diff --git a/src/heap/heap-inl.h b/src/heap/heap-inl.h
index bcc82beb6a..da6a481c4b 100644
--- a/src/heap/heap-inl.h
+++ b/src/heap/heap-inl.h
@@ -460,6 +460,14 @@ void Heap::CopyBlock(Address dst, Address src, int byte_size) {
             static_cast<size_t>(byte_size / kPointerSize));
 }
 
+void Heap::UpdateNewSpaceAllocationCounter() {
+  new_space_allocation_counter_ = NewSpaceAllocationCounter();
+}
+
+size_t Heap::NewSpaceAllocationCounter() {
+  return new_space_allocation_counter_ + new_space()->AllocatedSinceLastGC();
+}
+
 template <Heap::FindMementoMode mode>
 AllocationMemento* Heap::FindAllocationMemento(HeapObject* object) {
   // Check if there is potentially a memento behind the object. If
diff --git a/src/heap/heap.h b/src/heap/heap.h
index 24eba0fe8d..c9503d3e4d 100644
--- a/src/heap/heap.h
+++ b/src/heap/heap.h
@@ -1257,13 +1257,8 @@ class Heap {
     return static_cast<intptr_t>(total);
   }
 
-  void UpdateNewSpaceAllocationCounter() {
-    new_space_allocation_counter_ = NewSpaceAllocationCounter();
-  }
-
-  size_t NewSpaceAllocationCounter() {
-    return new_space_allocation_counter_ + new_space()->AllocatedSinceLastGC();
-  }
+  inline void UpdateNewSpaceAllocationCounter();
+  inline size_t NewSpaceAllocationCounter();
 
   // This should be used only for testing.
   void set_new_space_allocation_counter(size_t new_value) {
diff --git a/src/heap/spaces-inl.h b/src/heap/spaces-inl.h
index 1593ed87ae..db01d0eed9 100644
--- a/src/heap/spaces-inl.h
+++ b/src/heap/spaces-inl.h
@@ -243,6 +243,25 @@ bool NewSpace::FromSpaceContainsSlow(Address a) {
 bool NewSpace::ToSpaceContains(Object* o) { return to_space_.Contains(o); }
 bool NewSpace::FromSpaceContains(Object* o) { return from_space_.Contains(o); }
 
+size_t NewSpace::AllocatedSinceLastGC() {
+  const intptr_t age_mark_offset =
+      NewSpacePage::OffsetInPage(to_space_.age_mark());
+  const intptr_t top_offset =
+      NewSpacePage::OffsetInPage(allocation_info_.top());
+  const intptr_t age_mark_delta =
+      age_mark_offset >= NewSpacePage::kObjectStartOffset
+          ? age_mark_offset - NewSpacePage::kObjectStartOffset
+          : NewSpacePage::kAllocatableMemory;
+  const intptr_t top_delta = top_offset >= NewSpacePage::kObjectStartOffset
+                                 ? top_offset - NewSpacePage::kObjectStartOffset
+                                 : NewSpacePage::kAllocatableMemory;
+  DCHECK((allocated_since_last_gc_ > 0) ||
+         (NewSpacePage::FromLimit(allocation_info_.top()) ==
+          NewSpacePage::FromLimit(to_space_.age_mark())));
+  return static_cast<size_t>(allocated_since_last_gc_ + top_delta -
+                             age_mark_delta);
+}
+
 // --------------------------------------------------------------------------
 // AllocationResult
 
diff --git a/src/heap/spaces.cc b/src/heap/spaces.cc
index 54b7696835..3f42527461 100644
--- a/src/heap/spaces.cc
+++ b/src/heap/spaces.cc
@@ -1482,6 +1482,7 @@ bool NewSpace::AddFreshPage() {
   int remaining_in_page = static_cast<int>(limit - top);
   heap()->CreateFillerObjectAt(top, remaining_in_page, ClearRecordedSlots::kNo);
   pages_used_++;
+  allocated_since_last_gc_ += NewSpacePage::kAllocatableMemory;
   UpdateAllocationInfo();
 
   return true;
diff --git a/src/heap/spaces.h b/src/heap/spaces.h
index 6832458a16..c7866b64fb 100644
--- a/src/heap/spaces.h
+++ b/src/heap/spaces.h
@@ -542,6 +542,10 @@ class MemoryChunk {
     return reinterpret_cast<MemoryChunk*>(OffsetFrom(a) & ~kAlignmentMask);
   }
 
+  static intptr_t OffsetInPage(Address a) {
+    return reinterpret_cast<intptr_t>(a) & kPageAlignmentMask;
+  }
+
   static inline MemoryChunk* FromAnyPointerAddress(Heap* heap, Address addr);
 
   static inline void UpdateHighWaterMark(Address mark) {
@@ -2582,6 +2586,7 @@ class NewSpace : public Space {
         from_space_(heap, kFromSpace),
         reservation_(),
         pages_used_(0),
+        allocated_since_last_gc_(0),
         top_on_previous_step_(0),
         allocated_histogram_(nullptr),
         promoted_histogram_(nullptr) {}
@@ -2653,42 +2658,7 @@ class NewSpace : public Space {
   // Return the available bytes without growing.
   intptr_t Available() override { return Capacity() - Size(); }
 
-  size_t AllocatedSinceLastGC() {
-    bool seen_age_mark = false;
-    Address age_mark = to_space_.age_mark();
-    NewSpacePage* current_page = to_space_.first_page();
-    NewSpacePage* age_mark_page = NewSpacePage::FromAddress(age_mark);
-    NewSpacePage* last_page = NewSpacePage::FromAddress(top() - kPointerSize);
-    if (age_mark_page == last_page) {
-      if (top() - age_mark >= 0) {
-        return top() - age_mark;
-      }
-      // Top was reset at some point, invalidating this metric.
-      return 0;
-    }
-    while (current_page != last_page) {
-      if (current_page == age_mark_page) {
-        seen_age_mark = true;
-        break;
-      }
-      current_page = current_page->next_page();
-    }
-    if (!seen_age_mark) {
-      // Top was reset at some point, invalidating this metric.
-      return 0;
-    }
-    intptr_t allocated = age_mark_page->area_end() - age_mark;
-    DCHECK_EQ(current_page, age_mark_page);
-    current_page = age_mark_page->next_page();
-    while (current_page != last_page) {
-      allocated += NewSpacePage::kAllocatableMemory;
-      current_page = current_page->next_page();
-    }
-    allocated += top() - current_page->area_start();
-    DCHECK_LE(0, allocated);
-    DCHECK_LE(allocated, Size());
-    return static_cast<size_t>(allocated);
-  }
+  inline size_t AllocatedSinceLastGC();
 
   // Return the maximum capacity of a semispace.
   int MaximumCapacity() {
@@ -2722,7 +2692,10 @@ class NewSpace : public Space {
   // Get the age mark of the inactive semispace.
   Address age_mark() { return from_space_.age_mark(); }
   // Set the age mark in the active semispace.
-  void set_age_mark(Address mark) { to_space_.set_age_mark(mark); }
+  void set_age_mark(Address mark) {
+    to_space_.set_age_mark(mark);
+    allocated_since_last_gc_ = 0;
+  }
 
   // The allocation top and limit address.
   Address* allocation_top_address() { return allocation_info_.top_address(); }
@@ -2843,6 +2816,7 @@ class NewSpace : public Space {
   SemiSpace from_space_;
   base::VirtualMemory reservation_;
   int pages_used_;
+  intptr_t allocated_since_last_gc_;
 
   // Allocation pointer and limit for normal allocation and allocation during
   // mark-compact collection.