diff --git a/Libraries/LibWeb/Layout/Box.h b/Libraries/LibWeb/Layout/Box.h
index c08460f6c6..0e53216beb 100644
--- a/Libraries/LibWeb/Layout/Box.h
+++ b/Libraries/LibWeb/Layout/Box.h
@@ -52,6 +52,12 @@ public:
     float width() const { return m_size.width(); }
     float height() const { return m_size.height(); }
 
+    float border_box_width() const
+    {
+        auto border_box = box_model().border_box(*this);
+        return width() + border_box.left + border_box.right;
+    }
+
     float absolute_x() const { return absolute_rect().x(); }
     float absolute_y() const { return absolute_rect().y(); }
     Gfx::FloatPoint absolute_position() const { return absolute_rect().location(); }
diff --git a/Libraries/LibWeb/Layout/FormattingContext.cpp b/Libraries/LibWeb/Layout/FormattingContext.cpp
index 44d1e4bcfb..ba5c7ca610 100644
--- a/Libraries/LibWeb/Layout/FormattingContext.cpp
+++ b/Libraries/LibWeb/Layout/FormattingContext.cpp
@@ -96,7 +96,7 @@ static float greatest_child_width(const Box& box)
         }
     } else {
         box.for_each_child_of_type<Box>([&](auto& child) {
-            max_width = max(max_width, child.width());
+            max_width = max(max_width, child.border_box_width());
         });
     }
     return max_width;