system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How are casualties in battles/wars calculated?
gpt
You know how many people you send in. And you know how many people come back.

As for the other side, an educated guess. And it helps that battles usually have a winner who have to deal with the dead left behind by the loser.

But yeah, there can be a lot of guesswork and estimation involved.