Combo Feeding

My baby is 6 months old and exclusively breastfed. I just found out that I’m going to have to be away from him for 2 days next week (no option to take him with me) and I don’t think I’m going to have enough of a freezer stash to feed him with breast milk the whole time, even with pumping extra up until time to go. So if he ends up needing to be given formula, how much is he supposed to get in each bottle? I know the amounts with breast milk, but is formula the same? Or will he need more at each feed?