diff --git a/AK/IntrusiveList.h b/AK/IntrusiveList.h
index deadad169de..a5cdc77bbe7 100644
--- a/AK/IntrusiveList.h
+++ b/AK/IntrusiveList.h
@@ -51,6 +51,7 @@ public:
 
     void clear();
     [[nodiscard]] bool is_empty() const;
+    [[nodiscard]] size_t size_slow() const;
     void append(T& n);
     void prepend(T& n);
     void remove(T& n);
@@ -214,6 +215,17 @@ inline bool IntrusiveList<T, Container, member>::is_empty() const
     return m_storage.m_first == nullptr;
 }
 
+template<class T, typename Container, IntrusiveListNode<T, Container> T::*member>
+inline size_t IntrusiveList<T, Container, member>::size_slow() const
+{
+    size_t size = 0;
+    auto it_end = end();
+    for (auto it = begin(); it != it_end; ++it) {
+        ++size;
+    }
+    return size;
+}
+
 template<class T, typename Container, IntrusiveListNode<T, Container> T::*member>
 inline void IntrusiveList<T, Container, member>::append(T& n)
 {
diff --git a/Tests/AK/TestIntrusiveList.cpp b/Tests/AK/TestIntrusiveList.cpp
index 005642a9b6d..84f1d9f2c20 100644
--- a/Tests/AK/TestIntrusiveList.cpp
+++ b/Tests/AK/TestIntrusiveList.cpp
@@ -47,6 +47,7 @@ TEST_CASE(enumeration)
         actual_size++;
     }
     EXPECT_EQ(expected_size, actual_size);
+    EXPECT_EQ(expected_size, list.size_slow());
 
     size_t reverse_actual_size = 0;
     for (auto it = list.rbegin(); it != list.rend(); ++it) {