diff --git a/tools/witx/src/layout.rs b/tools/witx/src/layout.rs
index ca5457ec..506cec03 100644
--- a/tools/witx/src/layout.rs
+++ b/tools/witx/src/layout.rs
@@ -148,6 +148,7 @@ impl UnionDatatype {
             .map(|sa| sa.align)
             .max()
             .expect("nonzero variants");
+        let size = align_to(size, align);
         SizeAlign { size, align }
     }
 }