How much oatmeal do you use for an oatmeal bath for an elderly adult?

You don't use oatml: The preparation used for "oatmeal" baths is not oatmeal. That would produce a gooey mess. During the milling of oats, a fine flour like powder is created as a byproduct. This milling powder is collected, placed in single use packets, & sold to use as a wetting & anti-itch preparation. It is a good product & many patients enjoy it.

Colloidal oatmeal: For an oatmeal bath you can use unflavored instant oatmeal, quick oats, or regular slow cooking oats. Blend them at highest speed into a fine powder. You can use a blender, food processor or coffee grinder. Add ½ to 1 ½ cups to warm (not hot) water & soak for 15 to 20 minutes. Use 1/3rd cup oatmeal for babies.