diff --git a/Tests/LibWeb/Layout/expected/flex/specified-size-suggestion-with-box-sizing-border-box.txt b/Tests/LibWeb/Layout/expected/flex/specified-size-suggestion-with-box-sizing-border-box.txt
new file mode 100644
index 00000000000..c41d592c1c2
--- /dev/null
+++ b/Tests/LibWeb/Layout/expected/flex/specified-size-suggestion-with-box-sizing-border-box.txt
@@ -0,0 +1,5 @@
+Viewport <#document> at (0,0) content-size 800x600 children: not-inline
+ BlockContainer at (0,0) content-size 800x100 children: not-inline
+ Box
at (0,0) content-size 800x100 flex-container(row) children: not-inline
+ ImageBox
at (0,0) content-size 400x100 flex-item children: not-inline
+ ImageBox at (600,0) content-size 200x100 flex-item children: not-inline
diff --git a/Tests/LibWeb/Layout/input/flex/specified-size-suggestion-with-box-sizing-border-box.html b/Tests/LibWeb/Layout/input/flex/specified-size-suggestion-with-box-sizing-border-box.html
new file mode 100644
index 00000000000..0d8afc12309
--- /dev/null
+++ b/Tests/LibWeb/Layout/input/flex/specified-size-suggestion-with-box-sizing-border-box.html
@@ -0,0 +1,21 @@
+![]()
\ No newline at end of file
diff --git a/Userland/Libraries/LibWeb/Layout/FlexFormattingContext.cpp b/Userland/Libraries/LibWeb/Layout/FlexFormattingContext.cpp
index 803c62a8b11..57100e1ac6d 100644
--- a/Userland/Libraries/LibWeb/Layout/FlexFormattingContext.cpp
+++ b/Userland/Libraries/LibWeb/Layout/FlexFormattingContext.cpp
@@ -705,8 +705,10 @@ Optional FlexFormattingContext::specified_size_suggestion(FlexItem co
{
// If the item’s preferred main size is definite and not automatic,
// then the specified size suggestion is that size. It is otherwise undefined.
- if (has_definite_main_size(item.box))
- return inner_main_size(item.box);
+ if (has_definite_main_size(item.box)) {
+ // NOTE: We use get_pixel_{width,height} to ensure that CSS box-sizing is respected.
+ return is_row_layout() ? get_pixel_width(item.box, computed_main_size(item.box)) : get_pixel_height(item.box, computed_main_size(item.box));
+ }
return {};
}