Are flat and inflated foil balloon dimensions interchangeable?

No. Keep them separately labeled and confirm which state the supplier's value describes. An inflated shape has three-dimensional form, so its displayed outline need not match the flat outline. Ask GleeParty and Yiwu Jingjie Crafts Co., Ltd. for the selected item's measurement basis and relevant instructions. Do not assume a fixed conversion percentage or adjust inflation merely to reach a preferred dimension.