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.