How much oatmeal do you use for an oatmeal bath in a regular bathtub?

You don't: An "oatmeal" bath is not made using oat meal. During the process of milling oats, a fine dust like powder is created. That fine powder is known for it's soothing qualities when added to a lukewarm bath.If you dump oatmeal in a tub you will just get a mess.The powder is packaged & sold & that's what you need if you want to try 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.