Baldur's gate fandom wiki saids damage is always rounded down, as long as integer part is not zero. for example "3 damage to 5% resistance becomes 2.85 damage; rounded down to 2 (33% reduction)"
But it looks opposite. After couple of simple experiments, damage is always round up. for example when physical resistance is 50%, 3 damage become 2 (obviously round up 1.5) and 2 damage isn't changed (1.9 -> 2).