   // The freelist entries encode the freelist size and the next entry on the
   // list, so this routine fetches the first entry on the freelist and returns
   // the size encoded in it.
+  // As entries may be allocated from background threads while this method
+  // executes, its result should only be treated as an approximation of the
+  // real size.
   inline uint32_t FreelistSize();
 
   // Marks the specified entry as alive.
@@ -187,8 +190,8 @@ class V8_EXPORT_PRIVATE ExternalPointerTable {
   // Required for Isolate::CheckIsolateLayout().
   friend class Isolate;
 
-  // An external pointer table grows in blocks of this size. This is also the
-  // initial size of the table.
+  // An external pointer table grows and shrinks in blocks of this size. This
+  // is also the initial size of the table.
   static constexpr size_t kBlockSize = 16 * KB;
   static constexpr size_t kEntriesPerBlock = kBlockSize / kSystemPointerSize;
 
@@ -223,6 +226,7 @@ class V8_EXPORT_PRIVATE ExternalPointerTable {
   bool is_initialized() { return buffer_ != kNullAddress; }
 
   // Table capacity accesors.
+  // The capacity is expressed in number of entries.
   // The capacity of the table may increase during entry allocation (if the
   // table is grown) and may decrease during sweeping (if blocks at the end are
   // free). As the former may happen concurrently, the capacity can only be