diff --git a/Userland/Libraries/LibWeb/Streams/AbstractOperations.cpp b/Userland/Libraries/LibWeb/Streams/AbstractOperations.cpp
index 776dddb412..a6eb47dc77 100644
--- a/Userland/Libraries/LibWeb/Streams/AbstractOperations.cpp
+++ b/Userland/Libraries/LibWeb/Streams/AbstractOperations.cpp
@@ -187,6 +187,20 @@ bool readable_stream_has_default_reader(ReadableStream const& stream)
     return false;
 }
 
+// https://streams.spec.whatwg.org/#make-size-algorithm-from-size-function
+SizeAlgorithm extract_size_algorithm(QueuingStrategy const& strategy)
+{
+    // 1. If strategy["size"] does not exist, return an algorithm that returns 1.
+    if (!strategy.size)
+        return [](auto const&) { return JS::normal_completion(JS::Value(1)); };
+
+    // 2. Return an algorithm that performs the following steps, taking a chunk argument:
+    return [strategy](auto const& chunk) {
+        // 1. Return the result of invoking strategy["size"] with argument list « chunk ».
+        return WebIDL::invoke_callback(*strategy.size, JS::js_undefined(), chunk);
+    };
+}
+
 // https://streams.spec.whatwg.org/#validate-and-normalize-high-water-mark
 WebIDL::ExceptionOr<double> extract_high_water_mark(QueuingStrategy const& strategy, double default_hwm)
 {
diff --git a/Userland/Libraries/LibWeb/Streams/AbstractOperations.h b/Userland/Libraries/LibWeb/Streams/AbstractOperations.h
index 1e1e1281a5..5b9949a746 100644
--- a/Userland/Libraries/LibWeb/Streams/AbstractOperations.h
+++ b/Userland/Libraries/LibWeb/Streams/AbstractOperations.h
@@ -28,6 +28,7 @@ using WriteAlgorithm = JS::SafeFunction<WebIDL::ExceptionOr<JS::NonnullGCPtr<Web
 WebIDL::ExceptionOr<JS::NonnullGCPtr<ReadableStreamDefaultReader>> acquire_readable_stream_default_reader(ReadableStream&);
 bool is_readable_stream_locked(ReadableStream const&);
 
+SizeAlgorithm extract_size_algorithm(QueuingStrategy const&);
 WebIDL::ExceptionOr<double> extract_high_water_mark(QueuingStrategy const&, double default_hwm);
 
 void readable_stream_close(ReadableStream&);