To recalculate is to count or measure something again, using new information. You might need to recalculate your monthly budget after your landlord raises the rent.
If the vet says your cat is getting too fat, you'll have to recalculate how much you feed him to help him lose weight. And when you're planning a dinner party and your brother announces he's bringing five friends, you'll need to recalculate how much food to make. We can trace the verb calculate back to the Latin calculus, originally "pebble used as a reckoning counter." The prefix re- adds the sense of "do it again."