If one ate less than a kazayit bread, i know he does not have to wash. But does he have to bentch?
Actually, even in regard to washing for less than a "kzai'it" of bread there is a difference of opinion if one is required to wash. Therefore, the Mishna B'rura (קנח, ס"ק י) suggests to be stringent and to wash but without a bracha. In regard to bentching, one is required to bentch only if he ate a Kzai'it of bread.(שולחן ערוך או"ח קפ"ד, סעיף ו). However, it is interesting to point out that the Aruch Hashulchan (או"ח רב, ב) quotes the Ari z"l ,who says that one who wants to be truly pious should not leave himself in situations of question marks in regard to B'rachot and if in doubt as to bentching, one should eat a bit more so he is definitely required to bentch.